Q1: What will be the output of the program?

String s = 'ABC'; 
s.toLowerCase(); 
s += 'def'; 
System.out.println(s);

ANS:A - ABC

String objects are immutable. The object s above is set to 'ABC'. Now ask yourself if this object is changed and if so where - remember strings are immutable. Line 2 returns a string object but does not change the originag string object s, so after line 2 s is still 'ABC'. So what's happening on line 3? Java will treat line 3 like the following: s = new StringBuffer().append(s).append('def').toString(); This effectively creates a new String object and stores its reference in the variable s, the old String object containing 'ABC' is no longer referenced by a live thread and becomes available for garbage collection.