String1 === string2 make me confuse

Question

------------------------A-------------
String s1="hello "; //string "hello " ended with a blank
String s2="world";
String s3="hello world";
s1+=s2;
System.out.println(s3==s1);
------------------------B------------
String ss1="hello world";
String ss2="hello world";
System.out.println(ss1==ss2);
if the output of statement - A is false then why the output of statement - B is true?
plz ans...
thnx

Answer 1

To be honest, I can't tell you the underlying
mechanics,Strings are pooled internally meaning that only one copy of a String literal, such as "hello", is ever stored. So each String literal is uniquely define by an object reference. If you know what you're doing you can utilize this fact to use == to compare Strings. So even thought this "feature" has it's roots in an implementation technique (pooling) the bahaviour is defined by the language. This means, again if you know what you're doing, it'ss a perfectly kosher way to compare Strings using ==.
Pooling is also used by some Java implementations to store small Integer literals. Sun for example pools -127 to 128 I think.
Integer i1 = new Integer(5); // an object reference
Integer i2 = new Integer(5); // another object reference
if (i1==i2)  // my god they're equal ?
  System.out.println("Small Integer literals are pooled");The above == comparision "should" be false but istn't because of pooling.
The difference is that this behaviour is an implementation artefact and NOT part of the language so it can never be utilized the way String pooling can.