Converting Mutable class to Immutable class in java

 - Make class as final to ensure this class wont get extended

- Initialize all fields in Constructor

- Remove/Dont-Add Setters

- Make all mutable members as final , so that all these members gets initialised only once.

- Make all fields Private so that they are not accessible through object

- Special attention needs to be given for Mutable members of the class. Incase to return the value of mutable, then do deepcopy/clone and send it rather than the reference.

Found one good example: 

courtesy:https://howtodoinjava.com/java/basics/how-to-make-a-java-class-immutable/

https://www.informit.com/articles/article.aspx?p=20530

public final class ImmutableClass

{

 

    /**

    * Integer class is immutable as it does not provide any setter to change its content

    * */

    private final Integer immutableField1;

 

    /**

    * String class is immutable as it also does not provide setter to change its content

    * */

    private final String immutableField2;

 

    /**

    * Date class is mutable as it provide setters to change various date/time parts

    * */

    private final Date mutableField;

 

    //Default private constructor will ensure no unplanned construction of class

    private ImmutableClass(Integer fld1, String fld2, Date date)

    {

        this.immutableField1 = fld1;

        this.immutableField2 = fld2;

        this.mutableField = new Date(date.getTime());

    }

 

    //Factory method to store object creation logic in single place

    public static ImmutableClass createNewInstance(Integer fld1, String fld2, Date date)

    {

        return new ImmutableClass(fld1, fld2, date);

    }

 

    //Provide no setter methods

 

    /**

    * Integer class is immutable so we can return the instance variable as it is

    * */

    public Integer getImmutableField1() {

        return immutableField1;

    }

 

    /**

    * String class is also immutable so we can return the instance variable as it is

    * */

    public String getImmutableField2() {

        return immutableField2;

    }

 

    /**

    * Date class is mutable so we need a little care here.

    * We should not return the reference of original instance variable.

    * Instead a new Date object, with content copied to it, should be returned.

    * */

    public Date getMutableField() {

        return new Date(mutableField.getTime());

    }

 

    @Override

    public String toString() {

        return immutableField1 +" - "+ immutableField2 +" - "+ mutableField;

    }

}