Use a List<String>, such as an ArrayList<String>. It's dynamically growable, unlike arrays (see: Effective Java 2nd Edition, Item 25: Prefer lists to arrays).

import java.util.*;
//....

List<String> list = new ArrayList<String>();
list.add("1");
list.add("2");
list.add("3");
System.out.println(list); // prints "[1, 2, 3]"

If you insist on using arrays, you can use java.util.Arrays.copyOf to allocate a bigger array to accomodate the additional element. This is really not the best solution, though.

static <T> T[] append(T[] arr, T element) {
    final int N = arr.length;
    arr = Arrays.copyOf(arr, N + 1);
    arr[N] = element;
    return arr;
}

String[] arr = { "1", "2", "3" };
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3]"
arr = append(arr, "4");
System.out.println(Arrays.toString(arr)); // prints "[1, 2, 3, 4]"

This is O(N) per append. ArrayList, on the other hand, has O(1) amortized cost per operation.

See also

• Java Tutorials/Arrays
  • An array is a container object that holds a fixed number of values of a single type. The length of an array is established when the array is created. After creation, its length is fixed.
• Java Tutorials/The List interface