无法访问匿名嵌套子类的成员（实现接口）

Question

I have problem here. I write a piece of code like this：

package vh.Static;

public class Nesting {
    //static class Nested{}
    class Inner{}
    void method(){
        Inner inner = new Inner(){
            public int z =2;
            public int getZ(){
                return z;
            }
        };
        System.out.println(inner);
    }
    public static void main(String args[]){
        Nesting ne  =new Nesting();
        Inner inner = ne.new Inner(){
            public int z =1;
            public int getZ(){
                return z;
            }
        };
        System.out.println(inner);
        ne.method();
   }
}

I don't know how to get the var z definding in the Inner constructor block. help me,please! and tell me where is z local? in Inner? or Nesting

Answer 1

The answer to the original title of your question is really broad - you'll need to do some research:

• Interface is contract definition only, it has no implementation
• Class can have implementation
• A nested class is visible only to its containing class

But to address your specific example, in the code:

void method(){
    Inner inner = new Inner(){
        public int z =2;
        public int getZ(){
            return z;
        }
    };
}

This instantiates a new anonymous subclass of the nested class Inner , and then extending the anonymous subclass with a public field and a method.

The reason why the extended field + method can't be accessed is that the variable inner is of the base type Inner (to which nothing has been added - this is defined simply as class Inner{} ), and is not of the type of the anonymous subclass. As it stands you would need to resort to nasties like reflection to access the fields + properties in the anonymous subclass, outside of the anonymous subclass itself.

 System.out.println(inner.getZ()); // Does not compile

What can be done instead is to define abstract methods on Inner and then override these in the anonymous class:

abstract class Inner{
    public abstract int getZ();
}

void method(){
    Inner inner = new Inner(){
        private int z =2; // Private fields plz, use get / setters
        public int getZ(){
            return z;
        }
    };

You can now access the defined Inner methods anywhere within the Nesting class

Just one note on conventions - the docs refer to the inner class (your Inner ), as the nested class and the outer class (your Nesting ) as Outer .

Here's the same thing defining Inner as an interface which is then implemented anonymously:

class Nesting {
    interface Inner{
        int getZ();
    }
    void method(){
        Inner inner = new Inner(){
            private int z =2;
            public int getZ(){
                return z;
            }
            private int anotherInnerMethod(){
                // Can access non-interface items as it is part of the anon subclass
                return z * z;
            }
        };
        System.out.println(inner);
        // Access getZ() through the interface
        System.out.println(inner.getZ());
    }
    // Because this method is in class Nesting, it may still access Inner
    public static void main(String args[]){
        Nesting ne = new Nesting();
        ne.method();
        Inner inner = new Inner(){
            private int z =1;
            public int getZ(){
                return z;
            }
        };
        System.out.println(inner.getZ());
        ne.method();
    }
}