Consider an interface that you have developed calledDoIt
:Suppose that, at a later time, you want to add a third method topublic interface DoIt { void doSomething(int i, double x); int doSomethingElse(String s); }DoIt
, so that the interface now becomes:If you make this change, all classes that implement the oldpublic interface DoIt { void doSomething(int i, double x); int doSomethingElse(String s); boolean didItWork(int i, double x, String s); }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 extendsDoIt
:Now users of your code can choose to continue to use the old interface or to upgrade to the new interface.public interface DoItPlus extends DoIt { boolean didItWork(int i, double x, String s); }