Implement two interfaces in an anonymous class
JavaAndroidJava Problem Overview
I have two interfaces:
interface A {
void foo();
}
interface B {
void bar();
}
I am able to create anonymous instances of classes implementing either of these interfaces like so:
new A() {
void foo() {}
}
or:
new B() {
void bar() {}
}
I want to create an anonymous class that implements both interfaces. Something like (the fictitious):
new A implements B {
void foo() {}
void bar() {}
}
This obviously gives a compile error: "B cannot be resolved to a type".
The workaround is quite simple:
class Aggregate implements A, B {
void foo() {}
void bar() {}
}
I then use Aggregate
where ever I would have used the anonymous class.
I was wondering if it is even legal for an anonymous class to implement two interfaces.
Java Solutions
Solution 1 - Java
"An anonymous inner class can extend one subclass or implement one interface. Unlike non-anonymous classes (inner or otherwise), an anonymous inner class cannot do both. In other words, it cannot both extend a class and implement an interface, nor can it implement more than one interface. " (http://scjp.wikidot.com/nested-classes)
Solution 2 - Java
If you are determined to do this, you could declare a third interface, C:
public interface C extends A, B {
}
In this way, you can declare a single anonymous inner class, which is an implementation of C.
A complete example might look like:
public class MyClass {
public interface A {
void foo();
}
public interface B {
void bar();
}
public interface C extends A, B {
void baz();
}
public void doIt(C c) {
c.foo();
c.bar();
c.baz();
}
public static void main(String[] args) {
MyClass mc = new MyClass();
mc.doIt(new C() {
@Override
public void foo() {
System.out.println("foo()");
}
@Override
public void bar() {
System.out.println("bar()");
}
@Override
public void baz() {
System.out.println("baz()");
}
});
}
}
The output of this example is:
foo()
bar()
baz()
Solution 3 - Java
For save some keystrokes (for example if the interfaces have a lot of methods) you can do this:
abstract class Aggregate implements A, B {
}
new MyObject extends Aggregate {
void foo() {}
void bar() {}
}
Notice the key is to declare the Aggregate
as abstract.
Solution 4 - Java
Note that you can make a named local class that implements the two interfaces:
void method() {
class Aggregate implements A, B {
void foo() {}
void bar() {}
}
A a = new Aggregate();
B b = new Aggregate();
}
This save you from doing a class-level or top-level class declaration.
The result is called a local class. Local classes declared in instance methods are also inner classes, which means that they can reference the containing object instance.