String Handling :: Part 1

String Handling in java

In java, string is basically an immutable object. We will discuss about immutable string later. Let's first understand what is string and how we can create the string object.

Generally string is a sequence of characters. But in java, string is an object. String class is used to create string object.


There are two ways to create String object:

• By string literal
• By new keyword

By String Literal :
String literal is created by double quote.
For Example:

String s="Hello";


Each time you create a string literal, the JVM checks the string constant pool first. If the string already exists in the pool, a reference to the pooled instance returns. If the string does not exist in the pool, a new String object instantiates, then is placed in the pool.For example:


String s1="Welcome";
String s2="Welcome";//no new object will be created

In the above example only one object will be created.First time JVM will find no string object with the name "Welcome" in string constant pool,so it will create a new object.Second time it will find the string with the name "Welcome" in string constant pool,so it will not create new object whether will return the reference to the same instance.

String objects are stored in a special memory area known as string constant pool inside the Heap memory.

Why java uses concept of string literal?

To make Java more memory efficient (because no new objects are created if it exists already in string constant pool).


By new Keyword :

String s=new String("Welcome");//creates two objects and one reference variable


In such case, JVM will create a new String object in normal(non pool) Heap memory and the literal "Welcome" will be placed in the string constant pool.The variable s will refer to the object in Heap(non pool).



Immutable String:

In java, strings are immutable (unmodifiable) objects.For example


class Simple{
 public static void main(String args[]){

   String s="Sachin";
   s.concat(" Tendulkar");//concat() method appends the string at the end
   System.out.println(s);//will print Sachin because strings are immutable objects
 }
}


Output: Sachin


As you can see in the above figure that two objects will be created but no reference variable refers to "Sachin Tendulkar".But if we explicitely assign it to the reference variable, it will refer to "Sachin Tendulkar" object.For example:


class Simple{
 public static void main(String args[]){

   String s="Sachin";
   s=s.concat(" Tendulkar");
   System.out.println(s);
 }
}


Output: Sachin Tendulkar


Why string objects are immutable in java?

Because java uses the concept of string literal.Suppose there are 5 reference variables,all referes to one object "sachin".If one reference variable changes the value of the object, it will be affected to all the reference variables. That is why string objects are immutable in java.


                                           JAVA TUTORIALS HOMEPAGE