Identical strings comparison gives me false

Question

I have two identical strings, one in an array and one in a String variable. When I compare these IDENTICAL strings I get false every time. I have debugged and debugged, but I get the same result every time. Here is the code in question

String temp = ""+(num1*num2);
Boolean equal = temp == answers[i];

if(equal) {
    correct[i] = true;
    num_correct ++;
}else{
    correct[i] = false;
}

Again, I have debugged every minor detail of this program and I am 101% sure that the strings are IDENTICAL. Why is Java returning false on comparison?

Why are you using a Boolean (java class) instead of the native boolean type ? Also, why using the == operator instead of the equals() method ? — SirDarius, Oct 06 '10 at 15:57
We need a top ten FAQ list, == instead of equals comes up all the time. — Steve Kuo, Oct 06 '10 at 16:27
@Steve Kuo That's a great idea. Should suggest it on meta if it's not there already. — jdmichal, Oct 06 '10 at 17:15
Why the hell would you -1 me! == works on strings in every single programming language I've encountered. The possibility that it wouldn't work didn't even occur to me, nor does it to lots of other programmers. — Hubro, Oct 08 '10 at 03:25

score 25 · Accepted Answer · edited Aug 07 '12 at 18:59

25

When you use the == operator in Java with objects, you are attempting to compare object references. That is, is this object handle pointing to the EXACT same object as this other object handle. Unless the strings are interned, this will not work.

Use String.equals(Object) instead:

Boolean equal = temp.equals(answers[i]);

edited Aug 07 '12 at 18:59

BalusC

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answered Oct 06 '10 at 15:56

jdmichal

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There's no need to use `Boolean` instead of `boolean`. – Steve Kuo Oct 06 '10 at 16:28
1

Was just stating it as found in the OP. If it were mine, I would have used `final boolean`. – jdmichal Oct 06 '10 at 17:15

score 5 · Answer 2 · answered Oct 06 '10 at 15:57

You are doing reference comparison, not value comparison. When you use the == operator its checking to see if the references are equal, and they aren't. If you want to check whether the values are equal use the equals method.

boolean equal = temp.equals(answers[i]);

score 1 · Answer 3 · answered Oct 06 '10 at 16:00

== in java for strings is comparing to see if they are the same object, not the same string value. You should use .equals instead which will compare the value. == works sometimes because the strings can be interned and refer to the same object via reference even if created seperately through the same literal (so string b = "Hey" and string c = "Hey" end up being the same object in the background because "Hey" got interned to a hidden string object).

score -1 · Answer 4 · answered Oct 06 '10 at 16:18

-1

As others have shown you should use equals.

But I would also use the booleanValue of the Boolean object.

Here is your code correctly done

String temp = ""+(num1*num2);
Boolean equal = temp.equals(answers[i]);

if(equal.booleanValue()) {
    correct[i] = true;
    num_correct ++;
}else{
    correct[i] = false;
}

answered Oct 06 '10 at 16:18

Shervin Asgari

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Probably because using Boolean.booleanValue is much more complicated than *boolean*. – DJClayworth Oct 06 '10 at 17:26
and it would unboxed to the primitive boolean value anyway – David Oct 06 '10 at 17:26
I am not the one who used Boolean. I just used the same code as the OP, but improved it – Shervin Asgari Oct 06 '10 at 18:04
@David: (un)Boxing is very expensive – Shervin Asgari Oct 06 '10 at 18:04
which is why he should use boolean. But you don't gain anything (except longer code) from calling booleanValue as that is what the Java 5+ compiler inserts for you anyway. The overhead comes from using reference types instead of primitives. – David Oct 07 '10 at 13:43
I didn't say it was a good downvote, I just explained it. – DJClayworth Oct 12 '10 at 19:48

score -5 · Answer 5 · answered Oct 06 '10 at 15:56

-5

Does this help?

Boolean equal = (temp == answers[i]);

I'm not sure that would be an issue, but I always enclose my conditions in parenthesis.

answered Oct 06 '10 at 15:56

Dutchie432

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-1 for missing the problem completely – Erick Robertson Oct 06 '10 at 16:03

Identical strings comparison gives me false

5 Answers5