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  • (instead of printing)
Java
1
Print Hello World
Print a literal string on standard output 
System.out.println("Hello World");
Alternative implementation:
out.print("Hello, World!");
2
Print Hello 10 times
Loop to execute some code a constant number of times 
for(int i=0;i<10;i++)
  System.out.println("Hello");
Alternative implementation:
System.out.print("Hello\n".repeat(10));
Alternative implementation:
int i = 0;
while (i++ < 10) out.println("Hello");
Alternative implementation:
for (int i = 0; i++ < 10; out.println("Hello"));
Alternative implementation:
int i = 0;
do out.println("Hello");
while (i++ < 10);
Alternative implementation:
range(0, 10).forEach(x -> out.println("Hello"));
Alternative implementation:
generate(() -> "Hello%n")
    .limit(10)
    .forEach(out::printf);
3
Create a procedure
Like a function which doesn't return any value, thus has only side effects (e.g. Print to standard output) 
void finish(String name){
  System.out.println("My job here is done. Goodbye " + name);
}
Alternative implementation:
private void methodName() {
	System.out.println("Hello, World!");
}
Alternative implementation:
void f() { out.println("abc"); }
Alternative implementation:
interface F { void set(); }
F f = () -> out.println("abc");
f.set();
4
Create a function
Create a function which returns the square of an integer 
int square(int x){
  return x*x;
}
Alternative implementation:
Function<Integer,Integer> squareFunction = x -> x * x;
Alternative implementation:
int square(int a) { return a * a; }
Alternative implementation:
int square(int a) { return (int) pow(a, 2); }
Alternative implementation:
interface F { int get(int a); }
F square = x -> x * x;
Alternative implementation:
BigInteger square(BigInteger a) {
    return a.multiply(a);
}
5
Create a 2D Point data structure
Declare a container type for two floating-point numbers x and y 
class Point{
  double x;
  double y;
}
Alternative implementation:
class Point { double x, y; }
Alternative implementation:
public record Point(double x, double y) { }
6
Iterate over list values
Do something with each item x of the list (or array) items, regardless indexes. 
for (Item x: items) {
    doSomething(x);
}
Alternative implementation:
for(int i=0;i<items.length;i++){
	doSomething( items[i] );
}
Alternative implementation:
items.stream().forEach(item -> doSomething(item));
Alternative implementation:
for (T x : items) {}
Alternative implementation:
T t;
Iterator<T> i = items.iterator();
while (i.hasNext()) {
    t = i.next();
}
Alternative implementation:
T t;
Iterator<T> i = items.listIterator();
while (i.hasNext()) {
    t = i.next();
}
7
Iterate over list indexes and values
Print each index i with its value x from an array-like collection items 
int i, n;
for (i = 0, n = items.length; i < n; ++i)
    out.println(i + " = " + items[i]);
Alternative implementation:
int i, n;
for (i = 0, n = items.size(); i < n; ++i)
    out.println(i + " = " + items.get(i));
Alternative implementation:
int i = 0;
for (T t : items)
    out.println(i++ + " = " + t);
Alternative implementation:
range(0, items.length)
    .forEach(i -> {
        out.println(i + " = " + items[i]);
    });
8
Create a map (associative array)
Create a new map object x, and provide some (key, value) pairs as initial content. 
Map<String,Integer> x = new HashMap<>();
x.put("one", 1);
x.put("two", 2);
Alternative implementation:
final Map<String, Integer> x = new HashMap<String, Integer>() {{
    put("one", 1);
    put("two", 2);
    put("three", 3);
}};
Alternative implementation:
Map<String, Integer> x = of("x", 1, "y", 2);
Alternative implementation:
Entry<String, Integer> a = entry("x", 1),
                       b = entry("y", 2);
Map<String, Integer> x = ofEntries(a, b);
Alternative implementation:
Map<String, Integer> x = new HashMap<>(of("x", 1, "y", 2));
9
Create a Binary Tree data structure
The structure must be recursive because left child and right child are binary trees too. A node has access to children nodes, but not to its parent. 
class BinTree<T extends Comparable<T>>{
   T value;
   BinTree<T> left;
   BinTree<T> right;
}
Alternative implementation:
interface Tree<T>{
    boolean search(T x);
}
class Nil<T extends Comparable<? super T>> implements Tree<T>{
    public boolean search(T x) { return false; }
}
class Node<T extends Comparable<? super T>> implements Tree<T> {
 T item;  Tree<T> left; Tree<T> right;
 public Node(T i, Tree<T> l, Tree<T> r) {
     item=i;  left=l;  right=r; }
 public boolean search(T x) {
     int cmp = x.compareTo(item);
     return cmp==0 || (cmp<0 && left.search(x)) || (cmp>0 && right.search(x));
 }
10
Shuffle a list
Generate a random permutation of the elements of list x 
Collections.shuffle(x);
Alternative implementation:
shuffle(asList(x));
11
Pick a random element from a list
The list x must be non-empty. 
x.get((int)(Math.random()*x.size()))
Alternative implementation:
x.get(ThreadLocalRandom.current().nextInt(0, x.size()))
Alternative implementation:
if (!x.isEmpty()) {
    Random r = new Random();
    T t = x.get(r.nextInt(x.size()));
}
12
Check if list contains a value
Check if the list contains the value x.
list is an iterable finite container. 
boolean contains(int[] list, int x){
  for(int y:list)
    if( y==x )
      return true;
  return false;
}
Alternative implementation:
boolean <T> contains(T[] list, T x){
  if( x==null){
    for(T y:list)
      if( y==null )
        return true;
  }else{
    for(T y:list)
      if( x.equals(y) )
        return true;
  }
  return false;
}
Alternative implementation:
list.contains(x)
Alternative implementation:
asList(list).contains(x);
13
Iterate over map keys and values
Access each key k with its value x from an associative array mymap, and print them. 
for (Map.Entry<Object, Object> entry : mymap.entrySet()) {
    Object k = entry.getKey();
    Object x = entry.getValue();
    System.out.println("Key=" + k + ", Value=" + x);
}
Alternative implementation:
mymap.forEach((k,x) -> System.out.println("Key=" + k + ", Value=" + x));
Alternative implementation:
K k;
X x;
for (Entry<K, X> e : mymap.entrySet()) {
    k = e.getKey();
    x = e.getValue();
    out.println(e);
}
14
Pick uniformly a random floating point number in [a..b)
Pick a random number greater than or equals to a, strictly inferior to b. Precondition : a < b. 
double pick(double a, double b){
	return a + (Math.random() * (b-a));
}
Alternative implementation:
Random r = new Random();
double v = r.nextDouble(a, b);
Alternative implementation:
Random r = new Random();
double v = (r.nextDouble() * (b - a)) + a;
15
Pick uniformly a random integer in [a..b]
Pick a random integer greater than or equals to a, inferior or equals to b. Precondition : a < b. 
int pick(int a, int b){
	return a + new Random().nextInt(b - a + 1);
}
Alternative implementation:
int v = new Random().nextInt(a, b + 1);
16
Depth-first traversal of a binary tree
Call a function f on every node of binary tree bt, in depth-first infix order 
void dfs(BinTree bt) {
	if (bt.left != null) {
		dfs(bt.left);
        }
	f(bt);
	if (bt.right != null) {
		dfs(bt.right);
        }
}
Alternative implementation:
class BinTree {
	// ...

	void dfs() {
		if( left != null )
			left.dfs();
		f(this);
		if( right != null )
			right.dfs();
	}
}
Alternative implementation:
class BinTree<T> {
	// ...

	void dfs(Consumer<BinTree<T>> f) {
		if( left != null )
			left.dfs(f);
		f.accept(this);
		if( right != null )
			right.dfs(f);
	}
}
17
Create a Tree data structure
The structure must be recursive. A node may have zero or more children. A node has access to its children nodes, but not to its parent. 
class Tree<K,V> {
  K key;
  V deco;
  List<Tree<K,V>> children = new ArrayList<>();
}
19
Reverse a list
Reverse the order of the elements of the list x.
This may reverse "in-place" and destroy the original ordering. 
static <T> void reverse(List<T> x){
	int n = x.size();
	for(int i=0;i<n/2;i++){
		T tmp = x.get(i);
		x.set(i, x.get(n-i-1));
		x.set(n-i-1, tmp);
	}
}
Alternative implementation:
Collections.reverse(x);
Alternative implementation:
Collections.reverse(x);
Alternative implementation:
int i, m = x.size(), n = m-- / 2;
for (i = 0; i < n; ++i) swap(x, i, m - i);
Alternative implementation:
reverse(x);
20
Return two values
Implement a function search which looks for item x in a 2D matrix m.
Return indices i, j of the matching cell.
Think of the most idiomatic way in the language to return the two values at the same time. 
static class Position{
	int i;
	int j;
}

Position search(int[][] m, int x){
	for(int i=0;i<m.length;i++)
		for(int j=0;j<m[i].length;j++)
			if(m[i][j] == x){
				Position pos= new Position();
				pos.i = i;
				pos.j = j;
				return pos;
			}
	return null;
}
Alternative implementation:
record Cell(int i, int j) {}
<T> Cell search(T x, T m[][]) {
    int i, j, M = m.length, N;
    for (i = 0; i < M; ++i) {
        N = m[i].length;
        for (j = 0; j < N; ++j)
            if (m[i][j] == x)
                return new Cell(i, j);
    }
    return null;
}
21
Swap values
Swap the values of the variables a and b
 
T tmp = a;
a = b;
b = tmp;
22
Convert string to integer
Extract the integer value i from its string representation s (in radix 10) 
int i = Integer.parseInt(s);
Alternative implementation:
int i = new Integer(s).intValue();
Alternative implementation:
Integer i = Integer.valueOf(s, 10);
Alternative implementation:
int i = s.chars()
     .map(x -> x - '0')
     .reduce((a, b) -> (a * 10) + b)
     .getAsInt();
23
Convert real number to string with 2 decimal places
Given a real number x, create its string representation s with 2 decimal digits following the dot. 
String s = String.format("%.2f", x);
Alternative implementation:
String s = "%.2f".formatted(x);
Alternative implementation:
MathContext m = new MathContext(3, HALF_UP);
BigDecimal d = new BigDecimal(x, m);
String s = d.toPlainString();
Alternative implementation:
BigDecimal d = new BigDecimal(x);
String s = "%.2f".formatted(d);
Alternative implementation:
NumberFormat f = getNumberInstance();
f.setRoundingMode(HALF_UP);
f.setMaximumFractionDigits(2);
String s = f.format(x);
24
Assign to string the japanese word ネコ
Declare a new string s and initialize it with the literal value "ネコ" (which means "cat" in japanese) 
String s = "ネコ";
25
Send a value to another thread
Share the string value "Alan" with an existing running process which will then display "Hello, Alan" 
https://gist.github.com/f7c174c53efaba4d8575
Alternative implementation:
class Process {
    volatile String s;
    void f() throws InterruptedException {
        Thread a = new Thread(() -> {
            try {
                sleep(1_000);
                out.println("Hello, " + s);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        });
        a.start();
        sleep(500);
        s = "Alan";
    }
}
Process p = new Process();
p.f();
26
Create a 2-dimensional array
Declare and initialize a matrix x having m rows and n columns, containing real numbers. 
double[][] x = new double[m][n];
Alternative implementation:
BigDecimal x[][] = new BigDecimal[m][n];
27
Create a 3-dimensional array
Declare and initialize a 3D array x, having dimensions boundaries m, n, p, and containing real numbers. 
double[][][] x = new double[m][n][p];
Alternative implementation:
BigDecimal x[][][] = new BigDecimal[m][n][p];
28
Sort by a property
Sort the elements of the list (or array-like collection) items in ascending order of x.p, where p is a field of the type Item of the objects in items. 
Arrays.sort(items, new Comparator<Item>(){
	public int compare(Item a, Item b){
		return a.p - b.p;
	}
});
Alternative implementation:
Collections.sort(items, new Comparator<Item>(){
	@Override
	public int compare(Item a, Item b){
		return a.p - b.p;
	}
});
Alternative implementation:
items.stream().sorted(Comparator.comparing(x -> x.p))
Alternative implementation:
sort(items, comparing(x -> x.p));
Alternative implementation:
items.sort(comparing(a -> a.p));
29
Remove item from list, by its index
Remove i-th item from list items.
This will alter the original list or return a new list, depending on which is more idiomatic.
Note that in most languages, the smallest valid value for i is 0. 
items.remove(i);
Alternative implementation:
items.remove(i);
30
Parallelize execution of 1000 independent tasks
Launch the concurrent execution of the procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool. 
final ExecutorService executor = Executors.newFixedThreadPool(NB_THREADS);
for (int i = 1; i <= 1000; i++) {
  executor.submit(() -> f(i));
}
executor.shutdown();
31
Recursive factorial (simple)
Create the recursive function f which returns the factorial of the non-negative integer i, calculated from f(i-1) 
int f(int i) {
    if (i == 0)
        return 1;
    else
        return i * f(i - 1);
}
32
Integer exponentiation by squaring
Create function exp which calculates (fast) the value x power n.
x and n are non-negative integers. 
int exp(int x, int n){
	if(n==0)
		return 1;
	if(n==1)
		return x;
	if(n%2==0)
		return exp(x*x, n/2);
	else
		return x * exp(x*x, (n-1)/2);
}
33
Atomically read and update variable
Assign to the variable x the new value f(x), making sure that no other thread may modify x between the read and the write. 
synchronized(lock){
  x = f(x);
}
Alternative implementation:
volatile T x;
Alternative implementation:
class Example<T> {
    T x;
    Object lock = new Object();
    T read() {
        synchronized (lock) {
            return x;
        }
    }
    void write(T x) {
        synchronized (lock) {
            this.x = x;
        }
    }
}
34
Create a set of objects
Declare and initialize a set x containing unique objects of type T. 
Set<T> x = new HashSet<T>();
Alternative implementation:
Set<T> x = new HashSet<T>();
x.add(a);
x.add(b);
Alternative implementation:
Set<T> x = of(a, b, c);
35
First-class function : compose
Implement a function compose (A -> C) with parameters f (A -> B) and g (B -> C), which returns the composition function g ∘ f 
public Function<A, C> compose(Function<A, B> f, Function<B, C> g) {
   return x -> g.apply(f.apply(x));
}
Alternative implementation:
Function<Integer, Integer> addOne = i-> i + 1;
Function<Integer, String> toString = i-> i.toString();
Function<Integer, String> printIncremented = toString.compose(addOne);
Alternative implementation:
<A, B, C> Function<A, C> compose(Function<A, B> f, Function<B, C> g) {
    return f.andThen(g);
}
36
First-class function : generic composition
Implement a function compose which returns composition function g ∘ f for any functions f and g having exactly 1 parameter. 
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }
37
Currying
Transform a function that takes multiple arguments into a function for which some of the arguments are preset. 
IntBinaryOperator simpleAdd = (a, b) -> a + b;
IntFunction<IntUnaryOperator> curriedAdd = a -> b -> a + b;
System.out.println(simpleAdd.applyAsInt(4, 5));
System.out.println(curriedAdd.apply(4).applyAsInt(5));
38
Extract a substring
Find substring t consisting in characters i (included) to j (excluded) of string s.
Character indices start at 0 unless specified otherwise.
Make sure that multibyte characters are properly handled. 
String t = s.substring(i,j);
39
Check if string contains a word
Set the boolean ok to true if the string word is contained in string s as a substring, or to false otherwise. 
boolean ok = s.contains(word);
Alternative implementation:
boolean ok = s.indexOf(word) != -1;
40
Graph with adjacency lists
Declare a Graph data structure in which each Vertex has a collection of its neighbouring vertices. 
class Graph{
  List<Vertex> vertices;

  static class Vertex{
    int id;
    List<Vertex> neighbours;
  }
}
Alternative implementation:
class Graph{
  Set<Vertex> vertices;

  static class Vertex{
    int id;
    Set<Vertex> neighbours;
  }
}
41
Reverse a string
Create the string t containing the same characters as the string s, in reverse order.
The original string s must remain unaltered. Each character must be handled correctly regardless its number of bytes in memory. 
String t = new StringBuilder(s).reverse().toString();
Alternative implementation:
String t = "";
for (char c : s.toCharArray())
    t = c + t;
Alternative implementation:
char a[] = s.toCharArray(), c;
int i, m = a.length, n = m-- / 2, z;
for (i = 0; i < n; ++i) {
    c = a[i];
    a[i] = a[z = m - i];
    a[z] = c;
}
String t = new String(a);
Alternative implementation:
String t = s.chars()
    .mapToObj(x -> valueOf((char) x))
    .reduce((a, b) -> b + a)
    .get();
42
Continue outer loop
Print each item v of list a which is not contained in list b.
For this, write an outer loop to iterate on a and an inner loop to iterate on b. 
mainloop: for(int v:a){
	for(int w:b){
		if(v==w)
			continue mainloop;
	}
	System.out.println(v);
}
43
Break outer loop
Look for a negative value v in 2D integer matrix m. Print it and stop searching. 
mainloop: for(int i=0;i<m.length;i++)
	for(int j=0;j<m[i].length;j++)
		if(m[i][j]<0){
			System.out.println(m[i][j]);
			break mainloop;
		}
Alternative implementation:
   int i, j, M = m.length, N = m[0].length, x;
a: for (i = 0; i < M; ++i)
       for (j = 0; j < N; ++j)
           if ((x = m[i][j]) < 0) {
               out.println(x);
               break a;
           }
Alternative implementation:
int i, j, M = m.length, N = m[0].length, x;
boolean b = false;
for (i = 0; i < M; ++i) {
    for (j = 0; j < N; ++j)
        if ((x = m[i][j]) < 0) {
            out.println(x);
            b = true;
            break;
        }
    if (b) break;
}
44
Insert element in list
Insert the element x at position i in the list s. Further elements must be shifted to the right. 
s.add(i, x);
Alternative implementation:
int a = 0, n = s.length;
Class<?> c = s.getClass().getComponentType();
T t[] = (T[]) newInstance(c, n + 1);
arraycopy(s, a, t, a, i);
t[i] = x;
arraycopy(s, i, t, i + 1, n - i);
Alternative implementation:
int a = 0, n = s.length,
    t[] = new int[n + 1];
arraycopy(s, a, t, a, i);
t[i] = x;
arraycopy(s, i, t, i + 1, n - i);
45
Pause execution for 5 seconds
Sleep for 5 seconds in current thread, before proceeding with the next instructions. 
Thread.sleep(5000);
Alternative implementation:
TimeUnit.SECONDS.sleep(5);
46
Extract beginning of string (prefix)
Create the string t consisting of the 5 first characters of the string s.
Make sure that multibyte characters are properly handled. 
String t = s.substring(0,5);
47
Extract string suffix
Create string t consisting in the 5 last characters of string s.
Make sure that multibyte characters are properly handled. 
String t = s;
if (s.length()>= 5)
	t = s.substring(s.length()-5);
Alternative implementation:
int i = s.length() - 5;
if (isSurrogate(s.charAt(i))) --i;
String t = s.substring(i);
48
Multi-line string literal
Assign to variable s a string literal consisting in several lines of text, including newlines. 
String s = "This is a very long string which needs \n" +
           "to wrap across multiple lines because \n" +
           "otherwise my code is unreadable.";
Alternative implementation:
String s = """
This is a very long string which needs
to wrap across multiple lines because
otherwise my code is unreadable.
""";
Alternative implementation:
String s, n = lineSeparator();
s = "line 1" + n;
s = s + "line 2" + n;
s = s + "line 3" + n;
s = s + "line 4";
Alternative implementation:
String s;
StringBuilder b = new StringBuilder();
Formatter f = new Formatter(b);
f.format("line 1%n");
f.format("line 2%n");
f.format("line 3%n");
f.format("line 4");
f.flush();
s = b.toString();
49
Split a space-separated string
Build list chunks consisting in substrings of the string s, separated by one or more space characters. 
String chunks[] = s.split(" +");
Alternative implementation:
List<String> chunks = of(s.split(" +"));
Alternative implementation:
List<String> chunks = asList(s.split(" +"));
Alternative implementation:
Scanner t = new Scanner(s);
t.useDelimiter(compile(" +"));
List<String> chunks = t.tokens().toList();
t.close();
Alternative implementation:
Scanner t = new Scanner(s);
t.useDelimiter(compile(" +"));
String chunks[] = t.tokens()
    .toArray(String[]::new);
t.close();
Alternative implementation:
List<String> chunks
    = new ArrayList<>(of(s.split(" +")));
50
Make an infinite loop
Write a loop that has no end clause. 
for(;;){
	// Do something
}
Alternative implementation:
while(true) {
	// Do something	
}
Alternative implementation:
do {} while (true);
Alternative implementation:
while (true);
Alternative implementation:
for (;;);
51
Check if map contains key
Determine whether the map m contains an entry for the key k 
m.containsKey(k)
52
Check if map contains value
Determine whether the map m contains an entry with the value v, for some key. 
m.containsValue(v)
53
Join a list of strings
Concatenate elements of string list x joined by the separator ", " to create a single string y. 
String y = String.join(", ", x);
Alternative implementation:
String y = x.get(0);
int i = 1, n = x.size();
while (i < n)
    y = y + ", " + x.get(i++);
Alternative implementation:
String y = x.stream()
    .reduce((a, b) -> a + ", " + b)
    .get();
54
Compute sum of integers
Calculate the sum s of the integer list or array x. 
int s = 0;
for (int i : x) {
  s += i;
}
Alternative implementation:
BigInteger s = new BigInteger(valueOf(x[0]));
for (int i = 1, n = x.length; i < n; ++i)
    s = s.add(new BigInteger(valueOf(x[i])));
Alternative implementation:
BigInteger s = stream(x)
    .mapToObj(String::valueOf)
    .map(BigInteger::new)
    .reduce(BigInteger::add)
    .get();
Alternative implementation:
int i = 1, n = x.length, s = x[0];
while (i < n) s = s + x[i++];
Alternative implementation:
int s = of(x).sum();
55
Convert integer to string
Create the string representation s (in radix 10) of the integer value i. 
String s=((Integer)i).toString();
Alternative implementation:
String s = Integer.toString(i);
Alternative implementation:
String s = String.valueOf(i);
Alternative implementation:
String s = "" + i;
Alternative implementation:
String s = "%d".formatted(i);
Alternative implementation:
String s = "";
while (i != 0) {
    s = i % 10 + s;
    i = i / 10;
}
56
Launch 1000 parallel tasks and wait for completion
Fork-join : launch the concurrent execution of procedure f with parameter i from 1 to 1000.
Tasks are independent and f(i) doesn't return any value.
Tasks need not run all at the same time, so you may use a pool.
Wait for the completion of the 1000 tasks and then print "Finished". 
class Task implements Runnable {
	int i;
	Task(int i) {
		this.i = i;
	}
	@Override
	public void run() {
		f(i);
	}
}

ExecutorService executor = Executors.newFixedThreadPool(4);
for (int i = 1; i <= 1000; i++) {
	Task task = new Task(i);
	executor.submit(task);
}
executor.shutdown();
executor.awaitTermination(10L, TimeUnit.MINUTES);
System.out.println("Finished");
Alternative implementation:
ExecutorService executor = Executors.newFixedThreadPool(4);
for (int i = 1; i <= 1000; i++) {
	Task task = new Task(i);
	executor.submit(() -> f(i));
}
executor.shutdown();
executor.awaitTermination(10L, TimeUnit.MINUTES);
System.out.println("Finished");
57
Filter list
Create the list y containing the items from the list x that satisfy the predicate p. Respect the original ordering. Don't modify x in-place. 
List<T> y = x.stream().filter(p).toList();
Alternative implementation:
List<T> y = new ArrayList<>();
for (T t : x) if (p.test(t)) y.add(t);
58
Extract file content to a string
Create the string lines from the content of the file with filename f. 
byte[] encoded = Files.readAllBytes(Paths.get(f));
String lines = new String(encoded, StandardCharsets.UTF_8);
Alternative implementation:
File F = new File(f);
try (Scanner s = new Scanner(F)) {
    s.useDelimiter("\\R");
    String n = lineSeparator(),
           lines = s.tokens()
                    .collect(joining(n));
}
59
Write to standard error stream
Print the message "x is negative" to standard error (stderr), with integer x value substitution (e.g. "-2 is negative"). 
System.err.format("%d is negative\n",x);
Alternative implementation:
System.err.printf("%d is negative", x);
60
Read command line argument
Assign to x the string value of the first command line parameter, after the program name. 
String x = args[0];
Alternative implementation:
public static void main(String[] args) {
    if (args.length != 0) {
        String x = args[0];
    }
}
61
Get current date
Assign to the variable d the current date/time value, in the most standard type. 
Instant d = Instant.now();
Alternative implementation:
Date a = getInstance().getTime();
String d = "%tc".formatted(a);
Alternative implementation:
long a = currentTimeMillis();
String d = "%tc".formatted(a);
62
Find substring position
Set i to the first position of string y inside string x, if exists.

Specify if i should be regarded as a character index or as a byte index.

Explain the behavior when y is not contained in x. 
int i = x.indexOf(y);
63
Replace fragment of a string
Assign to x2 the value of string x with all occurrences of y replaced by z.
Assume occurrences of y are not overlapping. 
String x2 = x.replace(y, z);
64
Big integer : value 3 power 247
Assign to x the value 3^247 
BigInteger x = new BigInteger("3", 10).pow(247);
Alternative implementation:
BigInteger x = new BigInteger("3").pow(247);
65
Format decimal number
From the real value x in [0,1], create its percentage string representation s with one digit after decimal point. E.g. 0.15625 -> "15.6%" 
String s = new DecimalFormat("0.0%").format(x);
Alternative implementation:
String s = "%.1f%%".formatted(x * 100);
Alternative implementation:
NumberFormat f = getPercentInstance();
f.setMaximumFractionDigits(1);
f.setRoundingMode(HALF_UP);
String s = f.format(x);
66
Big integer exponentiation
Calculate the result z of x power n, where x is a big integer and n is a positive integer. 
BigInteger z = x.pow(n);
67
Binomial coefficient "n choose k"
Calculate binom(n, k) = n! / (k! * (n-k)!). Use an integer type able to handle huge numbers. 
static BigInteger binom(int N, int K) {
    BigInteger ret = BigInteger.ONE;
    for (int k = 0; k < K; k++) {
        ret = ret.multiply(BigInteger.valueOf(N-k))
                 .divide(BigInteger.valueOf(k+1));
    }
    return ret;
}
68
Create a bitset
Create an object x to store n bits (n being potentially large). 
BitSet x = new BitSet(n);
69
Seed random generator
Use seed s to initialize a random generator.

If s is constant, the generator output will be the same each time the program runs. If s is based on the current value of the system clock, the generator output will be different each time. 
Random r = new Random(s);
70
Use clock as random generator seed
Get the current datetime and provide it as a seed to a random generator. The generator sequence will be different at each run. 
Random rand = new Random(System.currentTimeMillis());
71
Echo program implementation
Basic implementation of the Echo program: Print all arguments except the program name, separated by space, followed by newline.
The idiom demonstrates how to skip the first argument if necessary, concatenate arguments as strings, append newline and print it to stdout. 
public class Echo {
    public static void main(final String... args) {
        out.println(join(" ", args));
    }
}
Alternative implementation:
class Echo {
    public static void main(String[] args) {
        out.print(join(" ", args));
    }
}
74
Compute GCD
Compute the greatest common divisor x of big integers a and b. Use an integer type able to handle huge numbers. 
BigInteger x = a.gcd(b);
75
Compute LCM
Compute the least common multiple x of big integers a and b. Use an integer type able to handle huge numbers. 
BigInteger a = new BigInteger("123456789");
BigInteger b = new BigInteger("987654321");
BigInteger x = a.multiply(b).divide(a.gcd(b));
76
Binary digits from an integer
Create the string s of integer x written in base 2.

E.g. 13 -> "1101" 
String s = Integer.toBinaryString(x);
77
Complex number
Declare a complex x and initialize it with value (3i - 2). Then multiply it by i. 
Complex x = new Complex(-2.0, 3.0);
x = x.multiply(Complex.I);
78
"do while" loop
Execute a block once, then execute it again as long as boolean condition c is true. 
do {
	someThing();
	someOtherThing();
} while(c);
Alternative implementation:
do f();
while (c);
79
Convert integer to floating point number
Declare the floating point number y and initialize it with the value of the integer x . 
int x = 10
float y = (float)x;
System.out.println(y);
Alternative implementation:
Float y = Float.parseFloat(String.valueOf(x));
Alternative implementation:
float y = x;
Alternative implementation:
float y = new BigDecimal(x).floatValue();
80
Truncate floating point number to integer
Declare integer y and initialize it with the value of floating point number x . Ignore non-integer digits of x .
Make sure to truncate towards zero: a negative x must yield the closest greater integer (not lesser). 
int y = (int)x;
81
Round floating point number to integer
Declare the integer y and initialize it with the rounded value of the floating point number x .
Ties (when the fractional part of x is exactly .5) must be rounded up (to positive infinity). 
long y = Math.round(x);
Alternative implementation:
NumberFormat f = getNumberInstance();
f.setRoundingMode(HALF_UP);
f.setMaximumFractionDigits(0);
int y = parseInt(f.format(x));
82
Count substring occurrences
Find how many times string s contains substring t.
Specify if overlapping occurrences are counted. 
int count = StringUtils.countMatches(s, t);
Alternative implementation:
Pattern pattern = Pattern.compile(Pattern.quote(t));
Matcher matcher = pattern.matcher(s);
int count = 0;
while(matcher.find()) count++;
Alternative implementation:
int z = 0;
Matcher m = compile(quote(t)).matcher(s);
while (m.find()) ++z;
Alternative implementation:
int z, i = z = 0;
Matcher m = compile(quote(t)).matcher(s);
while (m.find(i)) {
    ++z;
    i = m.start() + 1;
}
83
Regex with character repetition
Declare the regular expression r matching the strings "http", "htttp", "httttp", etc. 
Pattern r = Pattern.compile("htt+p");
Alternative implementation:
Pattern r = compile("ht{2,}p");
84
Count bits set in integer binary representation
Count number c of 1s in the integer i in base 2.

E.g. i=6 → c=2 
int c = Integer.bitCount(i);
Alternative implementation:
int c = new BigInteger(valueOf(i), 2).bitCount();
85
Check if integer addition will overflow
Write boolean function addingWillOverflow which takes two integers x, y and return true if (x+y) overflows.

An overflow may be above the max positive value, or below the min negative value. 
public boolean addingWillOverflow(int x, int y){
    boolean willOverFlow = false;
    if(x > 0 && y > 0){
        if(y > (Integer.MAX_VALUE - x)){
            willOverFlow = true;
        }
    }
    if(x < 0 && y < 0){
       if(y < (Integer.MIN_VALUE - x)){
           willOverFlow = true;
       }
    }
    return willOverFlow;
}
Alternative implementation:
boolean willOverflow(int x, int y) {
    try {
        addExact(x, y);
        subtractExact(x, y);
        return false;
    } catch (ArithmeticException e) {
        return true;
    }
}
86
Check if integer multiplication will overflow
Write the boolean function multiplyWillOverflow which takes two integers x, y and returns true if (x*y) overflows.

An overflow may reach above the max positive value, or below the min negative value. 
static boolean multiplyWillOverflow(int x, int y) {
	return Integer.MAX_VALUE/x < y;
}
Alternative implementation:
boolean b = Math.multiplyExact(x, y);
87
Stop program
Exit immediately.
If some extra cleanup work is executed by the program runtime (not by the OS itself), describe it. 
System.exit(0);
Alternative implementation:
throw new RuntimeException();
Alternative implementation:
throw new Error();
88
Allocate 1M bytes
Create a new bytes buffer buf of size 1,000,000. 
byte[] buf = new byte[1000000];
Alternative implementation:
byte buf[] = new byte[1_000_000];
Alternative implementation:
byte buf[] = new byte[(int) 1e6];
89
Handle invalid argument
You've detected that the integer value of argument x passed to the current function is invalid. Write the idiomatic way to abort the function execution and signal the problem. 
throw new IllegalArgumentException("Invalid value for x:" + x);
Alternative implementation:
<T> void f(T x) {
    Objects.requireNonNull(x);
}
Alternative implementation:
<T> void f(T x) throws Exception {
    if (x != value) throw new Exception();
}
Alternative implementation:
void f(int x) {
    Objects.checkIndex(x, 123);
}
90
Read-only outside
Expose a read-only integer x to the outside world while being writable inside a structure or a class Foo. 
public class Foo {
    private int x;

    public int getX() {
        return x;
    }
}
Alternative implementation:
public class Foo {
    private int x;
    private void set(int x) { this.x = x; }
    public int get() { return x; }
}
93
Pass a runnable procedure as parameter
Implement the procedure control which receives one parameter f, and runs f. 
static void control(Runnable f) {
    f.run();
}
Alternative implementation:
control: Callable = lambda f: f()
94
Print the type of a variable
Print the name of the type of x. Explain if it is a static type or dynamic type.

This may not make sense in all languages. 
System.out.println(((Object)x).getClass().getName());
Alternative implementation:
var x = "abc";
out.println(x.getClass().getSimpleName());
95
Get file size
Assign to variable x the length (number of bytes) of the local file at path. 
long x = new File(path).length();
96
Check string prefix
Set the boolean b to true if string s starts with prefix prefix, false otherwise. 
boolean b = s.startsWith(prefix);
97
Check string suffix
Set boolean b to true if string s ends with string suffix, false otherwise. 
boolean b = s.endsWith(suffix);
98
Epoch seconds to date object
Convert a timestamp ts (number of seconds in epoch-time) to a date with time d. E.g. 0 -> 1970-01-01 00:00:00 
Date d = new Date(ts * 1000);
Alternative implementation:
Instant d = Instant.ofEpochSecond(ts);
Alternative implementation:
LocalDateTime d = LocalDateTime.ofEpochSecond(ts, 0, ZoneOffset.UTC);
Alternative implementation:
long ts = currentTimeMillis();
String d = "%tc".formatted(ts);
Alternative implementation:
Date d = new Date(ts * 1000);
SimpleDateFormat f = new SimpleDateFormat();
f.applyPattern("yyyy-MM-dd HH:mm:ss");
String s = f.format(d);
99
Format date YYYY-MM-DD
Assign to the string x the value of the fields (year, month, day) of the date d, in format YYYY-MM-DD. 
String x = new SimpleDateFormat("yyyy-MM-dd").format(d);
Alternative implementation:
String x = String.format("%1$tY-%1$tm-%1$td", d)
Alternative implementation:
Date d = getInstance().getTime();
String s = "%tY-%<tm-%<td".formatted(d);
100
Sort by a comparator
Sort elements of array-like collection items, using a comparator c. 
Arrays.sort(items, c);
Alternative implementation:
Collections.sort(items, c);
101
Load from HTTP GET request into a string
Make an HTTP request with method GET to the URL u, then store the body of the response in the string s. 
String s = HttpClient.newHttpClient().send(HttpRequest.newBuilder()
                .uri(URI.create(u))
                .GET()
                .build(), HttpResponse.BodyHandlers.ofString())
                .body();
Alternative implementation:
HttpsURLConnection h = null;
try {
    h = (HttpsURLConnection) u.openConnection();
    h.setRequestMethod("GET");
    String s = new String(h.getInputStream().readAllBytes());
} catch (IOException e) {
    throw new RuntimeException(e);
} finally {
    if (h != null) h.disconnect();
}
105
Current executable name
1 
String s = System.getProperty("sun.java.command");
106
Get program working directory
Assign to string dir the path of the working directory.
(This is not necessarily the folder containing the executable itself) 
String path = this.getClass().getClassLoader().getResource("").getPath();
Alternative implementation:
String dir = new File("").getAbsolutePath();
Alternative implementation:
String dir = System.getProperty("user.dir");
108
Determine if variable name is defined
Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.) 
try {
    Field f = getClass().getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
109
Number of bytes of a type
Set n to the number of bytes of a variable t (of type T). 
<T> int bytes(T t) {
    return switch (t.getClass().getSimpleName()) {
        case "Boolean" -> 1;
        case "Byte" -> Byte.BYTES;
        case "Short" -> Short.BYTES;
        case "Character" -> Character.BYTES;
        case "Integer" -> Integer.BYTES;
        case "Float" -> Float.BYTES;
        case "Long" -> Long.BYTES;
        case "Double" -> Double.BYTES;
        default -> -1;
    };
}
110
Check if string is blank
Set the boolean blank to true if the string s is empty, or null, or contains only whitespace ; false otherwise. 
boolean blank = StringUtils.isBlank(s);
Alternative implementation:
boolean blank = s==null || s.strip().isEmpty();
Alternative implementation:
boolean blank = s==null || s.isBlank();
Alternative implementation:
boolean blank = s == null || s.isBlank();
111
Launch other program
From current process, run program x with command-line parameters "a", "b". 
Runtime.getRuntime().exec(new String[]{"x", "a", "b"});
Alternative implementation:
var b = new ProcessBuilder(of("x", "a", "b"));
try {
    b.start().waitFor();
} catch (Exception e) {
    throw new RuntimeException(e);
}
112
Iterate over map entries, ordered by keys
Print each key k with its value x from an associative array mymap, in ascending order of k. 
SortedMap<K, V> mymap = new TreeMap<>();
...
for(Map.Entry<K, V> e: mymap.entrySet())
	System.out.println("Key=" + e.getKey() + ", Value=" + e.getValue());
Alternative implementation:
List<K> keys = new ArrayList<>(mymap.keySet());
Collections.sort(keys);
for(K k: keys)
	System.out.println("Key=" + k + ", Value=" + mymap.get(k));
Alternative implementation:
var map = Map.of("a", 1, "d", 4, "c", 3, "b", 2);
new TreeMap<>(map).entrySet().forEach(System.out::println);
Alternative implementation:
mymap.entrySet().stream().sorted(Entry.comparingByKey()).forEach(System.out::println);
Alternative implementation:
for (Entry<K, V> e : new TreeMap<>(m).entrySet())
    out.println(e);
Alternative implementation:
mymap.entrySet()
    .stream()
    .sorted(comparingByKey())
    .forEach(e -> {
        K k = e.getKey();
        X x = e.getValue();
        out.println(e);
    });
113
Iterate over map entries, ordered by values
Print each key k with its value x from an associative array mymap, in ascending order of x.
Multiple entries may exist for the same value x. 
mymap.entrySet().stream()
    .sorted(Comparator.comparing(Map.Entry::getValue))
    .forEach(entry -> {
        K k = entry.getKey();
        X x = entry.getValue();
        System.out.println("k:" + k + ", x:" + x);
});
Alternative implementation:
mymap.entrySet()
    .stream()
    .sorted(comparingByValue())
    .forEach(out::println);
Alternative implementation:
mymap.entrySet()
    .stream()
    .sorted(comparingByValue())
    .forEach(e -> {
        K k = e.getKey();
        X x = e.getValue();
    });
115
Compare dates
Set boolean b to true if date d1 is strictly before date d2 ; false otherwise. 
boolean before = (d1.compareTo(d2) == -1);
116
Remove occurrences of word from string
Remove all occurrences of string w from string s1, and store the result in s2. 
String s2 = s1.replace(w, "");
117
Get list size
Set n to the number of elements of the list x. 
int n = x.size();
Alternative implementation:
int n = x.length;
118
List to set
Create the set y from the list x.
x may contain duplicates. y is unordered and has no repeated values. 
Set<T> y = new HashSet<>(x);
Alternative implementation:
Set<T> y = new HashSet<T>(x);
Alternative implementation:
Set<T> y = x.stream().collect(toSet());
Alternative implementation:
Set<T> y = copyOf(x);
Alternative implementation:
Set<T> y = new LinkedHashSet<>(x);
119
Deduplicate list
Remove duplicates from the list x.
Explain if the original order is preserved. 
Set<T> uniques = new HashSet<>(x);
x.clear();
x.addAll(uniques);
Alternative implementation:
x = new ArrayList<T>(new HashSet<T>(x));
Alternative implementation:
final HashSet<T> seen = new HashSet<T>();
final Iterator<T> listIt = x.iterator();
while (listIt.hasNext()) {
  final T curr = listIt.next();
  if (seen.contains(curr)) {
    listIt.remove();
  } else {
    seen.add(curr);
  }
}
Alternative implementation:
x = new ArrayList<>(new LinkedHashSet<>(x));
Alternative implementation:
Iterator<T> g;
T t;
int i, n;
for (i = 0, n = x.size(); i < n; ++i) {
    t = x.get(i);
    g = x.listIterator(i + 1);
    while (g.hasNext())
        if (g.next().equals(t)) {
            g.remove();
            --n;
        }
}
120
Read integer from stdin
Read an integer value from the standard input into the variable n 
Scanner in = new Scanner(System.in);
n = in.nextInt();
Alternative implementation:
Scanner in = new Scanner(System.in);
int n = in.nextInt();
Alternative implementation:
Scanner s = new Scanner(System.in);
int n = parseInt(s.nextLine());
s.close();
121
UDP listen and read
Listen UDP traffic on port p and read 1024 bytes into the buffer b. 
int len = 1024;
int p = 8888;
byte[] b = new byte[len];
try (DatagramSocket socket = new DatagramSocket(p)) {
  DatagramPacket packet = new DatagramPacket(b, len);
  socket.receive(packet);
}
122
Declare an enumeration
Create an enumerated type Suit with 4 possible values SPADES, HEARTS, DIAMONDS, CLUBS. 
enum Suit{
  SPADES,
  HEARTS,
  DIAMONDS,
  CLUBS;
}
123
Assert condition
Verify that predicate isConsistent returns true, otherwise report assertion violation.
Explain if the assertion is executed even in production environment or not. 
assert isConsistent() : "State consistency violated";
124
Binary search for a value in sorted array
Write the function binarySearch which returns the index of an element having the value x in the sorted array a, or -1 if no such element exists. 
static int binarySearch(final int[] arr, final int key) {
    final int index = Arrays.binarySearch(arr, key);
    return index < 0 ? - 1 : index;
}
Alternative implementation:
int i = binarySearch(a, x);
125
Measure function call duration
measure the duration t, in nanoseconds, of a call to the function foo. Print this duration. 
long t = System.nanoTime();
foo();
t = System.nanoTime() - t;
System.out.println(t + "ns");
Alternative implementation:
long t = nanoTime();
foo();
out.println(nanoTime() - t);
126
Multiple return values
Write a function foo that returns a string and a boolean value. 
static Object[] returnAnything() {
    return new Object[]{"a string", true};
}

public static void main (String[] args) {
    Object[] array = returnAnything();
    System.out.println(array[0]);
    System.out.println(array[1]);
}
Alternative implementation:
record Tuple(Object ... a) {}
Tuple foo() {
    return new Tuple("abc", true);
}
Alternative implementation:
Object[] foo() {
    return new Object[] { "abc", true };
}
128
Breadth-first traversing of a tree
Call a function f on every node of a tree, in breadth-first prefix order 
static void breadthFirstSearch(Node root, Consumer<Node> f) {
    Queue<Node> queue = new LinkedList<>();
    queue.offer(root);

    while(!queue.isEmpty()) {
        Node polled = queue.poll();
        f.accept(polled);
        polled.children.forEach(a -> queue.offer(a));
    }
}
131
Successive conditions
Execute f1 if condition c1 is true, or else f2 if condition c2 is true, or else f3 if condition c3 is true.
Don't evaluate a condition when a previous condition was true. 
if (c1) {
   f1();
} else if (c2) {
   f2();
} else if (c3) { 
   f3();
}
Alternative implementation:
if (c1) f1();
else if (c2) f2();
else if (c3) f3();
132
Measure duration of procedure execution
Run the procedure f, and return the duration of the execution of f. 
long clock(Runnable f) {
    long t0 = System.currentTimeMillis();
    f.run();
    long t1 = System.currentTimeMillis();
    return t1 - t0;
}
133
Case-insensitive string contains
Set boolean ok to true if string word is contained in string s as a substring, even if the case doesn't match, or to false otherwise. 
ok = s.toLowerCase().contains(word.toLowerCase());
Alternative implementation:
Pattern p = compile("(?i)" + quote(word));
boolean ok = p.matcher(s).find();
134
Create a new list
Declare and initialize a new list items, containing 3 elements a, b, c. 
List<T> items = new ArrayList<>();
items.add(a);
items.add(b);
items.add(c);
Alternative implementation:
List<T> items = Arrays.asList(a, b, c);
Alternative implementation:
var items = List.of(a, b, c);
Alternative implementation:
List<T> items = new ArrayList<>(of(a, b, c));
135
Remove item from list, by its value
Remove at most 1 item from list items, having the value x.
This will alter the original list or return a new list, depending on which is more idiomatic.
If there are several occurrences of x in items, remove only one of them. If x is absent, keep items unchanged. 
items.stream().findFirst().filter(item -> "x".equals(item)).ifPresent(removeIndex -> items.remove(removeIndex));
Alternative implementation:
T value;
for(int index = 0; index < items.size(); index++) {
	value = items.get(index);
	if(value.equals(x)) {
		items.remove(index);
		break;
	}
}
Alternative implementation:
<T> void remove(List<T> items, T x) {
    Iterator<T> i = items.listIterator();
    while (i.hasNext())
        if (i.next() == x) {
            i.remove();
            break;
        }
}
136
Remove all occurrences of a value from a list
Remove all occurrences of the value x from list items.
This will alter the original list or return a new list, depending on which is more idiomatic. 
items.removeAll(Collections.singleton(x));
Alternative implementation:
Iterator<T> i = items.listIterator();
while (i.hasNext())
    if (i.next().equals(x)) i.remove();
Alternative implementation:
items.removeIf(t -> t.equals(x));
137
Check if string contains only digits
Set the boolean b to true if the string s contains only characters in the range '0'..'9', false otherwise. 
boolean b = s.matches("[0-9]*");
Alternative implementation:
boolean b = s.matches("[0-9]+");
Alternative implementation:
boolean b = s.chars()
    .allMatch(Character::isDigit);
if (s.length() == 0) b = false;
Alternative implementation:
boolean b = s.matches("\\d+");
Alternative implementation:
String d = "0123456789";
int i, n = s.length();
boolean b = n != 0;
for (i = 0; i < n; ++i)
    if (d.indexOf(s.charAt(i)) == -1) {
        b = false;
        break;
    }
138
Create temp file
Create a new temporary file on the filesystem. 
File tempFile = File.createTempFile("prefix-", "-suffix");
tempFile.deleteOnExit();
139
Create temp directory
Create a new temporary folder on filesystem, for writing. 
File folder = new File("/path/");
folder.deleteOnExit();
140
Delete map entry
Delete from map m the entry having key k.

Explain what happens if k is not an existing key in m. 
map.remove(k);
141
Iterate in sequence over two lists
Iterate in sequence over the elements of the list items1 then items2. For each iteration print the element. 
items1.forEach(System.out::println);
items2.forEach(System.out::println);
Alternative implementation:
for (E e : items1) out.println(e);
for (E e : items2) out.println(e);
Alternative implementation:
Stream.concat(
	items1.stream(), 
	items2.stream()
).forEach(System.out::println);
142
Hexadecimal digits of an integer
Assign to string s the hexadecimal representation (base 16) of integer x.

E.g. 999 -> "3e7" 
String s = Integer.toHexString(x);
Alternative implementation:
String s = "%x".formatted(x);
Alternative implementation:
String s = HexFormat.of().toHexDigits(x);
143
Iterate alternatively over two lists
Iterate alternatively over the elements of the lists items1 and items2. For each iteration, print the element.

Explain what happens if items1 and items2 have different size. 
Iterator<String> iter1 = items1.iterator();
Iterator<String> iter2 = items2.iterator();
while (iter1.hasNext() || iter2.hasNext()) {
	if (iter1.hasNext()) {
		System.out.println(iter1.next());
	}
	if (iter2.hasNext()) {
		System.out.println(iter2.next());
	}
}
Alternative implementation:
IntStream.range(0, Math.max(items1.size(), items2.size()))
	.boxed()
	.flatMap(idx -> Stream.of(
		items1.size() > idx ? items1.get(idx) : null,
		items2.size() > idx ? items2.get(idx) : null
	))
	.filter(Objects::nonNull)
	.forEach(System.out::println);
Alternative implementation:
int i, A, B, n = max(A = a.size(), B = b.size());
for (i = 0; i < n; ++i) {
    if (i < A) out.println(a.get(i));
    if (i < B) out.println(b.get(i));
}
Alternative implementation:
Iterator<T> A = a.iterator(), B = b.iterator();
boolean x = A.hasNext(), y = B.hasNext();
while (x || y) {
    if (x) out.println(A.next());
    if (y) out.println(B.next());
    x = A.hasNext();
    y = B.hasNext();
}
144
Check if file exists
Set boolean b to true if file at path fp exists on filesystem; false otherwise.

Beware that you should not do this and then in the next instruction assume the result is still valid, this is a race condition on any multitasking OS. 
boolean b = new File(fb).exists();
145
Print log line with datetime
Print message msg, prepended by current date and time.

Explain what behavior is idiomatic: to stdout or stderr, and what the date format is. 
Logger LOGGER = Logger.getLogger(MyClass.class.getName());

LOGGER.info(msg);
Alternative implementation:
out.printf("%tc:  `%s`%n", currentTimeMillis(), msg);
146
Convert string to floating point number
Extract floating point value f from its string representation s 
double f = Double.parseDouble(s);
Alternative implementation:
Float f = Float.valueOf(s);
Alternative implementation:
Double f = Double.valueOf(s);
147
Remove all non-ASCII characters
Create string t from string s, keeping only ASCII characters 
String t = s.replaceAll("[^\\x00-\\x7F]", "");
Alternative implementation:
String t = "";
char c;
int i, n = s.length();
for (i = 0; i < n; ++i)
    if ((c = s.charAt(i)) < 0x80)
        t = t + c;
Alternative implementation:
String t = s.chars()
    .filter(c -> c < 0x80)
    .mapToObj(x -> valueOf((char) x))
    .collect(joining());
148
Read list of integers from stdin
Read a list of integer numbers from the standard input, until EOF. 
List<Integer> list = new ArrayList<>();
Scanner in = new Scanner(System.in);
while(in.hasNext())
    list.add(in.nextInt());
Alternative implementation:
Scanner s = new Scanner(System.in);
int a[] = s.tokens()
    .mapToInt(Integer::parseInt)
    .toArray();
s.close();
149
Rescue the princess
As an exception, this content is not under license CC BY-SA 3.0 like the rest of this website.
150
Remove trailing slash
Remove the last character from the string p, if this character is a forward slash / 
if (p.endsWith("/")) {
    p = p.substring(0, p.length() - 1);
}
Alternative implementation:
p = p.replaceAll("/$", "");
Alternative implementation:
int n = p.length() - 1;
if (n != 0 && p.charAt(n) == '/')
    p = p.substring(0, n);
Alternative implementation:
p = p.replaceAll("(?<!^)/+$", "");
151
Remove string trailing path separator
Remove last character from string p, if this character is the file path separator of current platform.

Note that this also transforms unix root path "/" into the empty string! 
int n = p.length() - 1;
if (n != 0 && p.charAt(n) == separatorChar)
    p = p.substring(0, n);
152
Turn a character into a string
Create string s containing only the character c. 
String s = String.valueOf(c);
Alternative implementation:
String s = Character.toString(c);
Alternative implementation:
String s = c + "";
Alternative implementation:
String s = = new Character(c).toString();
Alternative implementation:
String s = "%s".formatted(c);
153
Concatenate string with integer
Create the string t as the concatenation of the string s and the integer i. 
String t = s + i;
Alternative implementation:
String t = "%s%s".formatted(s, i);
154
Halfway between two hex color codes
Find color c, the average between colors c1, c2.

c, c1, c2 are strings of hex color codes: 7 chars, beginning with a number sign # .
Assume linear computations, ignore gamma corrections. 
String r1 = c1.substring(1,3);
String g1 = c1.substring(3,5);
String b1 = c1.substring(5,7);

String r2 = c2.substring(1,3);
String g2 = c2.substring(3,5);
String b2 = c2.substring(5,7);

String r = String.format("%02X", (Integer.parseInt(r1, 16)+Integer.parseInt(r2, 16))/2 );
String g = String.format("%02X", (Integer.parseInt(g1, 16)+Integer.parseInt(g2, 16))/2 );
String b = String.format("%02X", (Integer.parseInt(b1, 16)+Integer.parseInt(b2, 16))/2 );

String c = "#" + r + g + b;
Alternative implementation:
StringBuilder sb = new StringBuilder("#");
for(int i=0;i<3;i++) {
  String sub1 = c1.substring(1+2*i,3+2*i);
  String sub2 = c2.substring(1+2*i,3+2*i);
  int v1 = Integer.parseInt(sub1, 16);
  int v2 = Integer.parseInt(sub2, 16);
  int v = (v1 + v2)/2;
  String sub = String.format("%02X", v);
  sb.append(sub);
}
String c = sb.toString();
Alternative implementation:
Color x = decode(c1), y = decode(c2);
int r = (x.getRed()   + y.getRed())   / 2,
    g = (x.getGreen() + y.getGreen()) / 2,
    b = (x.getBlue()  + y.getBlue())  / 2,
    z = (r << 16) + (g << 8) + b;
String c = "#%06x".formatted(z);
155
Delete file
Delete from filesystem the file having path filepath. 
new File(filepath).delete();
Alternative implementation:
deleteIfExists(of(filepath));
156
Format integer with zero-padding
Assign to the string s the value of the integer i in 3 decimal digits. Pad with zeros if i < 100. Keep all digits if i ≥ 1000. 
String s = String.format("%03d", i);
Alternative implementation:
NumberFormat f = getIntegerInstance();
f.setMinimumIntegerDigits(3);
f.setGroupingUsed(false);
String s = f.format(i);
Alternative implementation:
String s = "%03d".formatted(i);
157
Declare constant string
Initialize a constant planet with string value "Earth". 
static final String planet = "Earth";
Alternative implementation:
final String planet = "Earth";
Alternative implementation:
final String planet = "Earth";
158
Random sublist
Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not. 
List<T> y = new ArrayList<>(x);
shuffle(y);
y = y.subList(0, k);
160
Detect if 32-bit or 64-bit architecture
Execute f32() if platform is 32-bit, or f64() if platform is 64-bit.
This can be either a compile-time condition (depending on target) or a runtime detection. 
switch(System.getProperty("sun.arch.data.model")) {
  case "64": 
    f64(); 
    break;
  case "32":
    f32();
    break;
}
161
Multiply all the elements of a list
Multiply all the elements of the list elements by a constant c 
elements = elements.stream().map(e -> e*c).collect(Collectors.toList());
Alternative implementation:
elements = elements.stream()
                   .map(x -> x * c)
                   .toList();
162
Execute procedures depending on options
execute bat if b is a program option and fox if f is a program option. 
var argsSet = Set.of(args);
if (argsSet.contains("b")) bat();
if (argsSet.contains("f")) fox();
Alternative implementation:
public static void main(String[] args) {
    for (String a : new LinkedHashSet<>(of(args)))
        switch (a) {
            case "b" -> bat();
            case "f" -> fox();
        }
}
163
Print list elements by group of 2
Print all the list elements, two by two, assuming list length is even. 
for(int i = 0; i < list.length; i += 2) {
  System.out.println(list[i] + ", " + list[i + 1]);
}
Alternative implementation:
range(0, list.size())
    .filter(i -> i % 2 == 0)
    .mapToObj(i -> list.get(i) + ", " + list.get(i + 1))
    .forEach(out::println);
165
Last element of list
Assign to the variable x the last element of the list items. 
int x = items[items.length - 1];
Alternative implementation:
T x = items.getLast();
166
Concatenate two lists
Create the list ab containing all the elements of the list a, followed by all the elements of the list b. 
var ab = new ArrayList<>(a);
ab.addAll(b);
Alternative implementation:
List<?> ab = new ArrayList<>(a) {{ addAll(b); }};
167
Trim prefix
Create the string t consisting of the string s with its prefix p removed (if s starts with p). 
String t = s.replaceFirst("^" + p, "");
Alternative implementation:
int i = s.indexOf(p), n = p.length();
t = i == 0 ? s.substring(n) : s;
Alternative implementation:
String t = s.replaceFirst('^' + quote(p), "");
168
Trim suffix
Create string t consisting of string s with its suffix w removed (if s ends with w). 
String t = s;
int index = s.lastIndexOf(w);
if (index + w.length() == s.length()) {
    t = s.substring(0, index);
}
Alternative implementation:
int i = s.lastIndexOf(w),
    m = s.length(), n = w.length();
t = i == m - n ? s.substring(0, i) : s;
Alternative implementation:
String t = s.replaceAll(quote(w) + '$', "");
169
String length
Assign to the integer n the number of characters of the string s.
Make sure that multibyte characters are properly handled.
n can be different from the number of bytes of s. 
int n = s.length();
170
Get map size
Set n to the number of elements stored in mymap.

This is not always equal to the map capacity. 
int n = mymap.size();
171
Add an element at the end of a list
Append the element x to the list s. 
s.add(x);
172
Insert an entry in a map
Insert value v for key k in map m. 
m.put(k, v);
173
Format a number with grouped thousands
Number will be formatted with a comma separator between every group of thousands. 
String.format("%,d", 1000000);
Alternative implementation:
NumberFormat f = new DecimalFormat("#,###");
String s = f.format(1_234);
Alternative implementation:
DecimalFormat f = (DecimalFormat) getNumberInstance();
f.setGroupingSize(3);
String s = f.format(1_000);
Alternative implementation:
out.printf("%,d", new BigInteger("1234"));
Alternative implementation:
out.printf("%,f", new BigDecimal("1234.5"));
Alternative implementation:
out.printf("%,f", 1_234.5);
174
Make HTTP POST request
Make a HTTP request with method POST to the URL u 
String s = HttpClient.newHttpClient().send(HttpRequest.newBuilder()
                        .uri(URI.create(u))
                        .POST(HttpRequest.BodyPublishers.ofString(content))
                        .build(), HttpResponse.BodyHandlers.ofString())
                .body();
175
Bytes to hex string
From the array a of n bytes, build the equivalent hex string s of 2n digits.
Each byte (256 possible values) is encoded as two hexadecimal characters (16 possible values per digit). 
String s = "";
for (byte b : a)
    s = s + "%02x".formatted(b);
Alternative implementation:
var s = HexFormat.of().formatHex(a);
Alternative implementation:
String s = range(0, n)
    .mapToObj(x -> a[x])
    .map("%02x"::formatted)
    .collect(joining());
176
Hex string to byte array
From hex string s of 2n digits, build the equivalent array a of n bytes.
Each pair of hexadecimal characters (16 possible values per digit) is decoded into one byte (256 possible values). 
byte[] a = new BigInteger(s, 16).toByteArray();
Alternative implementation:
public static byte[] hexToByteArray(String s) {
        int len = s.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(s.charAt(i), 16) << 4)
                    + Character.digit(s.charAt(i+1), 16));
        }
        return data;
    }
Alternative implementation:
int i, n = s.length();
byte a[] = new byte[n / 2];
for (i = 0; i < n; i = i + 2)
    a[i / 2] = (byte) fromHexDigits(s, i, i + 2);
177
Find files for a list of filename extensions
Construct a list L that contains all filenames that have the extension ".jpg" , ".jpeg" or ".png" in directory D and all its subdirectories. 
static List<String> L = new ArrayList<>();
record Find(File d, String ... t) {
    Find { get(d, t); }
    void get(File d, String ... t) {
        File a[] = d.listFiles();
        if (a == null) return;
        for (File f : a)
            if (f.isDirectory()) get(f, t);
            else asList(t).forEach(s -> {
                if (f.getName().endsWith(s))
                    L.add(f.getName());
            });
    }
}
178
Check if point is inside rectangle
Set boolean b to true if if the point with coordinates (x,y) is inside the rectangle with coordinates (x1,y1,x2,y2) , or to false otherwise.
Describe if the edges are considered to be inside the rectangle. 
boolean b = x <= x2 && x >= x1 && y <= y2 && y >= y1;
Alternative implementation:
double w = x2 - x1, h = y2 - y1;
Rectangle2D r = new Rectangle2D.Double(x1, y1, w, h);
boolean b = r.contains(x, y);
Alternative implementation:
int w = x2 - x1, h = y2 - y1;
Rectangle r = new Rectangle(x1, y1, w, h);
boolean b = r.contains(x, y);
179
Get center of a rectangle
Return the center c of the rectangle with coördinates(x1,y1,x2,y2) 
double[] c = {(x1+x2)/2,(y1+y2)/2};
Alternative implementation:
double w = x2 - x1, h = y2 - y1;
Rectangle2D r = new Rectangle2D.Double(x1, y1, w, h);
double c[] = { r.getCenterX(), r.getCenterY() };
Alternative implementation:
int w = x2 - x1, h = y2 - y1;
Rectangle r = new Rectangle(x1, y1, w, h);
double c[] = { r.getCenterX(), r.getCenterY() };
180
List files in directory
Create the list x containing the contents of the directory d.

x may contain files and subfolders.
No recursive subfolder listing. 
final File directory = new File(d);
final File[] x = directory.listFiles();
Alternative implementation:
String x[] = new File(d).list();
184
Tomorrow
Assign to t a string representing the day, month and year of the day after the current date. 
String t = LocalDate.now().plusDays(1).toString();
185
Execute function in 30 seconds
Schedule the execution of f(42) in 30 seconds. 
ScheduledExecutorService executorService = Executors.newSingleThreadScheduledExecutor();
executorService.schedule(() -> _f(42), delayInSeconds, TimeUnit.SECONDS);
Alternative implementation:
Thread t = new Thread(() -> {
    try {
        sleep(30_000);
        f(42);
    } catch (InterruptedException e) {
        throw new RuntimeException(e);
    }
});
t.start();
186
Exit program cleanly
Exit a program cleanly indicating no error to OS 
System.exit(0);
187
Disjoint Set
Disjoint Sets hold elements that are partitioned into a number of disjoint (non-overlapping) sets. 
public class DisjointSetElement {
  private DisjointSetElement representative = this;

  public DisjointSetElement getRepresentative() {
    if (representative != this) {
      representative = representative.getRepresentative();
    }
    return representative;
  }

  public void union(DisjointSetElement other) {
    other.getRepresentative().representative = getRepresentative();
  }
}
189
Filter and transform list
Produce a new list y containing the result of the function T applied to all elements e of the list x that match the predicate P. 
x.stream().filter(P).map(T).collect(Collectors.toList());
Alternative implementation:
List<T> y = x.stream()
             .filter(P)
             .map(T)
             .toList();
191
Check if any value in a list is larger than a limit
Given a one-dimensional array a, check if any value is larger than x, and execute the procedure f if that is the case 
for(int i = 0; i<a.length;i++) {
	if(a[i]>x) {
		f();
		break;
	}
}
Alternative implementation:
for (int i : a)
    if (i > x) {
        f();
        break;
    }
Alternative implementation:
if (of(a).anyMatch(i -> i > x)) f();
192
Declare a real variable with at least 20 digits
Declare a real variable a with at least 20 digits; if the type does not exist, issue an error at compile time. 
String pi = "3.14159265358979323846";
MathContext m = new MathContext(21, HALF_UP);
BigDecimal a = new BigDecimal(pi, m);
193
Transpose a two-dimensional matrix
Declare two two-dimensional arrays a and b of dimension n*m and m*n, respectively. Assign to b the transpose of a (i.e. the value with index interchange). 
Class<?> c = a.getClass().getComponentType()
                         .getComponentType();
T b[][] = (T[][]) newInstance(c, m, n);
int y, x;
for (y = 0; y < m; ++y)
    for (x = 0; x < n; ++x)
        b[y][x] = a[x][y];
Alternative implementation:
int b[][] = new int[m][n], x, y;
for (y = 0; y < m; ++y)
    for (x = 0; x < n; ++x)
        b[y][x] = a[x][y];
195
Pass a two-dimensional array
Pass an array a of real numbers to the procedure (resp. function) foo. Output the size of the array, and the sum of all its elements when each element is multiplied with the array indices i and j (assuming they start from one). 
public static void foo(int[][] a){
       System.out.println("Array a has a size of " + a.length+" by " + a[0].length);
       int sum = 0;
       for (int i = 0; i<a.length; i++){
           for (int j = 0; j<a[0].length; j++){
               sum += ((i+1)*(j+1)*a[i][j]);
           }
       }
       System.out.println("The sum of all elements multiplied by their indices is: " + sum);
    }
Alternative implementation:
void foo(double a[][]) {
    int i, j, m = a.length, n;
    double z = 0, t;
    out.println(m);
    for (i = 0; i < m; ++i) {
        n = a[i].length;
        for (j = 0; j < n; ++j) {
            z = (z + ((t = a[i][j]) * (i + 1)));
            z = (z + (t * (j + 1)));
        }
    }
    out.println(z);
}
196
Pass a sub-array
Given an integer array a of size n, pass the first, third, fifth and seventh, ... up to the m th element to a routine foo which sets all these elements to 42. 
void foo(int a[], int m) {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
}
Alternative implementation:
interface F { void set(int a[], int m); }
F f = (a, m) -> {
    for (int i = 1; i <= m; a[i] = 42, i = i + 2);
};
197
Get a list of lines from a file
Retrieve the contents of file at path into a list of strings lines, in which each element is a line of the file. 
List<String> lines = Files.readAllLines(new File(path).toPath());