Immutable Class - Java

1. Class should be made final - avoiding overriding of methods by sub classes
2. All fields should be final and private - avoiding fields access outside the class and avoiding the  changes in the field values
3. No setters
4. Make sure that mutable fields getters will return a defensive copy from their getter method, there by avoiding the state to be mutated

public final class Immutable {

    private final Integer immutableinteger;

    private final String immutableString;

    private final Date mutableDate;

    public Immutable(Integer immutableinteger, String immutableString, Date mutableDate) {

        this.immutableinteger = immutableinteger;

        this.immutableString = immutableString;

        this.mutableDate = mutableDate;

    }

    public Integer getImmutableinteger() {

        return immutableinteger;

    }

    public String getImmutableString() {

        return immutableString;

    }

    public Date getMutableDate() {

        return new Date(mutableDate.getTime());

    }

}

public class ImmutableDemo {

    public static void main(String[] args) {

        Immutable immutable1 = new Immutable(100,"First String",new Date());

        System.out.println(immutable1.getImmutableinteger()+ " " + immutable1.getImmutableString()+ " " + immutable1.getMutableDate());

        modifyImmutable(immutable1.getImmutableinteger(),immutable1.getImmutableString(),immutable1.getMutableDate());

        System.out.println(immutable1.getImmutableinteger()+ " " + immutable1.getImmutableString()+ " " + immutable1.getMutableDate());

        Immutable immutable2 = new Immutable(200,"Second String",new Date());

        System.out.println(immutable2.getImmutableinteger()+ " " + immutable2.getImmutableString()+ " " + immutable2.getMutableDate());

    }

    private static void modifyImmutable(Integer immutableInteger, String immutableString, Date mutableDate) {

        immutableInteger = 1000;

        immutableString = "ModifiedString";

        mutableDate.setTime(11111);

    }

}

Output:

100 First String Mon Jun 16 18:31:10 IST 2014

100 First String Mon Jun 16 18:31:10 IST 2014

200 Second String Mon Jun 16 18:31:10 IST 2014