What

Prototype Pattern is a creational pattern to create new objects by cloning existed objects. Usually we use consutructor of class to create new instances, but you can also clone one from old object, then copy all the fields.

When

In what situation, we need use cloning to create new object? Can’t we just use constructors? Imagine you have an object, and you don’t know what class the object belongs to, how do you create another object of that class? Yes, you can clone one from the original object. Another possible place to apply prototype pattern is you want a new object but it’s very expensive to call the constructor. To clone one maybe a better solution.

How

When we use prototype pattern, we should define a Prototype interface, whatever can be cloned must implement this Prototype interface. In the client, we use clone to get new instance of the concrete classes.

(From http://www.wikiwand.com/en/Prototype_pattern)

Where

Look at the class diagrams above again, is the Prototype interface very like Object class in Java? Yes, the Object use this pattern by default. But in Java Object, the clone method is protected, so yo can’t call it directly. What you need to do is extends Object(every class must do that), override clone method, and implements Cloneable interface. The Cloneable interface don’t have any method, it’s just a mark interface, indicating objects of the class which implements it can be cloned. If you didn’t implements this Cloneable interface, a CloneNotSupportedException wiil be thrown when you call clone method.