Immutable
means cannot be changed or altered. String instance once created are immutable.
But still you can perform all type of operations like appending, reversing
the string etc. Each time you perform any of these operations, a new string
instance is formed and the original string on which operation is performed is
unchanged.
The
below example program justifies the immutable nature of string:
|
public class
MyStringImmutable
{
public static void main(String[]
args)
{
String str1="Coding"; // string literal.
str1=str1.concat(" Unlimited");
System.out.println(str1);
}
}
|
Output:
|
Coding Unlimited
|
Now
lets us see what happens when the above program is executed:
String str1="Coding";
After
the execution of above line a new string instance “Coding” is formed and placed in
string constant pool and
variable str1 has the reference to the instance.
str1=str1.concat("Unlimited");
After the execution of above line the string
instance “Coding”
is concatenated with “Unlimited” to form a new string instance “Coding Unlimited” and it is also placed in string constant
pool. The variable str1 now refers to “Coding Unlimited”.
What
is the advantage of immutable nature of String?
Suppose consider there are 3 reference
variables, all referring to one object “Coding”. If anyone of the 3 reference
variables tries to change the value of string object and if suppose string was
mutable object then change will affect all three reference variable, this may
not be desired in all circumstances. So to avoid such situation string objects
are immutable in java.
