String creation in java

Question

In java String can be created by using new operator or by using + and +=. So, does all these string creation techniques check whether the string already exist in the string pool. If they dint then which String creation technique will check the pool.

Answer 1

No they don't.

Simple example :

    String s = new String("hell");
    String hello = "hello";
    s += 'o';
    System.out.println(hello == hello.intern()); //True
    System.out.println(s == s.intern());         //False
    System.out.println(s == hello);              //False
    System.out.println(s.intern() == hello);     //True
    //To sum up : s != s.intern() == hello.intern() == hello

This code on ideone

Here the new version of "s" isn't the internal version of "hello"

If you want to have the pool version of a specific string, you can use the intern() method (just as I did above). This way you're sure to have the same reference.

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Resources :

• Javadoc : String.intern() (you might think that the link points to valueOf(double), but the intern() method is right after ;) )

Answer 2

Only string constants and literals are automatically interned. If you're concatenating or otherwise creating strings, you need to actually call the intern() method. See http://download.oracle.com/javase/1.4.2/docs/api/java/lang/String.html#intern().