Java Virtual Machine maintains an internal list of references for interned Strings ( pool of unique Strings) to avoid duplicate String objects in heap memory. Whenever the JVM loads String literal from class file and executes, it checks whether that String exists in the internal list or not. If it already exists in the list, then it does not create a new String and it uses reference to the existing String Object. JVM does this type of checking internally for String literal but not for String object which it creates through ‘new’ keyword. You can explicitly force JVM to do this type of checking for String objects which are created through ‘new’ keyword using String.intern() method. This forces JVM to check the internal list and use the existing String object if it is already present.
So the conclusion is, JVM maintains unique String objects for String literals internally. Programmers need not bother about String literals but they should bother about String objects that are created using ‘new’ keyword and they should use intern() method to avoid duplicate String objects in heap memory which in turn improves java performance. see the following section for more information.
Have a look at the following code snippet.
String s1 = new String("Java"); String s2 = new String("Java"); System.out.println(s1 == s2);//false System.out.println(s1.equals(s2));//true String t1= "JavaSE"; String t2 = "JavaSE"; System.out.println(t1 == t2); //true System.out.println(t1.equals(t2));//true String p1 = new String("JavaEE"); p1 = p1.intern();//Forced to use string literal value, from String Pool String p2 = "JavaEE"; p2 = p2.intern();//Forced to use string literal value, from String Pool System.out.println(p1 == p2); System.out.println(p1.equals(p2)); String m = new String("JavaFX"); m = m.intern();//Forced to use string literal value, from String Pool String n = new String("JavaFX"); n = n.intern();//Forced to use string literal value, from String Pool System.out.println(m == n);//true System.out.println(m.equals(n));//true