How to resize an array in Java

In Java, arrays cannot be resized dynamically. One approach is to use java.util.ArrayList (or java.util.Vector) instead of a native array. Another (faster) solution is to re-allocate the array with a different size and to copy the contents of the old array to the new array. The generic function resizeArray (below) can be used to do that.

Update: Starting with Java 1.6, there is a library function that does the same: java.utils.Arrays.copyOf().

* Reallocates an array with a new size, and copies the contents
* of the old array to the new array.
* @param oldArray  the old array, to be reallocated.
* @param newSize   the new array size.
* @return          A new array with the same contents.
private static Object resizeArray (Object oldArray, int newSize) {
   int oldSize = java.lang.reflect.Array.getLength(oldArray);
   Class elementType = oldArray.getClass().getComponentType();
   Object newArray = java.lang.reflect.Array.newInstance(
         elementType, newSize);
   int preserveLength = Math.min(oldSize, newSize);
   if (preserveLength > 0)
      System.arraycopy(oldArray, 0, newArray, 0, preserveLength);
   return newArray; }

// Test routine for resizeArray().
public static void main (String[] args) {
   int[] a = {1, 2, 3};
   a = (int[])resizeArray(a, 5);
   a[3] = 4;
   a[4] = 5;
   for (int i=0; i<a.length; i++)
      System.out.println(a[i]); }

How to resize a two-dimensional array

Two-dimensional arrays in Java are arrays of arrays. To resize a two-dimensional array, the resizeArray function must be applied to the outer array and to all the nested arrays.

  int a[][] = new int[2][3];
  a = (int[][])resizeArray(a, 20);
    // new array is [20][3]
  for (int i=0; i<a.length; i++) {
     if (a[i] == null)
        a[i] = new int[30];
      else a[i] = (int[])resizeArray(a[i], 30); }
   // new array is [20][30]

Author: Christian d'Heureuse (,
License: Free / LGPL