How can I dynamically add items to a Java array?

Question

In PHP, you can dynamically add elements to arrays by the following:

$x = new Array();
$x[] = 1;
$x[] = 2;

After this, $x would be an array like this: {1,2}.

Is there a way to do something similar in Java?

Answer 1

Apache Commons has an ArrayUtils implementation to add an element at the end of the new array:

/** Copies the given array and adds the given element at the end of the new array. */
public static <T> T[] add(T[] array, T element)

Answer 2

You can dynamically add elements to an array using Collection Frameworks in JAVA. collection Framework doesn't work on primitive data types.

This Collection framework will be available in "java.util.*" package

For example if you use ArrayList,

Create an object to it and then add number of elements (any type like String, Integer ...etc)

ArrayList a = new ArrayList();
a.add("suman");
a.add(new Integer(3));
a.add("gurram");

Now you were added 3 elements to an array.

if you want to remove any of added elements

a.remove("suman");

again if you want to add any element

a.add("Gurram");

So the array size is incresing / decreasing dynamically..

Answer 3

You probably want to use an ArrayList for this -- for a dynamically sized array like structure.

Answer 4

This is a simple way to add to an array in java. I used a second array to store my original array, and then added one more element to it. After that I passed that array back to the original one.

    int [] test = {12,22,33};
    int [] test2= new int[test.length+1];
    int m=5;int mz=0;
    for ( int test3: test)        
    {          
    test2[mz]=test3; mz++;        
    } 
    test2[mz++]=m;
    test=test2;

    for ( int test3: test)

    {
      System.out.println(test3);
    }

Answer 5

Arrays in Java have a fixed size, so you can't "add something at the end" as you could do in PHP.

A bit similar to the PHP behaviour is this:

int[] addElement(int[] org, int added) {
    int[] result = Arrays.copyOf(org, org.length +1);
    result[org.length] = added;
    return result;
}

Then you can write:

x = new int[0];
x = addElement(x, 1);
x = addElement(x, 2);

System.out.println(Arrays.toString(x));

But this scheme is horribly inefficient for larger arrays, as it makes a copy of the whole array each time. (And it is in fact not completely equivalent to PHP, since your old arrays stays the same).

The PHP arrays are in fact quite the same as a Java HashMap with an added "max key", so it would know which key to use next, and a strange iteration order (and a strange equivalence relation between Integer keys and some Strings). But for simple indexed collections, better use a List in Java, like the other answerers proposed.

If you want to avoid using List because of the overhead of wrapping every int in an Integer, consider using reimplementations of collections for primitive types, which use arrays internally, but will not do a copy on every change, only when the internal array is full (just like ArrayList). (One quickly googled example is this IntList class.)

Guava contains methods creating such wrappers in Ints.asList, Longs.asList, etc.