Extending an interface

An interface can extend another interface via the extends keyword.

public interface BasicResourceService {
    Resource getResource();
}

public interface ExtendedResourceService extends BasicResourceService {
    void updateResource(Resource resource);
}

Now a class implementing ExtendedResourceService will need to implement both getResource() and updateResource().

Extending multiple interfaces

Unlike classes, the extends keyword can be used to extend multiple interfaces (Separated by commas) allowing for combinations of interfaces into a new interface

public interface BasicResourceService {
    Resource getResource();
}

public interface AlternateResourceService {
    Resource getAlternateResource();
}

public interface ExtendedResourceService extends BasicResourceService, AlternateResourceService {
    Resource updateResource(Resource resource);
}

In this case a class implementing ExtendedResourceService will need to implement getResource(), getAlternateResource(), and updateResource().