Difference in string comparison result b/w == and String#replace with ==

Question

Possible Duplicate:
String comparison and String interning in Java

I have small doubt regarding String comparisons in Java, consider the following code:

if("String".replace('t','T') == "String".replace('t','T')) {
  System.out.println("true");
}
else {
  System.out.println("false");
}

The above code always print's false, where as if I try like this:

if("STring" == "STring") {
  System.out.println("true");
}
else {
  System.out.println("false");
}

It will always print me true. Yes, I know String comparisons should be done with String.equals() or equalsIgnoreCase() method. But this is one of the question was asked in interview and I am confused. Can anyone guide me on this behavior?

As per my knowledge, in code snippet 1, "String.replace('t','T') is returning object, so object comparisons returns in false. Am I right?

Rohit Jain · Accepted Answer · 2013-01-08T07:47:27.810

5

"String.replace('t','T') is returning object, so object comparisons returns in false. Am I right?

Yes, as for this case, you are right. String#replace(or any method of String class for that matter), will return a new String object (You can guess why? Immutability). And thus you would have to do the comparison using equals method, to compare their contents.

Now, in the second case: -

"STring" == "STring"

You are comparing two string literals. Now, since String literals are interned in Java, so both the literals are same (in the sense, they point to the same memory location), and hence == comparison gives you true.

The difference in comparison using == and equals is that, == compares the reference value - i.e value of memory location of objects, which will be different for two different string objects, as you are having in first case. Whereas, equals compares the actual content in those objects.

edited Jan 08 '13 at 07:47

answered Jan 08 '13 at 07:41

Rohit Jain

209,639
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Thanks Rohit Jain, I said same in interview, and my assumption was correct.. :) – Pradeep Simha Jan 08 '13 at 07:43
@PradeepSimha.. You're welcome :) So, what was the reply of the interviewer? Sure he must have said something? – Rohit Jain Jan 08 '13 at 07:45
nothing, just he said "ok".. :D – Pradeep Simha Jan 08 '13 at 07:47

score 4 · Answer 2 · answered Jan 08 '13 at 07:38

4

"String.replace('t','T') is returning object, so object comparisons returns in false. Am I right?

Yes, == compares object references, and your first code is comparing two different objects.

As far as the second code is concerned its due to string interning.

answered Jan 08 '13 at 07:38

Habib

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please read through this article, http://javadude.com/articles/passbyvalue.htm . This would even be an answer to the whole question aswell. There is no pass by reference – SomeJavaGuy Jan 08 '13 at 08:00

score 1 · Answer 3 · answered Jan 08 '13 at 07:42

ok lets do it like this, your both String objects "String" are referering to the same object. So they are "basicly" equal. That is a thing the compiler does for you

but the method replace, does create and return a new String object, and that is why your second code is not equal.

score 0 · Answer 4 · answered Jan 08 '13 at 07:41

0

Java always compares the basic types (int, byte, etc) or references for objects when using ==.

The java compiler optimizes the two string constants you entered to use the same object, thus the same reference, thus the == return true

answered Jan 08 '13 at 07:41

Rob Audenaerde

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score -1 · Answer 5 · answered Jan 08 '13 at 07:39

DO this way

("String".replace('t','T').Tostring() ==  ("String".replace('t','T')).ToString()

This will solve your problem because the replace statement should be converted to string before eveluation.

You can also user the String.Equals for this or better you use ignore case as you mention in your question.

score -1 · Answer 6 · answered Jan 08 '13 at 07:39

-1

Try this:

if(string1.equals(string2)){

...

}

answered Jan 08 '13 at 07:39

pidel

65
9

Yes, but my doubt was "why it is returning false" – Pradeep Simha Jan 08 '13 at 07:41

Difference in string comparison result b/w == and String#replace with ==

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