strange effect with string comparisation with ==

Question

I know that we shouldn't compare Strings with == and its better to use equals.
For so far I learned this should all be false.
So why does the first method return true?

private String ab = "AB";
private String ab2 = "A" + "B";
private String a = "A";
private String b = "B";
private String ab3 = a + b;

public void test () {
    System.out.println("ab == ab2" + ab==ab2);
    System.out.println("ab == ab3" + ab==ab3);
    System.out.println("ab == a+b" + ab==(a+b));
}

"For so far I learned this should all be false." Nope. Due to string literal interning... — Jon Skeet, Mar 07 '14 at 14:48
Thats why I put for so far, we are always learning ;) (and some seniors here didn't know it to) — chillworld, Mar 07 '14 at 14:51

score 5 · Accepted Answer · answered Mar 07 '14 at 14:49

5

Because the concatenation of literal Strings are compiled into a single String, which will also be interned in the String pool. This code:

private String ab2 = "A" + "B";

is compiled to

private String ab2 = "AB";

ab and ab2 point to the same literal String "AB", thus they're ==s.

Still, you should not trust comparison of Strings with ==, you should always compare their equality using equals method.

answered Mar 07 '14 at 14:49

Luiggi Mendoza

85,076
16
154
332

Like I said in mine first sentence that we shouldn't compare with == but equals. – chillworld Mar 07 '14 at 14:55
@chillworld yes but that line is posted for future readers that won't read all your question and will go directly to the code. – Luiggi Mendoza Mar 07 '14 at 14:59
oke np ;) and thx for the explication. – chillworld Mar 07 '14 at 15:01

score 0 · Answer 2 · answered Mar 07 '14 at 14:51

0

It is called String interning:

http://en.wikipedia.org/wiki/String_interning

To summarize, since strings are immutable in Java, JVM optimizes by creating only one object for two string literals that are equal. Therefore, comparison returns true.

answered Mar 07 '14 at 14:51

Selcuk

57,004
12
102
110

strange effect with string comparisation with ==

2 Answers2