Changing Array Size : Array Basics « Collections « Java Tutorial

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Once an array is created, its size cannot be changed. If you want to change the size, you must create a new array and populates it using the values of the old array.

public class MainClass {

  public static void main(String[] args) {
    int[] numbers = { 1, 2, 3 };

    int[] temp = new int[4];
    int length = numbers.length;
    for (int j = 0; j < length; j++) {
      temp[j] = numbers[j];
    }
    numbers = temp;

  }

}