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Trail: Learning the Java Language
Lesson: Interfaces and Inheritance
Section: Interfaces
Interfaces and Inheritance
Interfaces
Defining an Interface
Implementing an Interface
Using an Interface as a Type
Rewriting Interfaces
Summary of Interfaces
Questions and Exercises
Inheritance
Overriding and Hiding Methods
Hiding Fields
Using the Keyword super
Object as a Superclass
Writing Final Classes and Methods
Abstract Methods and Classes
Summary of Inheritance
Questions and Exercises
Home Page > Learning the Java Language > Interfaces and Inheritance
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Rewriting Interfaces

> Consider an interface that you have developed called `DoIt`:

> >     public interface DoIt {
> >        void doSomething(int i, double x);
> >        int doSomethingElse(String s);
> >     }

> Suppose that, at a later time, you want to add a third method to `DoIt`, so that the interface now becomes:

> >     public interface DoIt {

> >        void doSomething(int i, double x);
> >        int doSomethingElse(String s);
> >        boolean didItWork(int i, double x, String s);

> >     }

> If you make this change, all classes that implement the old `DoIt` interface will break because they don't implement the interface anymore. Programmers relying on this interface will protest loudly.

> Try to anticipate all uses for your interface and to specify it completely from the beginning. Given that this is often impossible, you may need to create more interfaces later. For example, you could create a `DoItPlus` interface that extends `DoIt`:

> >     public interface DoItPlus extends DoIt {

> >        boolean didItWork(int i, double x, String s);

> >     }

> Now users of your code can choose to continue to use the old interface or to upgrade to the new interface.

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