equals() method and == are equal or not equal?

Question

I know that Object.equals() compares the value of object in heap memory. and == compares the references.

But when I am running my code I am getting both as equal.

public class test3 {
    public static void main(String args[]){
        test2 ts = new test2();
        test2 tss = new test2();

        if(ts.a == tss.a){
            System.out.println("they are equal");
        }else
            System.out.println("they are unequal..");

        if(ts.a.equals(tss.a)){
            System.out.println("equal");
        }else
            System.out.println("not equal..");
    }
}

public class test2 { String a= "soumya"; }

Your variable `a` is probably a value. `ts.a == tss.a` is then a value equals another value. If you instead do `ts == tss` you'll get the reference to ts does not equals the reference of tss. Another thing to think about is that if a is static, ts.a will always equal to tss.a — user2687506, Aug 27 '14 at 08:55
can you provide implementation of class `test2`, as you are not comparing objects but fields — user902383, Aug 27 '14 at 08:56
test2 is another class in the same package having a declared as — Soumya, Aug 27 '14 at 08:57
@user2363747 can you please post also the constructor for test2? — Olimpiu POP, Aug 27 '14 at 09:00

score 3 · Answer 1 · edited May 23 '17 at 10:29

3

Looks like a rebus, but nothing suprising. In java there are several reference types which have cache (pool) of their values in some intervals. Such types are e.g.: String, Integer (for values -128...127).

So, I suppose, your test2 class looks like:

class test2 {
    String a = "x" ;
}

or

class test2 {
    Integer a = 1;
}

or something similar.

About these caches you can read:

Integer: Integers caching in Java

String: What is the Java string pool and how is "s" different from new String("s")?

edited May 23 '17 at 10:29

Community

1
1

answered Aug 27 '14 at 09:01

Andremoniy

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@user2687506 Sorry?? Have you ever heard about `Java String pool`? Sure, we are talking about classical JVM implementation (Oracle e.g.) – Andremoniy Aug 27 '14 at 09:04
Yes, sorry, deleted my comment. Without the references to Java String Pool the guy posting could easily think that == is a good way to compare strings (which it certainly isn't). The main reason is that if you have some kind of "unkown" string this will not be fetched from the pool even if it's the same. It only works for "constants". – user2687506 Aug 27 '14 at 09:07

NoDataFound · Answer 2 · 2014-08-27T09:52:39.673

1

Since your updated your question with the definition of class test2, the answer is now clear: String's constant are stored into an internal cache, using intern().

And this part:

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification

That's why reference and equality (using equals) returns both true.

edited Aug 27 '14 at 09:52

answered Aug 27 '14 at 08:54

NoDataFound

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JaskeyLam · Answer 3 · 2014-08-28T04:02:43.513

Most of the case, ==(defaults to the Object#equals(Object o) ) returns the same as equals()

Please check the equals(Object obj)of Object in the source code:

public boolean equals(Object obj) {
    return (this == obj);
}

They return true if and only if their references are one and the same.

But sometimes, they may not be the same, if the equals() has been overrided.The most common case is that String has overrided the equals() to make it s1.equals(s2) return true as long as s1 and s2 have the same literal value.

Please check the below output:

public static void main(String[] args) {
    String s1=new String("123");
    String s2=new String("123");
    Object o1=new Object();
    Object o2=new Object();

    System.out.println(s1.equals(s2));//true, since String override the equals() method
    System.out.println(s1==s2);//false

    System.out.println(o1.equals(o2));//false
    System.out.println(o1==o2);//false
}

equals() method and == are equal or not equal?

3 Answers3