Question

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What makes reference comparison (==) work for some strings in Java?

I know this has been asked before, but in spite of recommendations to use .equals() instead of the == comparison operator, I found that == works all the time:

String s1 = "Hello";
String s2 = "Hello";
System.out.println(s1 == s2); // true

Can anyone give me an example of the == operator failing?

Answer 1

This is because you’re lucky. The == operator in Java checks for reference equality: it returns true if the pointers are the same. It does not check for contents equality. Identical strings found at compile-time are collapsed into a single String instance, so it works with String literals, but not with strings generated at runtime.

For instance, "Foo" == "Foo" might work, but "Foo" == new String("Foo") won’t, because new String("Foo") creates a new String instance, and breaks any possible pointer equality.

More importantly, most Strings you deal with in a real-world program are runtime-generated. User input in text boxes is runtime-generated. Messages received through a socket are runtime-generated. Stuff read from a file is runtime-generated. So it’s very important that you use the equals method, and not the == operator, if you want to check for contents equality.