Question

I have the following query , I was going through Java immutable class concept and come up with the following analysis..

• All the fields must be private and preferably final
• Ensure the class cannot be overridden – make the class final, or use static factories and keep constructors private
• Fields must be populated from the Constructor/Factory
• Don’t provide any setters for the fields
• Watch out for collections. Use Collections.unmodifiable*.
• Also, collections should contain only immutable Objects
• All the getters must provide immutable objects or use defensive copying
• Don’t provide any methods that change the internal state of the Object.

Now I have the following class..

public final class Bill {

    private final int amount;
    private final DateTime dateTime;
    private final List<Integers> orders;

}

Please advise how it can be made as immutable class.

Answer 1

Your class as it is is immutable. Now you probably want to add a few methods:

public final class Bill {

    private final int amount;
    private final DateTime dateTime;
    private final List<Integers> orders;

    public Bill(int amount, DateTime dateTime, List<Integer> orders) {
        this.amount = amount; //primitive type: ok
        this.dateTime = dateTime; //joda.DateTime is immutable: ok
        this.orders = new ArrayList<Integer> (orders); //make a copy as the caller could modify the list at its end
    }

    // no method that adds or removes from the list

   public List<Integer> getOrders() {
       return Collections.unmodifiableList(orders); //defensive copy
   }
}

Alternatively, you can use this.orders = Collections.unmodifiableList(orders); in the constructor and return it from getOrders(): return orders;, which enforces the fact that you should not modify that list, even within your class.