push a new item into the end of the array

Question

So I want to Push a new item onto the end of the array (the numbers 1-9). I was told by a friend that the code I have written is correct, but when I run it on eclipse, nothing happens. What do I need to do? Should I just print the array under the main block? thanks.

public static void main(String[] args) {
    long[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
}

public static long[] push(long[] array, long item) {

    // Add one item to an array
    long cellsToAdd = 1;
    long[] array2 = new long[array.length + 1];

    // copy each item in array2 -> array3
    for (int i = 0; i < array.length; i++) {
        array[i] = array2[i];
    }

    array2[array2.length - 1] = item;

    System.out.println("Array: ");
    // Print the array to the console
    for (long p : array2) {
        System.out.println(p);
    }

    return array2;
}

Note also that Java arrays have fixed length. You *can't* push anything onto the end. The code you present instead creates a new, longer array with the desired contents. There's nothing inherently wrong with that, but it's important to appreciate the difference between that and what you asked. — John Bollinger, Oct 14 '16 at 16:09
Possible duplicate of [equivalent to push() or pop() for arrays?](http://stackoverflow.com/questions/4537980/equivalent-to-push-or-pop-for-arrays) — Frederic Klein, Oct 14 '16 at 16:24

Mr. Polywhirl · Answer 1 · 2016-10-14T16:36:50.337

You can use System.arraycopy to create a new array and increase its size.

Since you are using the primitive long type, you would need to copy-paste this logic for each primitive (int, float, double, etc) if you want to support these types.

public static void main(String[] args) {
    long[] digs = { 0, 1, 2, 3, 5 }                       
    long[] digs2 = push(digs, 6);
    long[] digs3 = pushAll(digs2, new long[] { 7, 8, 9 });

    System.out.println(Arrays.toString(digs));   // [0, 1, 2, 3, 5]
    System.out.println(Arrays.toString(digs2));  // [0, 1, 2, 3, 5, 6]
    System.out.println(Arrays.toString(digs3));  // [0, 1, 2, 3, 5, 6, 7, 8, 9]

    // Generic Example
    Long[] genArr = push(new Long[] { 0L, 1L }, new Long(3L), Long.class);

    // or Long[] genArr = push(new Long[] { 0L, 1L }, new Long(3L));

    System.out.println(Arrays.toString(genArr)); // [0, 1, 3] 
}

Push

public static long[] push(long[] a, long b) {
    long[] result = new long[a.length + 1]; 
    System.arraycopy(a, 0, result, 0, a.length); 
    result[a.length] = b;
    return result;
}

Push All

public static long[] pushAll(long[] a, long[] b) {
    long[] result = new long[a.length + b.length]; 
    System.arraycopy(a, 0, result, 0, a.length); 
    System.arraycopy(b, 0, result, a.length, b.length); 
    return result;
}

Generic Push

public static <E> E[] push(E[] a, E b, Class<E> classType) {
    @SuppressWarnings("unchecked")
    E[] result = (E[]) Array.newInstance(classType, a.length + 1);
    System.arraycopy(a, 0, result, 0, a.length); 
    result[a.length] = b;
    return result;
}

Optional

// Convenience, so that you don't have to pass in the class.
public static Long[] push(Long[] a, Long b) {
    return push(a, b, Long.class);
}

score 1 · Answer 2 · answered Oct 14 '16 at 16:37

1

Aside from actually calling the function like others mentioned, your for loop copys from array2 to array, while it should be the other way around

answered Oct 14 '16 at 16:37

Simon Eismann

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Basil Bourque · Answer 3 · 2016-10-14T16:34:05.333

0

Call the method

You defined a method but did not call it.

public static void main(String[] args) {
    long[] array = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    array = MyClassName.push( array , 10L );
}

If you are merely practicing to learn, well enough. But know that this kind of functionality is already provided for you in the Java Collections framework.

edited Oct 14 '16 at 16:34

answered Oct 14 '16 at 16:24

Basil Bourque

303,325
100
852
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