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1)Declarations and Access Control

Objective 4

State the legal return types for any method given the declarations of all related methods in this or parent classes.

Note on this objective

This seems to be a rather obscurely phrased objective. It appears to be asking you to understand the difference between overloading and overriding.

To appreciate this objective you need a basic understanding of overloading and overriding of methods. This is covered in

Section 6: Overloading Overriding Runtime Type and Object Orientation

Methods in the same class

By related methods I assume that the objective means a method with the same name. If two or more methods in the same class have the same name, the method is said to be overloaded. You can have two methods in a class with the same name but they must have different parameter types and order.

It is the parameter order and types that distinguish between any two versions of overloaded method. The return type does not contribute towards distinguishing between methods.

The following code will generate an error at compile time, the compiler sees amethod as an attempt to define the same method twice. It causes an error that will say something like

method  redefined with different return type: void amethod(int)
was int amethod(int)

class Same{
public static void main(String argv[]){
       Over o = new Over();
        int iBase=0;
        o.amethod(iBase);
        }
        //These two cause a compile time error
        public void amethod(int iOver){
                System.out.println("Over.amethod");
        }
       public int amethod(int iOver){
                System.out.println("Over int return method");
                return 0;
        }
}

The return data type does not contribute towards distinguishing between one method and another.

Methods in a sub class

You can overload a method in a sub class. All that it requires is that the new version has a different parameter order and type. The parameter names are not taken into account or the return type.

If you are going to override a method, ie completely replace its functionality in a sub class, the overriding version of the method must have exactly the same signature as the version in the base class it is replacing. This includes the return type. If you create a method in a sub class with the same name and signature but a different return type you will get the same error message as in the previous example. i.e.

method  redefined with different return type: void amethod(int) 
was int amethod(int)

The compiler sees it as a faulty attempt to overload the method rather than override it.

Questions

Question 1)

Given the following class definition

public class Upton{
public static void main(String argv[]){
        }
        public void amethod(int i){}
        //Here
}

Which of the following would be legal to place after the comment //Here ?
1) public int amethod(int z){}
2) public int amethod(int i,int j){return 99;}
3) protected void amethod(long l){ }

4) private void anothermethod(){}

Question 2)

Given the following class definition

class Base{
        public void amethod(){
        System.out.println("Base");
        }
}
public class Hay extends Base{
public static void main(String argv[]){
        Hay h = new Hay();
        h.amethod();
        }
}

Which of the following methods in class Hay would compile and cause the program to print
out the string "Hay"

1) public int amethod(){ System.out.println("Hay");}
2) public void amethod(long l){ System.out.println("Hay");}
3) public void amethod(){ System.out.println("Hay");}
4) public void amethod(void){ System.out.println("Hay");}

Answers

Answer to Question 1)

2) public int amethod(int i, int j) {return 99;}
3) protected void amethod (long l){}
4) private void anothermethod(){}

Option 1 will not compile on two counts. One is the obvious one that it claims to return an integer. The other is that it is effectivly an attempt to redefine a method within the same class. The change of name of the parameter from i to z has no effect and a method cannot be overriden within the same class.

Answer to Question 2)

3) public void amethod(){ System.out.println("Hay");}

Option 3 represents an overriding of the method in the base class, so any zero parameter calls will invoke this version.

Option 1 wll return an error indicating you are attempting to redefine a method with a different return type. Although option 2 will compile the call to amethod() invoke the Base class method and the string "Base" will be output.Option 4 was designed to catch out those with a head full of C/C++, there is no such thing as a void method parameter in Java.

Last updated
10 Nov 2000
copyright © Marcus Green 2000
most recent copy at http://www.jchq.net