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Samuel Fernandes
Samuel Fernandes

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Posted on    May-12-2015 1:59 AM

 Java Java  String 

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I've been using the == operator in my program to compare all my strings so far. However, I ran into a bug, changed one of them into .equals() instead, and it fixed the bug.

Is == bad? When should it and should it not be used? What's the difference?

Mayank Tripathi
Mayank Tripathi

Total Post:397

Posted on    May-12-2015 7:04 AM

== tests for reference equality.

.equals() tests for value equality.

Consequently, if you actually want to test whether two strings have the same value you should use .equals().

There are however a few situations where you can guarantee that two strings with the same value will be represented by the same object because of String interning. Those cases are specified by the Java Language Specification.

== is for testing whether two strings are the same object.

// These two have the same value
new String("test").equals("test") // --> true 
// ... but they are not the same object
new String("test") == "test" // --> false 
// ... neither are these
new String("test") == new String("test") // --> false 
// ... but these are because literals are interned by 
// the compiler and thus refer to the same object
"test" == "test" // --> true 
// concatenation of string literals happens at compile time,
// also resulting in the same object
"test" == "te" + "st" // --> true
// but .substring() is invoked at runtime, generating distinct objects
"test" == "!test".substring(1) // --> false
// interned strings can also be recalled by calling .intern()
"test" == "!test".substring(1).intern() // --> true
It is important to note that == is a bit cheaper than equals() (a single reference comparison instead of a method call), thus, in situations where it is applicable (i.e. you can guarantee that you are only dealing with interned strings) it can present an important performance improvement. However, these situations are rare.

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