create a instance of local inner class

Question

case I

 class Outer {    

    void outerMethod() {
        Inner i = new Inner();    //compile error : cannot find symbol
        i.innerMethod();

        class Inner {
            void innerMethod() {
                System.out.println("inner class method...");
            }
        }       
    }
}

case II

class Outer {    

    void outerMethod() {
        Inner i = new Inner();
        i.innerMethod();        
    }
    class Inner {
            void innerMethod() {
                System.out.println("inner class method...");
            }
    }   
}

there are two separate class files in these two cases. but one is getting compile error and other one is fine. what is the reason for that ?

Eyal Schneider · Accepted Answer · 2014-04-08 08:40:54Z

3

Local classes have different scope rules. From the JLS, section 6.3:

"The scope of a local class declaration immediately enclosed by a block (§14.2) is the rest of the immediately enclosing block, including its own class declaration. "

In your first example, you call the constructor of Inner class before the scope of this inner class, therefore it is illegal.

TO illustrate this in your code:

void outerMethod() {
    // ...        
    // ...

    // Beginning of legal usage scope of the local class
    class Inner {
        void innerMethod() {
            System.out.println("inner class method...");
        }
    }
    // ...
    // ...
    // End of legal usage scope of the local class
}

edited Apr 8, 2014 at 8:40

answered Apr 8, 2014 at 8:27

Eyal Schneider

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4 Comments

Kalhan Over a year ago

thank you for the answer sir , i am having difficulty to understand this could you please illustrate this :)

Kalhan Over a year ago

sir then what is the legal scope of the case II this may be a very novice question :) and can u give me a little about scope ? thank you :)

Dtex Over a year ago

It seems like standard variable scope rule to me. If you declare a variable at the end of a method, do you expect to be able to use it at the start of it? If you declare a class field at the bottom of a class, do you expect to be able to use it in every method of the class?

Eyal Schneider Over a year ago

@Dtex: except that local classes also contain their own scope, where self-references are legal.

Rahul Tripathi · Accepted Answer · 2014-04-08 08:25:50Z

1

Your case II is working because you have done the proper scoping

class Outer {    

    void outerMethod() {
        Inner i = new Inner();
        i.innerMethod();        
    }   --> You have the scope change here in this case
    class Inner {
            void innerMethod() {
                System.out.println("inner class method...");
            }
    }   
}

ie, local inner class must be defined before using it.

In the first case when you instantiate your Inner class, it is unknown and hence resulted in compile time error

answered Apr 8, 2014 at 8:25

Rahul Tripathi

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Comments

Boris Brodski · Accepted Answer · 2014-04-08 08:30:15Z

1

In case 1 you have to define a class first and then use it. The reason for this is the context of the class Inner. Consider this:

 class Outer {    

    void outerMethod() {
        Inner i = new Inner();    //compile error : cannot find symbol
        i.innerMethod();

        final int localVar = 1;
        class Inner {
            void innerMethod() {
                System.out.println("inner class method... localVar = " + localVar );
            }
        }       
        Inner i = new Inner();    // No Error
        i.innerMethod();
    }
}

As you can see, the class Inner uses local variable defined earlier and though can only be used later on in the code.

answered Apr 8, 2014 at 8:30

Boris Brodski

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Comments

Patrick · Accepted Answer · 2014-04-08 08:26:11Z

0

In your first case, when you instantiate your Inner class, it's unknown. Just change the order of your instructions :

void outerMethod() {
   class Inner {
       void innerMethod() {
           System.out.println("inner class method...");
       }
   }   

   Inner i = new Inner();
   i.innerMethod();
}

answered Apr 8, 2014 at 8:26

Patrick

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Comments

Ankur Shanbhag · Accepted Answer · 2014-04-08 08:26:16Z

0

The local inner class must be defined withing the method before using it.

Try this:

class Outer {    

    void outerMethod() {
        class Inner { // Inner class definition 
            void innerMethod() {
                System.out.println("inner class method...");
            }
        }       

        Inner i = new Inner();    // No compilation error 
        i.innerMethod();

    }
}

answered Apr 8, 2014 at 8:26

Ankur Shanbhag

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Comments

Miron Balcerzak · Accepted Answer · 2014-04-08 08:26:59Z

0

in case 1 - think of the class as variable declaration (since it only exists in the scope of this method), you are trying to assign variable "inner" when it still not exists, move the class declaration to the top of the method and it will compile.

 class Outer {    

    void outerMethod() {
        class Inner {
            void innerMethod() {
                System.out.println("inner class method...");
            }
        }   
        Inner i = new Inner();
        i.innerMethod();   
    }
}

answered Apr 8, 2014 at 8:26

Miron Balcerzak

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