String objects' behavior

Question

Hello,
I have two classes
testA
public class testA {
public static void main(String args[]) {
String s1 = "abc";
String s2 = "abc";
if(s1 == s2)
System.out.println(1);
else
System.out.println(2);
if(s1.equals(s2))
System.out.println(3);
else
System.out.println(4);
testB
public class testB {
public static void main(String args[]) {
String s1 = "abc";
String s2 = new String("abc");
if(s1 == s2)
System.out.println(1);
else
System.out.println(2);
if(s1.equals(s2))
System.out.println(3);
else
System.out.println(4);
Why is the first test (s1==s2) evaluating to true (haven't we got two string objects here??) and the second evaluating to false?
Thanks in advance,
Balteo.

Answer 1

The JVM performs some trickery while instantiating string literals to increase performance and decrease memory overhead. The JVM can reuse and share string references because strings are immutable and, therefore, by definition thread-safe. In fact string objects will be stored in a pool whenever you create a string object JVM will check if it exists in the pool otherwise it creates. If it exists in the pool then JVM gets it from the pool and there is no harm in doing so, because strings are immutable - can't be changed. When you explicitly create a string object like String str = new String()...It will create a new object (it may or maynot exist in the pool). The equals() method of Object class does the indepth comparison i.e it compares the actual objects unlike the object reference in the case of == operator.
I would like to tell you to visit the Javaworld site (URL: http://www.javaworld.com/javaqa/2002-09/01-qa-0906-strings.html?#) where similar questions are answered.
I hope this helps out you better.
Sree Ramakrishna
That's a nice design; although a design you must be aware of and treat accordingly.