Difference between String ,StringBuffer & StringBuilder?

Run this simple java program and create scenerio of String object creation in your mind.You will see huge difference than expectation.

                               I would recommend to first try on your own then follow the solution .

As you execute the above line of codes in your Java Runtime Environment .As compiler

read the lines .In  case of 

                                   String a= new String("Hello");

       As it will allow compiler to create a new string object .It is always mandatory for 

compiler to allocate a memory space in heap whenever new keyword is used.So String

class object created successfully in heap with reference variable a.And a very important point 

need to be noted down that here  "new String("Hello');"  It will make a literal object in String 

pool with value "Hello".So total number of object created equals two.so if you print value of a

using System.out.println(a);  It will print Hello as reference variable for object a contain same 

value. Similarly it happens with  the next line  

                                     String b= new String("Java");

it also creates the same scenerio as String referenced above.But in case of next following line 

                                     String c="HelloJava";

In this case there will be a literal stored in constant pool.It also rise three most important question.

Que:What is difference between  constant pool and heap?

Que:What makes String immutable & why?

Que:How JVM utilize the need of constant pool?

As next lines of code executed it work as follow.In the given case when evaluation for

                                   System.out.print("c==a+b");

Here a & b are object of type String .While c is an string literal which contain value "HelloJava".

So when + operator work between two different object .It does not evaluate result equal to any 

String literal which is stored in Constant pool. So for "c==a+b" it will result to boolean result 

which comes out to be false.So only false will be printed.

In the following next line while evaluating  c=="Hello"+"Java";

It will work as operator overloading between two String literals.And as describe "Hello" &

"Java" both are stored in Constant bool which concat and result equals to c.

So true will pe printed  in System.out.print(c=="Hello"+"Java");

StringBuffer is faster than String when performing simple concatenations. In String

 manipulation code, character strings are routinely concatenated.