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Difference between String and StringBuffer

1) A string created by String class is immutable i.e. it's original value cannot be modified. But a string of characters created by StringBuffer class can be modified, hence it is mutable.

2) Trying to modify a string created by String class leaves the original String unmodified and instead a new String is created, which leads to memory waste, while, no memory is wasted when a string created by StringBuffer is modified. Let's understand these concepts.

Modification of String object.

let us understand by a short code -

String s1= new String("Hello"); //Statement1
String s2= new String("World"); //Statement2
s1= s1.concat(s2); 		//Statement3

Figure 1 represents the Heap memory after executing the above three statements. Dashed line indicates a deleted reference.

• Statement1 creates a String object Hello, it is initially pointed by reference variable s1.
• Statement2 creates a String object World, pointed by reference variable s2.
• Statement3 calls concat() method to modify the String pointed s1 by adding String pointed by s2 to it.
• Doing so gives us a new String object HelloWorld, now pointed by s1. The original String Hello which we wanted to modify is not modified and now it is not even referenced by any variable(see Figure 1), leading to memory waste.

Modification of StringBuffer object.

The memory waste doesn't happen when we try to modify the a string of characters created by StringBuffer class because StringBuffer works directly with original object to modify it.
let us understand by a coding example -

StringBuffer s1= new StringBuffer("Hello");  // Statement1
StringBuffer s2= new StringBuffer("World");  // Statement2
s1.append(s2);		  		     // Statement3

Figure 2 represents the Heap memory after executing the above three statements.

• Statement1 creates a StringBuffer object "Hello", referenced by sb1.
• Statement2 creates a StringBuffer object "World", referenced by sb2.
• Statement3 has modified StringBuffer object "Hello" by appending "World" to it, giving us the modified StringBuffer object "HelloWorld". There is no loss of original object as we have modified the original StringBufffer object.

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StringBuffer example

Here in program below, we are trying to modify string of character created using StringBuffer class by calling its append() method.

// StringBuffer Class Example

class StringBufferEx
{
public static void main(String... ar)
{
StringBuffer sb= new StringBuffer("Keep");

System.out.println("Original String is : "+ sb);
sb.append("Learning");
System.out.println("Modified String is : "+ sb);
}
}

Output is :

Original String is : Blue
Modified String is : BlueSky

Program Analysis

• We have created a StringBuffer object that points to a string of characters Keep.
• We have called append() method to append a String Learning to our StringBuffer object.
• Post the call to append() method, the original StringBuffer object points to the modified string of characters KeepLearning, pointed by reference sb hence, no object is lost

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