How to create Immutable class?

There are many immutable classes like String, Boolean, Byte, Short, Integer, Long, Float, Double etc. In short, all the wrapper classes and String class is immutable. We can also create immutable class by creating final class that have final data members as the example given below:

Example to create Immutable class

In this example, we have created a final class named Employee. It have one final datamember, a parameterized constructor and getter method.

ImmutableDemo.java

public final class Employee
{  
final String pancardNumber;  
public Employee(String pancardNumber)
{  
this.pancardNumber=pancardNumber;  
}
public String getPancardNumber(){  
return pancardNumber;  
}  
}  
public class ImmutableDemo
{
public static void main(String ar[])
{
Employee e = new Employee("ABC123");
String s1 = e.getPancardNumber();
System.out.println("Pancard Number: " + s1);
}
}

Output:

Pancard Number: ABC123

The above class is immutable because:

The instance variable of the class is final i.e. we cannot change the value of it after creating an object.
The class is final so we cannot create the subclass.
There is no setter methods i.e. we have no option to change the value of the instance variable.

These points makes this class as immutable.

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