How to define a member interface in a static context in java?

Question

The member interface can only be defined inside a top-level class or interface or in a static context.

Case A: Interface within a top-level class works perfectly

package multiplei.interfaces.test;

public class InterfaceBetweenClass {

    interface Foo {
        void show();
    }

    class InnerClass implements Foo{
        public void show(){
            System.out.println("Inner Class implements Foo");
        }
    }

    public static void main(String[] args) {
        new InterfaceBetweenClass().new InnerClass().show();
    }

}

Case B: Interface within an interface works good.

public interface Creatable {
    interface Foo{
        void show();
    }}

Case C: I know it sounds stupid that why would anyone define an interface in a static context. But it give me the same error message when i try to define the interface in static context.

package multiplei.interfaces.test;

public class InterfaceBetweenClass {
    public static void main(String[] args) {
        interface Foo {  //Line 5
            void show(); 
        }
    }

}}

But line number 5 gives me the following error message "The member interface Foo can only be defined inside a top-level class or interface or in a static context." Please help me out with this If an interface can be defined in static context then how?

Even if it were possible it would not make any sense to create an interface within a method. This interface would not be visible outside the methods scope, not even in anonymous inner classes create later in the same method. — Timothy Truckle
– Timothy Truckle, Commented Mar 13, 2017 at 8:28
@TimothyTruckle I don't know about that. It seems to be the same situation as with "local classes" and those are allowed within methods: stackoverflow.com/questions/2428186/… — Thilo
– Thilo, Commented Mar 13, 2017 at 8:39
@Thilo 'It seems to be the same situation as with "local classes"' not quite. You create local class for immediate use within the same method (and nowhere else) whereas you create interfaces for use at some other place instead of some concrete class implementing it. — Timothy Truckle
– Timothy Truckle, Commented Mar 13, 2017 at 8:42
@TimothyTruckle You might create a "local interface" and have three "local classes" implement it. The calling code (also in the same method) could benefit from that. Note that you can have abstract local classes and local classes can extend from other local classes. — Thilo
– Thilo, Commented Mar 13, 2017 at 9:06

Thilo · Accepted Answer · 2017-03-13 08:09:27Z

5

You cannot define interfaces within methods.

I think the scenario the error message is referring to is defining an interface inside an inner class (which can be done, but only if that is a static inner class):

class A{
   static class X{
     interface Y{}
   }
}

answered Mar 13, 2017 at 8:09

Thilo

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1 Comment

Yati Sawhney - Yates Over a year ago

Static Nested Classes are just like top level classes so that should be covered in Case A I guess :(

Clara · Accepted Answer · 2020-07-18 08:45:52Z

0

For interface, both nested-interface and member-interface are the same thing (#opposite to member-class). Moreover, member-interface is an interface which is DIRECTLY enclosed by another class or another interface. Therefore, local interface DOES NOT exist.

answered Jul 18, 2020 at 8:45

Clara

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