1. Don't provide any methods that modify the object 
2. Ensure that no methods may be overridden. This prevents 
careless or malicious subclasses from compromising the 
immutable behavior of the class. Preventing
method overrides is generally done by making the class final
3. Make all fields final. 
4. Make all fields private. This prevents clients from 
modifying fields directly.

Make the class as final..
like
public final class String

Steps:
 1. Create class as final class
 2. Make all the properties i.e instance variable as final.
 3. Avoid or make the least access for the setter method 
for properties.

declare calss as a final...

1. Declare the Class as final (this inherently makes all
member function final)
2. Declare member variables private (not necessarily final,
because private instance variable are accessed via public
member functions only, e.g. in java.lang.String class,
variable "hash" is declared as "private int hash;" with no
final keyword).