comparing concatinated strings and normal strings

Question

when i am comparing a string that is concatinated and comparing the objects of the concatinated string and normal string its showing false. I thought that the Strig literal "kalyansreekar" is already present in String constant pool. can soomeone explain me why this is happening and also the implementation code of string concatination. Thank you.

String a="kalyan";
String b=a+"sreekar";
String c="kalyansreekar";
System.out.println(b==c);

"I expect the result to be true, but the actual output is false"

score 0 · Answer 1 · answered Sep 05 '19 at 13:03

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Here b==c compares the reference of the string not the content of the string.
If you want to compare value of string then use b.equals(c) instead of b==c

answered Sep 05 '19 at 13:03

sandip

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i don't want to compare values. I thought both the references b and c will point to same string literal "kalyansreekar" string in constant pool and so the output should be true but its not. – Kalyan Sreekar Jhade Sep 05 '19 at 13:06
@KalyanSreekarJhade well then you just tested it and saw it was not the case. Use `intern()` to make it so. – Matthieu Sep 05 '19 at 18:52
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But that doesn't mean they're pointing to the same place in memory, they are separate objects. – Richard Barker Sep 05 '19 at 18:52

Matthieu · Answer 2 · 2019-09-05T19:11:14.233

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The + on String internally uses a StringBuilder that returns a new String instance (on Java <= 8) or uses invokedynamic (on Java >= 9).

Code constants like "kaylan", "sreekar" and "kalyansreekar" are interned in the JVM String pool. If you really want to compare using references, then try:

String a="kalyan";
String b=a+"sreekar";
b=b.intern(); // Puts it in the String pool
String c="kalyansreekar";
System.out.println(b==c); // true

edited Sep 05 '19 at 19:11

answered Sep 05 '19 at 18:56

Matthieu

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`+` on String doesn't use a `StringBuilder`. – Johannes Kuhn Sep 05 '19 at 18:58
@JohannesKuhn it does on my setup when I debug through it. What does it use on yours? – Matthieu Sep 05 '19 at 18:59
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It uses `invokedynamic`. – Johannes Kuhn Sep 05 '19 at 19:00
@JohannesKuhn interesting. I'll check on that and include a link if I find one. – Matthieu Sep 05 '19 at 19:01
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It was changed in Java 9, with [JEP 280](https://openjdk.java.net/jeps/280). – Johannes Kuhn Sep 05 '19 at 19:03
@JohannesKuhn that makes sense: I'm still on Java 8 (and sometimes 7...) ;) I thought it might be used in some other JVM languages like Scala. – Matthieu Sep 05 '19 at 19:08

comparing concatinated strings and normal strings

2 Answers2