Why is "F" + "alse" not == "False"?

Question

Possible Duplicate:
How do I compare strings in Java?

I cant understand why the declared variables is not the same.

ex code:

 String firstPart = "F";
 String whole = "False";
 String connected = firstPart + "alse";
 System.out.println(connected == whole);

Now this produce a boolean and I thought it would be 'true' BUT it is not, It comes out as false and I don't understand why.

Can someone explain this?

+1. A fair enough question if you don't know about `equals()`, although using "False" as the example confuses the issue (people may be marking you down thinking you want to comparing strings with booleans) — SDC, Jan 15 '13 at 13:40
It is a common mistake. But this question has probably been asked THOUSANDS of times before on SO. — Stephen C, Jan 15 '13 at 13:41
If you make `firstPart` final it will be the same object so `==` will be true. Otherwise only the *contents* will be the same so you have to use `equals` — Peter Lawrey, Jan 15 '13 at 13:48
Strings in Java are immutable. So string a + string b will never == string c. Likely (string a + string b) will be allocated on the stack, while string c will be in the string pool. — Mikkel Løkke, Jan 15 '13 at 13:58
The question is - if you're writing this kind of stuff in your code how will somebody else understand it? — Mukus, Mar 06 '14 at 22:54
@TejaswiRana This is a homework question to illustrate how the compiler works rather then to point out the many other errors with the syntax ;) Since i learned alot it was not a bad question in it self. — user1501127, Mar 07 '14 at 10:57

score 20 · Answer 1 · answered Jan 15 '13 at 13:38

20

You are comparing references, not the values.

You'll need to use equals:

connected.equals(whole);

answered Jan 15 '13 at 13:38

Femaref

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Brian Agnew · Accepted Answer · 2013-01-15T17:20:26.310

17

This

String connected = firstPart + "alse";

creates a new String object, with a new underlying char array and a new reference.

Consequently when you compare references (using '=='') you won't get a match. If you compare the actual object contents using equals() then you'll get the result you want (since String.equals() compares the contents of the underlying char arrays)

edited Jan 15 '13 at 17:20

answered Jan 15 '13 at 13:39

Brian Agnew

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Just because others will see this, due to String interning, if you make the String assignments final, then (connected == whole) is indeed true. – Quaternion Jan 24 '19 at 22:45

score 6 · Answer 3 · answered Jan 15 '13 at 13:38

6

You should compare strings using equals(). Like so:

System.out.println(connected.equals(whole));

answered Jan 15 '13 at 13:38

Ivaylo Strandjev

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score 1 · Answer 4 · answered Jan 15 '13 at 13:41

When you use "==" to compare the strings it means that you want to compare their reference values but not the values they have, which in this case will return false.

you need to use equals method which will compare strings based upon the values they are holding..

score 1 · Answer 5 · answered Jan 15 '13 at 13:42

1

You should use .equals() method to compare string...

answered Jan 15 '13 at 13:42

Kanagaraj M

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score 1 · Answer 6 · answered Jan 15 '13 at 13:42

Essentially, only PDTs can be checked with ==. These are byte, short, int, long, float, double, boolean, char and of course, references. As the two references you get are different objects, the result would be false.

For any ADT (or object), you have to make use of the equals() method. All Java objects have them and you have to implement it for your own objects as well. For the String object this has been done for you and the characters are compared for to test equality.

score 1 · Answer 7 · answered Jan 15 '13 at 13:42

Java ain't JavaScript - the comparison operators are different:

In java == means "are the two objects the same instance?", which corresponds to JavaScript's === operator.

In java .equals() for String means "are the characters the same", which corresponds to JavaScript's == operator.

score 1 · Answer 8 · answered Jan 15 '13 at 13:43

Rule of thumb: == compares object references, not object values. Overriding equals is the standard way to define equality amongst objects (String included since it is a class, not a primitive).

firstPart, whole and connected define a new String each, so their references are different even if connected and whole match the same String in the pool, because == will not contemplate value.

Why is "F" + "alse" not == "False"?

8 Answers8