How can this equality statement be false?

Question

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Equal strings aren't equal (==) in Java?

From Eclipse's Display window:

 messages.get(i).getMsg() == lastMsg
 (boolean) false
 messages.get(i).getMsg().length()
 (int) 14
 lastMsg.length()
 (int) 14
 messages.get(i).getMsg()
 (java.lang.String) INSERT QUARTER
 lastMsg
 (java.lang.String) INSERT QUARTER

Fairly new to Java. How can the first statement be false? Am I missing something ridiculously simple here?

score 5 · Accepted Answer · answered Dec 27 '11 at 08:10

When you compare strings in Java, you should do that by calling the equals() method, not by using the == operator.

The == operator does not test if the value or content of two objects is the same. It simply checks if the two expressions on both sides of the == refer to the exact same object. If you have two String objects that have the same content, but that are distinct objects, then == will return false.

(There is a minor complication to this though: the compiler does some clever tricks so that if you use string literals, == will actually return true).

String msg = messages.get(i).getMsg();
System.out.println(msg == lastMsg); // false
System.out.println(msg.equals(lastMsg)); // true

If you use string literals, they will end up as the same object not only thanks to clever compiler tricks, but as a result of the language spec. — Thilo, Dec 27 '11 at 08:13
Yes @Thilo, I didn't want to go into that too much here as it would probably just confuse AngryHacker. — Jesper, Dec 27 '11 at 08:14

score 1 · Answer 2 · answered Dec 27 '11 at 08:09

You're comparing strings with == - which means you're actually seeing whether the two string references are references to the same string object.

It looks like you've actually got two distinct string objects which are equal - so you need to use equals:

messages.get(i).getMsg().equals(lastMsg) // Should print true

This is a very common mistake for those new to Java, so don't feel bad :) Operators can't be overloaded in Java, so for reference types, == always compares the references themselves, a sort of identity comparison rather than an equivalence comparison.

score 1 · Answer 3 · answered Dec 27 '11 at 08:10

1

== compares the references and not the string content. If you want to compare strings in java use String.equals:

lastMsg.equals(messages.get(i).getMsg())

answered Dec 27 '11 at 08:10

Howard

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score 1 · Answer 4 · answered Dec 27 '11 at 08:10

1

When dealing with String in Java and checking for equality you should be using equals and not ==

answered Dec 27 '11 at 08:10

Eugene

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score 1 · Answer 5 · answered Dec 27 '11 at 08:11

1

"==" operator compares objects' references. use method equals() to compare objects themselves

answered Dec 27 '11 at 08:11

Andrey Atapin

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score 1 · Answer 6 · edited May 23 '17 at 11:48

So it looks like messages.get(i).getMsg() returns a String and lastMsg is also a String. If you use the == to test equality, you're actually comparing the instances and it's returning false because the 2 Strings are 2 different instances eventhough the String that they represent are the same. You need to use Object's .equals() method to compare the quality of Objects. See also: Java String.equals versus ==

score 1 · Answer 7 · answered Dec 27 '11 at 08:12

1

If you are coming from the C/C++ world, then this is a classic Java "gotcha".

The comparison operator (==) is generally "true" when the objects on both sides of the == are the same object, and not because the members of both objects are equivalent.

answered Dec 27 '11 at 08:12

selbie

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How can this equality statement be false?

7 Answers7