Immutable_Object & Class

** What is immutable class..??
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-> Immutable class is a class which once created, it’s contents can not be changed.
-> Immutable objects are the objects whose state can not be changed once constructed.
   e.g. String class.
-> All wrapper classes in java.lang are immutable –
   String, Integer, Boolean, Character, Byte, Short, Long, Float, Double.

*** Why use immutable class..??
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-> 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.

-> In case of String object, just because of SCP a single object is referred by multiple references.
   by using one reference if we are allowed to change the content then remaining refereces will be impacted.
   to overcome this problem some people made string object as immutable.
   ....
-> But in case of StringBuffer for every requirment a separate object will be created.
   by using one reference if we are allowed to change the content then remaining refereces won't be impacted.
   hence immutability concept is not required for StringBuffer object.

***How to create an immutable class???
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-> Create a final class.
   Set the values of properties using constructor only.
   Make the properties of the class final and private.
   Do not provide any setters for these properties.
   If the instance fields include references to mutable objects, don't allow those objects to be changed:
   Don't provide methods that modify the mutable objects.
   Don't share references to the mutable objects.
   Never store references to external, mutable objects passed to the constructor;
   if necessary, create copies, and store references to the copies. Similarly,
   create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

-> Ex..

public final class FinalPersonClass {
    private final String name;
    private final int age;

    public FinalPersonClass(final String name, final int age) {     
     this.name = name;
     this.age = age;
    }
    public int getAge() {
      return age;
    }
    public String getName() {
     return name;
    }
  }