Question

If Strings are immutable in Java, then how can we write as:

String s = new String();
s = s + "abc";

Answer 1

Your string variable is NOT the string. It’s a REFERENCE to an instance of String.

See for yourself:

String str = "Test String";
System.out.println( System.identityHashCode(str) ); // INSTANCE ID of the string

str = str + "Another value";
System.out.println( System.identityHashCode(str) ); // Whoa, it's a different string!

The instances the str variable points to are individually immutable, BUT the variable can be pointed to any instance of String you want.

If you don’t want it to be possible to reassign str to point to a different string instance, declare it final:

final String str = "Test String";
System.out.println( System.identityHashCode(str) ); // INSTANCE ID of the string

str = str + "Another value"; // BREAKS HORRIBLY