Java Collection How to - Check Capacity

Question

We would like to know how to check Capacity.

Answer

The capacity is the number of elements the array list can hold before the internal data structure has to resize.

Use the ensureCapacity() method to check that the internal data structure has enough capacity before adding elements:

public void ensureCapacity(int minimumCapacity)

import java.util.ArrayList;
/*from   w w w . ja  v  a 2 s.  c o  m*/
public class MainClass {
  public static void main(String[] a) {

    ArrayList list = new ArrayList();
    list.add("A");
    list.ensureCapacity(10);

    System.out.println(list.size());
  }
}

The code above generates the following result.

After adding all of the elements, call the trimToSize() method

public void trimToSize()

The trimToSize() method makes sure that there is no unused space in the internal data structure.

import java.util.ArrayList;
//from  w ww  . j  av a2s . c  o  m
public class MainClass {
  public static void main(String[] a) {

    ArrayList list = new ArrayList();

    list.add("A");
    list.ensureCapacity(10);
    list.trimToSize();
  }
}