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Resize an arrayTag(s): Language


Arrays cannot be resized dynamically. If you want a dynamic data structure with random access, you use a Collection (Map, ArrayList,...).

If you need to expand, you can use System.arraycopy() method to copy the content of an array to another one. 

import java.lang.reflect.Array;

public class ArrayUtils {

  private ArrayUtils() {}

  public static Object expand(Object a) {
    Class cl = a.getClass();
    if (!cl.isArray()) return null;
    int length = Array.getLength(a);
    int newLength = length + (length / 2); // 50% more
    Class componentType = a.getClass().getComponentType();
    Object newArray = Array.newInstance(componentType, newLength);
    System.arraycopy(a, 0, newArray, 0, length);
    return newArray;
  }

  public static void main (String arg[]) {
    String s[] = new String[20];
    System.out.println("The s array length is " + s.length); // 20
    s = (String[])ArrayUtils.expand(s);
    System.out.println("The s array length is " + s.length); // 30

    int i[] = {1 ,2 ,3, 4};
    System.out.println("The i array length is " + i.length); // 4
    i = (int[])ArrayUtils.expand(i);
    System.out.println("The i array length is " + i.length); // 6
  }

}

But a better way is to use a Vector or an ArrayList. ArrayList is roughly equivalent to Vector, except that it is unsynchronized. 

import java.util.ArrayList;

public class ArrayListDemo {
  public static void main (String arg[]) {
    ArrayListDemo x = new ArrayListDemo();
    x.doit1();
    x.doit2();
  }

  public void doit1() {
    // single dimension
    ArrayList list = new ArrayList();
    list.add("a");
    list.add("b");
    int size = list.size();  // 2
    System.out.println("Array 1 " +list.get(0);   // a
  }

  public void doit2() {
    // multi dimensions
    ArrayList list = new ArrayList();
    ArrayList l1 = new ArrayList();
    l1.add("a");
    ArrayList l2 = new ArrayList();
    l2.add("b");
    ArrayList l3 = new ArrayList();
    l3.add("c");

    list.add(l1);
    list.add(l2);
    list.add(l3);

    int size1 = list.size();  // 3
    int size2 = ((ArrayList)list.get(0)).size();  // 1
    System.out.println("Array 2 "
      + ((ArrayList)list.get(1)).get(0));   // b
  }
}