compare two objects whether they are equals:

Question

I have a list with two Box objects which are not the same (They have differ in id, address, quantity) and I need to get the index of the last item in the list which is 1 but I am always getting that the both objects are equal and the index of the last object is 0 and not 1:

Why are the both objects the same and how to get the index of the last box in the list? Am I missing something?

System.out.println("##--## boxList size:  " + boxList.size());
if( boxList.get(0).equals(boxList.get(1))){
    System.out.println("##--## the both boxes are equals");
}

for(Box boxB: boxList){
    System.out.println("##--## boxB id: " + boxB.id + " box address: " + boxB.store.address + " ,box quantity: " + boxB.quantity + " x: " + boxList.indexOf(boxB));         
}

Box lastFirstLoop = boxList.get(1);
int indexTemp = boxList.indexOf(lastFirstLoop); 
System.out.println("##--## indexTemp: " + indexTemp );

The output is:

##--## boxList size: 2
##--## the both boxes are equals
##--## boxB id: 1513682911061 box address: DS/1-1-1-1/A ,box quantity: 120 x: 0
##--## boxB id: 1513682911062 box address: DS/1-1-2-1/A ,box quantity: 18 x: 0
##--## indexTemp: 0

Edit:

@Override
public boolean equals(final Object other) {
    if (!(other instanceof Box)) {
        return false;
    }
    return new EqualsBuilder().appendSuper(super.equals(other)).isEquals();
}


/**
 * {@inheritDoc}
 */
@Override
public int hashCode() {
    if (this.hashCode == 0) {
        this.hashCode = new HashCodeBuilder().appendSuper(super.hashCode()).toHashCode();
    }
    return this.hashCode;
}

have you checked the equals method to verify what is compared? — Stultuske, Feb 12 '18 at 12:43
The `Box` class will have an `equals` method, it should show you how equality is determined between boxes. If it doesn't, search online for "how to override equals method" — byxor, Feb 12 '18 at 12:47
ok I did not implemented the equals and hashCode methods in the Box class. — window, Feb 12 '18 at 12:49
https://www.sitepoint.com/implement-javas-equals-method-correctly/ — c0der, Feb 12 '18 at 12:50
If you didn't implement those methods, `Object` do it but should not return `true` since both `Box` are different instance... with different references. You probably badly implement those instead. Provide class `Box` please. — AxelH, Feb 12 '18 at 12:51
@M.Prokhorov: it does not always retrun true I misunderstood user7. I meant it always retruns true in my case. I added the implementation of the both methods. — window, Feb 12 '18 at 13:07
@window, with attached implementation, the only way you can get the second line printed is if `list.get(0)` and `list.get(1)` both return the exact same `Box`, which, however, is not true according to the later printed lines. So you clearly aren't showing the full picture here. — M. Prokhorov, Feb 12 '18 at 13:13
What base class is Box derived from? And how is the base class equals() method defined? — DodgyCodeException, Feb 12 '18 at 13:34

score 0 · Answer 1 · answered Feb 12 '18 at 12:50

Assuming the "Box" is your custom class you should take a look here: https://stackoverflow.com/a/27609/6705466

It guides how to override equals(Object o) and hashCode(). You always should override those both together, and whenever equals returns true the hashCode should be the same.

My other suggestion is, add a little if statement before accessing index in the list (Just to avoid IndexOutOufBoundsException, possibly null check would be a nice to have)

Jayesh Choudhary · Answer 2 · 2018-02-12T14:17:47.937

0

First of all you need to **Override** your equals() method since **Box** is your Custom class. If you want to use .equals() method properly. Example is shown below: 

    class Box {
    /*your class body*/

    // Overriding equals() to compare two Box objects
    @Override
    public boolean equals(Object o) {
        Box box = (Box) o;
        if(id.equals(box.id)){
            return true;
        }
        return false;
    }
    }

And for indexOf() method you need to override hashCode() method.Shown below:

@Override
public int hashCode() {
   return Objects.hash(id);
}

edited Feb 12 '18 at 14:17

answered Feb 12 '18 at 13:18

Jayesh Choudhary

748
2
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after overriding them I can do the following: `item.equals(lastItem)` where the item and lastItem are box objects? – window Feb 12 '18 at 13:25
@Jayesh have you tested this code? It won't compile. – DodgyCodeException Feb 12 '18 at 13:54
@DodgyCodeException thanks for feedback. I have updated my ans. – Jayesh Choudhary Feb 12 '18 at 14:07
@window yes, After overriding these two methods your code will work fine. – Jayesh Choudhary Feb 12 '18 at 14:09
1

One more caveat: `if (id==box.id)` works ok if `id` is a primitive type such as `int` or `long`. If it is a reference type such as `String` or `Long`, then you should use `if (id.equals(box.id))`. – DodgyCodeException Feb 12 '18 at 14:16

compare two objects whether they are equals:

2 Answers2