Table of Contents

1\. Rules to create immutable classes
2\. Java immutable class example
3\. Benefits of making a class immutable
5\. Summary

import java.util.Date;

/**
* Always remember that your instance variables will be either mutable or immutable.
* Identify them and return new objects with copied content for all mutable objects.
* Immutable variables can be returned safely without extra effort.
* */
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;
	}
}

class TestMain
{
	public static void main(String[] args)
	{
		ImmutableClass im = ImmutableClass.createNewInstance(100,"test", new Date());
		System.out.println(im);
		tryModification(im.getImmutableField1(),im.getImmutableField2(),im.getMutableField());
		System.out.println(im);
	}

	private static void tryModification(Integer immutableField1, String immutableField2, Date mutableField)
	{
		immutableField1 = 10000;
		immutableField2 = "test changed";
		mutableField.setDate(10);
	}
}

100 - test - Tue Oct 30 21:34:08 IST 2012
100 - test - Tue Oct 30 21:34:08 IST 2012