How to compare arrays in Java

Using the operator == in Java sometimes gets into a bug when do string comparations. To fix that bug some people use .equals(). Here is the explanation to when and where use the == operator or equals().

== 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() (except in a few situations where you can guarantee that two strings with the same value will be represented by the same object eg: String interning). == 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.