Adding a new Value to an already existing Array (not using ArrayList)

Question

I want to add a new Value to my already existing Array. I made a new Array with the length of the already existing one and added one more Place for the new Value.

public class Measurement {

    private int[] data;

    public Measurement() {
        data = new int[0];
    }

    public static void addValue(int value) {
        int [] a = new int [data.length + 1];
        for(int i = 0; i < data.length; i++) {
            a[i] = data[i];
            a[i + 1] = value;
        }
        for(int i: a) {
            System.out.println(i);
        }
    }
}

I tried it with an already initialized Array and it worked, but I don't know what it doesn't with my existing try.

int[] a = {1,3,4,5,6,9};
        int value = 10;
        int[] b = new int[a.length + 1];
        for(int i = 0; i < a.length; i++) {
            b[i] = a[i];
            b[i + 1] = value;
    }
    for (int i : b) {
        System.out.println(i);
    }

Your current code wouldn't compile because addValue is a static method, and data is an instance field. — Jon Skeet
– Jon Skeet, Commented Oct 29, 2021 at 11:04
Your first loop will not run even once for empty array as i < data.length will be false immediately. meaning a[i + 1] = value; will also never run. But i don't see the logic in adding the extra element in the loop anyway. just do a[a.length-1] = value; after your loop to add the extra value. — user6073886
– user6073886, Commented Oct 29, 2021 at 11:07

Oleg Cherednik · Accepted Answer · 2021-10-29 11:16:10Z

1

public class Measurement {

    private int[] data = new int[0];

    public void addValue(int value) {
        int[] arr = new int[data.length + 1];
        System.arraycopy(data, 0, arr, 0, data.length);
        arr[arr.length - 1] = value;
        data = arr;
    }

}

Alternative way:

public class Measurement {

    private int[] data = new int[0];

    public void addValue(int value) {
        data = Arrays.copyOf(data, data.length + 1);
        data[data.length - 1] = value;
    }

}

edited Oct 29, 2021 at 11:16

answered Oct 29, 2021 at 11:10

Oleg Cherednik

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2 Comments

Neo Over a year ago

Thank you, so just for me to understand: data = Arrays.copyOf(data, data.length + 1); is there to make a new Array with the a new index and with data[data.length - 1] = value; i Am looking at the new created array and adding the new Value to the created extra space?

vsfDawg Over a year ago

@Goof Yes. Per the javadoc of copyOf(int[],int): "Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length."

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