Question

01    class SubClass extends SuperClass {}
02    class AppSuperClass {
03        /**
04         * @param superClass
05         */
06        public void print(SuperClass superClass) {
07            System.out.println("AppSuperClass:superclass is parameter");
08     
09        }
10        /**
11         * @param subClass
12         */
13        public void print(SubClass subClass) {
14            System.out.println("AppSuperClass:subclass is parameter");
15   
16        }
17    }
18   
19    class AppSubClass extends AppSuperClass {
20      /**
21       * @param superClass
22       */
23      public void print(SuperClass superClass) {
24          System.out.println("AppSubClass:superclass is parameter");
25   
26      }
27      /**
28       * @param subClass
29       */
30      public void print(SubClass subClass) {
31          System.out.println("AppSubClass:subclass is parameter");
32   
33      }
34  }
35  public class OverloadedTest {
36      public static void main(String[] args) {
37          AppSuperClass appSuperClass = new AppSuperClass();
38          AppSuperClass appSubClass = new AppSubClass();
39          SuperClass superClass = new SuperClass();
40          SuperClass subClassInstance = new SubClass();
41          /*
42           * Making request to print AppSuperClass
43           *  1. Passing SuperClass instance
44           *  2. Passing SubClass instance (*make note of the type) <img src="http://s0.wp.com/wp-includes/images/smilies/icon_smile.gif?m=1304052800g" alt=":)" class="wp-smiley">
45           */
46   
47          appSuperClass.print(superClass);
48          appSuperClass.print(subClassInstance);
49          /*
50           * Above is repeated with AppSubClass instance
51           */
52          appSubClass.print(superClass);
53          appSubClass.print(subClassInstance);
54      }
55   
56  }

When I run this I get

       AppSuperClass:superclass is parameter
       AppSuperClass:superclass is parameter
       AppSubClass:superclass is parameter
       AppSubClass:superclass is parameter

How can I get

           AppSuperClass:superclass is parameter
           AppSuperClass:subclass is parameter
           AppSubClass:superclass is parameter
           AppSubClass:subclass is parameter

as o/p without changing the type of any of the objects?

Answer 1

Your question isn’t very clear, but it sounds like you’re basically after execution-time overloading, which simply doesn’t exist in Java. Overloads are entirely resolved at compile-time. One option is this:

public void print(SuperClass superClass) {
    if (superClass instanceof SubClass) {]
        print((SubClass) superClass);
        return;
    }
    System.out.println("AppSuperClass:superclass is parameter");
}

Note that you’d need to do this in the override as well, or have a template method which does this, and a separate printImpl(SuperClass) method which can be overridden in the subclass.

To take this to the logical extreme, you might have:

class AppSuperClass {

    public final void print(SuperClass superClass) {
        if (superClass is SubClass) {
            printImpl((SubClass) superClass);
        } else {
            printImpl(superClass);            
        }
    }

    protected void printImpl(SuperClass superClass) {
        ...
    }

    protected void printImpl(SubClass subClass) {
       ...
    }
}

AppSubClass would then only override printImpl (one or both overloads).

EDIT: As noted in comments, an alternative is to use the Visitor pattern. It’s not a pattern I’m terribly fond of, but if you can modify SuperClass and SubClass to know about AppSuperClass (or an interface it implements) it could work for you.