How can I wrap the int[] into an object and implement equals()?

Question

I am trying to see how can I test if an element, with the same value as one already inside my Stack, is in fact in my stack.

ex.

int aa[] = {5,1};
int bb[] = {3,4};
int cc[] = {3,4};

Stack stack = new Stack();

stack.push(aa);
stack.push(bb);

System.out.println(stack.contains(cc));
>>>false

If I understand correctly this is false because the int[] objects are pointers, and becuase they are pointing at two different arrays, they are considered unequal. (From this answer)

I'm trying to wrap the int[] into an object and implement equals and hashCode, as was done in the answer to the other question but I'm getting a Cannot resolve symbol 'myArray' on o.myArray.length != myArray.length and int i = 0; i < o.myArray.length; i++. I also don't undertand why I need / where hashCode() is used.

Can someone tell me what I'm doing wrong or if there is a better solution?

import java.util.Arrays;

public class IntArray {

    public int[] myArray;

    public IntArray () {
        myArray = new int[0];
    }

    public IntArray (int[] array) {
        myArray = array;
    }

    public int[] getArray() {
        return myArray;
    }

    public int hashCode() {
        return Arrays.hashCode(myArray);
    }

    public boolean equals(Object o) {

        if (!(o instanceof IntArray))
            return false;

        if (o.myArray.length != myArray.length)
            return false;
        else {
            for (int i = 0; i < o.myArray.length; i++) {
                if (myArray[i] != myArray[i]) {
                    return false;
                }
            }
            return true;
        }
    }
}

You need to cast `o` to `IntArray`. But why not use `Arrays.equals()`? — shmosel, Jul 30 '19 at 23:20
like `if (Arrays.equals(o.myArray, myArray))` ? I'm sorry I don't understand, how do I cast `o` to `IntArray` — Bn.F76, Jul 30 '19 at 23:22
I think the real question here is this: are you always testing pairs, and what kind of values do you want in that `Stack`? Paris of int(s), or just int(s)? Basically, if `aa` was `{1, 3}` and `bb` was `{4, 5}` should it match `{3, 4}`? — Elliott Frisch, Jul 30 '19 at 23:25
Like `return o instanceof IntArray && Arrays.equals(((IntArray)o).myArray, this.myArray);` — shmosel, Jul 30 '19 at 23:25

cyborg · Accepted Answer · 2019-07-31T11:02:09.633

I shall leave the question of a better way of doing this and explain what you are being told.

o is always of type Object in the equals method. The Object class does not have a myArray field - hence the error. That you checked o's an instance of IntArray previously using instanceof and that o must be at least an IntArray due to that and it defines a myArray field is not enough - you are required to cast it explictly:

((IntArray)o).myArray.length;
IntArray a = (IntArray)o;
a.myArray.length;

So basically that's it as far as that goes - you are not being explicit. I would have expected any syntax checking editor to have picked that up really so if that is approriate consider getting any one of the myriad available to avoid such mistakes.

How can I wrap the int[] into an object and implement equals()?

1 Answers1