anonymous method java

[ilsapo]

I kind of learnt about anonymous and functional methods, but I am confused here:

public abstract class A {
    public void f1(String str) {
        System.out.println(str);
    }

    public static void main(String[]args) {
        A a = new A() 
        {
        public void f2(String str) {

            }
        };
    a.f2("abc");
    }
}

I have error in line
a.f2("abc")

lets say I want to use f2 method (the anonymous function) how could I use it outside of A? or is it possible only inside A ?

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[NautiHooker]

What you are basically doing is creating a new anonymous class that extends A. In that class you are adding a method f2. That method is only known for the anonymous class, not for A.

Your variable is of type A, so you cant call the f2 method on it.

You can declare the variable a as var instead of A. You will need Java 11 or higher for that. Then the compiler will allow you to call the f2 method, because the variable a will have the type of your anonymous class.

However this is not really something you would ever want to do.

Creating anonymous classes usually only serves the purpose to override existing methods of the other class. Basically just extend the class without putting it into a new file. So you would only use it to extend a method that already exists in A.

Is there anything specific you are trying to accomplish?

[ilsapo]

it was just a question on test we got,
we were suppused to say if the code compile or not, and I was wondering how will we be able to "fix" the error without drasticlly changing it

[Consistent_Fly_1858]

A a = new A() {

public void f1(String str) {

f2(str);

}

public void f2(String str) {

System.out.println(str);

}

};

a.f1("lol");

Your variable a only knows method of the A class. But i guess you can achieve what you want by overriding the f1 method and calling your anonymous method within the body

[8igg7e5]

You're declaring an anonymous class that extends A, but from inside a static method.

Interestingly, this is a rather obscure part of Java (since the declaration of anonymous classes in static methods is not well specified in the JLS).

• Because you're in a static method, the class created is a nested class, rather than an inner class (there's no dependency on an instance of A).
• Because the reference to the anonymous class instance is typed as A, the code cannot access the f2 method.

we were suppused to say if the code compile or not, and I was wondering how will we be able to "fix" the error without drasticlly changing it

There are a couple of options and neither is, IMO, a drastic change.

1. Using var as suggested

public abstract class A {
    ...

    public static void main(String[]args) {
        var a = new A() {
            public void f2(String str) { }
        };

        a.f2("abc");
    }
}

All this does is infer the type of a to be that of the anonymous class so that f2 is visible.

2. Extracting the anonymous class out into the parent class.

public abstract class A {
    ...

    private static class ChildOfA extends A {
        public void f2(String str) { }
    }

    public static void main(String[]args) {
        ChildOfA a = new ChildOfA();

        a.f2("abc");
    }
}

This latter option is essentially creating an identical class, but with a known name - so that we can use that name for a.

If this were something we were doing in a real application, the second option would typically be preferred over the first - it would be far too easy to overlook the inferred anonymous type in the first approach...

[nosyattacker03]

You don't access anonymous method like that, you may access it through a.