Java String intern()

Java String intern() is a native method. When the intern() method is invoked on a String object, if the String Pool already has a String with the same value, then the reference of String from the Pool is returned. Otherwise, this string object is added to the pool and the reference is returned.

public class StringIntern {
     public static void main(String args[]) {
         String s1 = new String("abc"); // goes to Heap Memory, like other objects
         String s2 = "abc"; // goes to String Pool
         String s3 = "abc"; // again, goes to String Pool
         // Let's check out above theories by checking references
         System.out.println("s1==s2? " + (s1 == s2)); // false
         System.out.println("s2==s3? " + (s2 == s3)); // true
         // Let's call intern() method on s1 now
         s1 = s1.intern(); // this should return the String with same value, BUT from String Pool
         // Let's run the test again
         System.out.println("s1==s2? " + (s1 == s2)); // true
     }
 }

Output : 
 s1==s2? false
 s2==s3? true
 s1==s2? true

• When we are using new operator, the String is created in the heap space. So “s1” object is created in the heap memory with value “abc”.
• When we create string literal, it’s created in the string pool. So “s2” and “s3” are referring to string object in the pool having value “abc”.
• In the first print statement, s1 and s2 are referring to different objects. Hence s1==s2 is returning false.
• In the second print statement, s2 and s3 are referring to the same object in the pool. Hence s2==s3 is returning true.
• Now, when we are calling s1.intern(), JVM checks if there is any string in the pool with value “abc” is present? Since there is a string object in the pool with value “abc”, its reference is returned.
• Notice that we are calling s1 = s1.intern(), so the s1 is now referring to the string pool object having value “abc”.
• At this point, all the three string objects are referring to the same object in the string pool. Hence s1==s2 is returning true now.