Specify that a method argument must inherit from one class and implement an interface

Question

I am writing a method which requires that one of its arguments descend from a particular class ( MyClass ) and implement an interface ( MyInterface ).

A crude way of doing this would be

public void doStuff(MyClass arg0) {
    if (!(arg0 instanceof MyInterface))
        throw new IllegalArgumentException("arg0 must implement MyInterface");
    // do whatever we need to do
}

Note that MyClass does not implement MyInterface , and both are classes that I import as they are.

Is there a more elegant way of doing this, preferably one that would already flag errors at build time?

Answer 1

You can do that with the following generic method:

public <T extends MyClass & MyInterface> void doStuff(T arg) { ... }

Assuming the following classes (and interfaces)

class MyClass {}
interface MyInterface {}

class A extends MyClass {}
class B implements MyInterface {}
class C extends MyClass implements MyInterface {}

the following two statements are illegal (compiler error)

doStuff(new A());
doStuff(new B());

whereas the following statement will compile

doStuff(new C());

See JLS §4.4 (Type Variables) for more information about the somehwat weird type variable declaration.