I don't understand this ("string" == "string") example

Question

I found this java code on a java tutorial page:

if ("progress" == evt.getPropertyName())

http://download.oracle.com/javase/tutorial/uiswing/examples/components/index.html

How could this work? I thought we HAVE TO use the equals() method for this situation (string.equals("bla"))? Could we use equals() here too? Would it be better? Any ideas?

Edit: So IF equals() would be better, then I really don't understand why a serious oracle tutorial page didn't use it? Also, I don't understand why it's working because I thought a string is an object. If I say object == object, then that's a big problem.

Answer 1

Yes, equals() would definitely be better and correct. In Java, a pool of string constants is maintained and reused intelligently for performance. So this can work, but it is only guaranteed if evt.getPropertyName() is assured to return constants.

Also, the more correct version would be "progress".equals(evt.getPropertyName()), in case evt.getPropertyName() is null. Note that the implementation of String.equals starts with using == as a first test before doing char-by-char comparison, so performance will not be much affected versus the original code.

Answer 2

Which demo are we looking at?

This explains equals() vs ==

http://www.java-samples.com/showtutorial.php?tutorialid=221

It is important to understand that the equals( ) method and the == operator perform two different operations. As just explained, the equals( ) method compares the characters inside a String object. The == operator compares two object references to see whether they refer to the same instance. The following program shows how two different String objects can contain the same characters, but references to these objects will not compare as equal:

So in your particular example, it is comparing the reference to see if they are the same reference, not to see if the string chars match I believe.