The Subs Bench

How do I subclass a list element declared in my interface?

In a current project, we’re looking at our strong types having both interfaces and concrete implementations. There’s a reason for this that’s NOT about making things mockable.

Read More: 1Z0-816: Oracle Java SE 11 Programmer II

What’s nice and obvious is that this is possible:

public interface Bar {

}

public interface Foo {

    Bar getBar();

}

public class BarImpl implements Bar {

}

public class FooImpl implements Foo {

    public BarImpl getBar() { ... }

}

Despite the fact that the FooImpl subclass returns a subclass of the interface’s declared getter everything compiles happily, because a subclass can be substituted for a base class and still meet the Liskov substitution principle.

However, there’s a problem if you try to do this:

public interface Foo {

   List<Bar> getBars();

}

public class FooImpl implements Foo {

   // compiler error...

   public List<BarImpl> getBars() { ... }

}

In the Java generic type system, List<BarImpl> is not a subclass of List<Bar>. The folks who designed this stuff are pretty clever, and there’s probably a very good technical reason why this is the case.

The question is – can it be solved?

public interface Foo<T extends Bar> {

    // we know this is "at least" a Bar

    List<T> getBars();

}

public class FooImpl implements Foo<BarImpl> {

    // compiles great

    public List<BarImpl> getBars() { ... }

}

For lots of List or Optional of things, this might seem a bit cumbersome, but it is how it’s done… apparently.

Source: javacodegeeks.com