Why the below objects are not equals in Java?

Question

IDE: Eclipse; Language: Java core

package p1; public class StringTestA { /** * @param args */ public static void main(String[] args) { Object o1 = new StringTestA(); Object o2 = new StringTestA(); StringTestA o3 = new StringTestA(); StringTestA o4 = new StringTestA(); if (o1.equals(o2)) { System.out.println("Object o1 and o2 are eqals"); } if (o3.equals(o4)) { System.out.println("Object o3 and o4 are eqals"); } } public boolean equals(StringTestA other) { System.out.println("StringTestA equals mathod reached"); return true; } }

Output:

 StringTestA equals method reached Object o3 and o4 are equals

No output if equals in not overridden.

Question: why System.out.println("Object o1 and o2 are eqals"); line is not printed as equals is returning true;

Mark Peters · Accepted Answer · 2014-10-04 09:13:11Z

8

You are not overriding equals(Object). The argument must be an Object, not StringTestA. You are instead overloading equals (creating a different method with the same name).

Always annotate methods you wish to override with @Override. Doing so will cause a compile error if you happen to make a mistake in the method declaration, as you did here.

@Override public boolean equals(Object obj) { //... }

answered Oct 4, 2014 at 9:13

Mark Peters

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4 Comments

Andrea Borgogelli Avveduti Over a year ago

If you override the equals(), you MUST also override hashCode(). Otherwise a violation of the general contract for Object.hashCode will occur, which can have unexpected repercussions when your class is in conjunction with all hash-based collections.

Mark Peters Over a year ago

@AndreaBorgogelliAvveduti: While this is true and good advice, it is tangential to the question. For all we know, OP did override hashCode but didn't include it because it had nothing to do with the question.

T.J. Crowder Over a year ago

I wouldn't call it irrelevant. It is only tangentially related. I'd certainly have mentioned it as a side comment at the end of the answer, e.g., "Side note: If you're overriding equals, you have to override hashCode too." (+1, btw, good explanation, particularly re the annotation.)

Mark Peters Over a year ago

@T.J.Crowder: You're right. It's a good reminder. But it belongs as a comment on the question, not an answer.

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