equals and hashcode question

[Adolfo Eloy]

Hello everyone.

Trying a mock exam from coderanch, I fall into the question below:

"Given the following class House, which code fragments can be added to complete the equals() method without breaking the contract for hashCode()?"

public class House {
	private int windows;
	private int doors;
	private int chimneys;
	public House(int windows, int doors, int chimneys) {
		this.windows = windows;
		this.doors = doors;
		this.chimneys = chimneys;
	}
	public int hashCode() { return windows + doors; }
	public boolean equals(Object obj) {
		if (this == obj) return true;
		if (obj == null) return false;
		if (getClass() != obj.getClass()) return false;
		final House other = (House) obj;
		return // <-- insert code here
	}
}

I've selected two correct answers, but there is an answer provided by the mock that I think it is wrong. That is:

return windows == other.windows && doors == other.doors && chimneys == other.chimneys

I don't think the statement above are true, because running the code below, I get the output

hashCode: true
equals: false

public class TestHouse {
	public static void main(String[] args) {
		House h1 = new House(10, 5, 8);
		House h2 = new House(10, 5, 9);
		System.out.println("hashCode: " + h1.hashCode() == h2.hashCode());
		System.out.println("equals: " + h1.equals(h2));
	}
}

tks

[Tom Reilly]

The documentation states

equal objects must have equal hash codes

It does not say that unequal objects must have unequal hash codes.

[Sandra Bachan]

Hash Code determines which bucket an Object goes into. There can be different Objects in the same bucket.


Object 1 and Object 2 can evaluate to the same Hash Code and end up in the same bucket. However, Object 1 is not equal to Object 2.

[Adolfo Eloy]

Thanks for all reply.

But that's the point. When I was working on this question, I firstly was convinced about the concept that having two instances of the same class may have the same hashcode and be meaningfully unequal.
So, at this mock from coderanch, there was another answer that leads to the same situation (same hascode values and not equals) that follows:

return windows + doors + chimneys == other.windows + other.doors + other.chimneys 

Regarding this, the answers that follows must be considered valid to not break the hashCode's contract am I right?

return windows + doors + chimneys == other.windows + other.doors + other.chimneys

return windows == other.windows && doors == other.doors && chimneys == other.chimneys

tks

[Stephan van Hulst]

return windows + doors + chimneys == other.windows + other.doors + other.chimneys

This one will violate the contract. Consider the following:

House h1 = new House(5, 4, 6);
House h2 = new House(5, 3, 7);

Equals gives true, hashCodes are not equal.

[Adolfo Eloy]

Thanks to all. I understand now.