Interning String in Java

String class of Java API has a native method, intern().  This method is used for internalizing Java String. It's a process of creating a String pool where String objects with equal values are stored only once.

Method signature: 
public native String intern()

Straight from Java Doc

Returns a canonical representation for the string object.A pool of strings, initially empty, is maintained privately by the class String.

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.

It follows that for any two strings s and t, 

 s.intern() == t.intern() is true if and only if s.equals(t) is true.

All literal strings and string-valued constant expressions are interned.

public static void main(String[] args){
        String s = "abc";
        String d = "abc".intern();
        String f = new String("abc").intern();
        String g = new String("abc");
       
        System.out.format("  s == d : %b", (s==d));
        System.out.format("; s == f : %b", (s==f));
        System.out.format("; s == g : %b", (s==g));
        System.out.format("; s.equals(f) : %b", s.equals(f));
        System.out.format("; s.equals(g) : %b", s.equals(g));
    }

Ouptput :  s == d : true; s == f : true; s == g : false; s.equals(f) : true; s.equals(g) : true

So all strings except g in above case are interned.

Related Post : Changes to intern() method in Java 7