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Programming-Idioms

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  • Java

Idiom #306 Ensure list capacity

Preallocate memory in the list x for a minimum total capacity of 200 elements.

This is not possible in all languages. It is only meant as a performance optimization, should not change the length of x, and should not have any effect on correctness.

import java.util.ArrayList;
import java.util.List;
List<T> x = new ArrayList<>(200);
import java.util.ArrayList;
ArrayList<?> x = new ArrayList<>();
x.ensureCapacity(200);

The default capacity is 10.
with Ada.Containers.Vectors;
declare
   X : Vector;
begin
   Reserve_Capacity (X, Capacity => 200);
end;
New implementation...
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programming-idioms.org 
List<T> x = new ArrayList<>(200);
ArrayList<?> x = new ArrayList<>();
x.ensureCapacity(200);
declare
   X : Vector;
begin
   Reserve_Capacity (X, Capacity => 200);
end;
if cap(x) < 200 {
	y := make([]T, len(x), 200)
	copy(y, x)
	x = y
}
x = slices.Grow(x, 200)
x.capacity := 200;
SetLength(a, 200);
my @list = (undef) x 200;
x.reserve(200);
x := Array new: 200.
"#(nil nil nil nil nil ...)"