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The Essence of OOP Using Java, Polymorphism Based on Overloaded Methods

  • January 31, 2002
  • By Richard G. Baldwin
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Java Programming Notes #1608


Preface

This lesson is one of a series of lessons designed to teach you about the essence of Object-Oriented Programming (OOP) using Java.

The first lesson in the group was entitled The Essence of OOP Using Java, Objects, and Encapsulation.  That lesson, and each of the lessons following that one, has provided explanations of certain aspects of the essence of Object-Oriented Programming using Java.  The previous lesson was entitled The Essence of OOP using Java, Inheritance, Part 2.

Necessary and significant aspects

This miniseries will describe and discuss the necessary and significant aspects of OOP using Java.

If you have a general understanding of computer programming, you should be able to read and understand the lessons in this miniseries, even if you don't have a strong background in the Java programming language.

Viewing tip

You may find it useful to open another copy of this lesson in a separate browser window.  That will make it easier for you to scroll back and forth among the different listings while you are reading about them.

Supplementary material

I recommend that you also study the other lessons in my extensive collection of online Java tutorials.  You will find those lessons published at Gamelan.com.  However, as of the date of this writing, Gamelan doesn't maintain a consolidated index of my Java tutorial lessons, and sometimes they are difficult to locate there.  You will find a consolidated index at Baldwin's Java Programming Tutorials.

Preview

Previous lessons introduced overloading and overriding methods.  This lesson concentrates on the use of method overloading to achieve compile-time polymorphism.

Every class in Java is a direct or indirect subclass of the class named Object.  Methods defined in the class named Object are inherited into all other classes.  They may be overridden to make their behavior more appropriate to objects instantiated from the new class.

Overloaded methods have the same name and different formal argument lists.  They may or may not have the same return type.

Polymorphism manifests itself in Java in the form of multiple methods having the same name.  This lesson concentrates on method overloading, sometimes referred to as compile-time polymorphism.  Subsequent lessons concentrate on method overriding, sometimes referred to as runtime polymorphism.

Overloaded methods may all be defined in the same class, or may be defined in different classes as long as those classes have a superclass-subclass relationship.

Discussion and Sample Code

Three concepts

In an earlier lesson, I explained that most books on OOP will tell you that in order to understand OOP, you must understand the following three concepts:

  • Encapsulation
  • Inheritance
  • Polymorphism
I agree with that assessment.

Encapsulation and inheritance

Previous lessons in this series have explained Encapsulation and Inheritance.  This lesson will tackle the somewhat more complex topic of Polymorphism.

Overloading and overriding methods

In the lessons on inheritance, you learned a little about overloading and overriding methods (you will learn more about these concepts as you progress through these lessons).  This lesson concentrates on the use of overloaded methods to achieve compile-time polymorphism.

Real-world scenarios

The sample programs that I used in the previous lessons in this series dealt with two kinds of car radios:

  • Plain car radios
  • Car radios having built-in tape players
I couched those programs in a real-world scenario in an attempt to convince you that encapsulation and inheritance really do have a place in the real world.

Programs were fairly long

However, even though those programs were simple in concept, they were relatively long.  This made them somewhat difficult to explain due simply to the amount of code involved.

Keep it short and simple

At this point, I am going to back away from real-world scenarios and begin using sample programs that are as short and as simple as I know how to make them, while still illustrating the important points regarding polymorphism.  My objective is make the polymorphic concepts as clear as possible without having those concepts clouded by other programming issues.

I will simply ask you to trust me when I tell you that polymorphism has enormous application in the real world.

A little more on inheritance

There is another aspect of inheritance that I didn't explain in the previous lessons in this series.

Every class extends some other class

Every class in Java extends some other class.  If you don't explicitly specify the class that your new class extends, it will automatically extend the class named Object.

A class hierarchy

Thus, all classes in Java exist in a class hierarchy where the class named Object forms the root of the hierarchy.

Some classes extend Object directly, while other classes are subclasses of Object further down the hierarchy.

Methods in the Object class

The class named Object defines default versions of the following methods:

  • clone()
  • equals(Object obj)
  • finalize()
  • getClass()
  • hashCode()
  • notify()
  • notifyAll()
  • toString()
  • wait()
  • wait(long timeout)
  • wait(long timeout, int nanos)
As you can see, this list includes three overloaded versions of the method named wait (same name, different formal argument lists).

Every class inherits these methods

Because every class is either a direct or indirect subclass of Object, every class in Java, (including new classes that you define), inherit these eleven methods.

To be overridden ...

Generally speaking, many of these eleven methods are intended to be overridden for various purposes.  However, some of them, such as getClass, notify, and wait, are intended to be used directly without overriding.

What is polymorphism?

The meaning of the word polymorphism is something like one name, many forms.

How does Java implement polymorphism?

Polymorphism manifests itself in Java in the form of multiple methods having the same name.

In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).

In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

Three distinct forms of polymorphism

From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:

  • Method overloading
  • Method overriding through inheritance
  • Method overriding through the Java interface
Method overloading

I will begin the discussion of polymorphism with method overloading, which is the simpler of the three.  I will cover method overloading in this lesson and will cover polymorphism based on overridden methods and interfaces in subsequent lessons.

Method overloading versus method overriding

Don't confuse method overloading with method overriding.

Java allows you to have two or more method definitions in the same scope with the same name, provided that they have different formal argument lists.

More specifically, here is what Roberts, Heller, and Ernest have to say about overloading methods in their excellent book entitled The Complete Java 2 Certification Study Guide:

"A valid overload differs in the number or type of its arguments.  Differences in argument names are not significant.  A different return type is permitted, but is not sufficient by itself to distinguish an overloading method."
Similarly, here is what they have to say about method overriding:
"A valid override has identical argument types and order, identical return type, and is not less accessible than the original method.  The overriding method must not throw any checked exceptions that were not declared for the original method."
You should read these two descriptions carefully and make certain that you recognize the differences.

Compile-time polymorphism

Some authors refer to method overloading as a form of compile-time polymorphism, as distinguished from run-time polymorphism.

This distinction comes from the fact that, for each method invocation, the compiler determines which method (from a group of overloaded methods) will be executed, and this decision is made when the program is compiled. (In contrast, I will tell you later that the determination of which overridden method to execute isn't made until runtime.)

Selection based on the argument list

In practice, the compiler simply examines the types, number, and order of the parameters being passed in a method invocation, and selects the overloaded method having a matching formal argument list.

A sample program

I will discuss a sample program named Poly01 to illustrate method overloading.  A complete listing of the program can be viewed in Listing 4 near the end of the lesson.

Within the class and the hierarchy

Method overloading can occur both within a class definition, and vertically within the class inheritance hierarchy.  (In other words, an overloaded method can be inherited into a class that defines other overloaded versions of the method.)  The program named Poly01 illustrates both aspects of method overloading.

Class B extends class A, which extends Object

Upon examination of the program, you will see that the class named A extends the class named Object.  You will also see that the class named B extends the class named A.

The class named Poly01 is a driver class whose main method exercises the methods defined in the classes named A and B.

Once again, this program is not intended to correspond to any particular real-world scenario.  Rather, it is a very simple program designed specifically to illustrate method overloading.

Will discuss in fragments

As is my usual approach, I will discuss this program in fragments.

The code in Listing 1 defines the class named A, which explicitly extends Object.
 

class A extends Object{
  public void m(){
    System.out.println("m()");
  }//end method m()
}//end class A

Listing 1

Redundant code

Recall that explicitly extending Object is not required (but it also doesn't hurt anything).

By default, the class named A would extend the class named Object automatically, unless the class named A explicitly extends some other class.

The method named m()

The code in Listing 1 defines a method named m().  Note that this version of the method has an empty argument list (it doesn't receive any parameters when it is executed).  The behavior of the method is simply to display a message indicating that it has been invoked.

The class named B

Listing 2 contains the definition for the class named B.  The class named B extends the class named A, and inherits the method named m defined in the class named A.
 

class B extends A{
  public void m(int x){
    System.out.println("m(int x)");
  }//end method m(int x)
  //---------------------------------//
  
  public void m(String y){
    System.out.println("m(String y)");
  }//end method m(String y)
}//end class B

Listing 2

Overloaded methods

In addition to the inherited method named m, the class named B defines two overloaded versions of the method named m:

  • m(int x)
  • m(String y)
(Note that each of these two versions of the method receives a single parameter, and the type of the parameter is different in each case.)

As with the version of the method having the same name defined in the class named A, the behavior of each of these two methods is simply to display a message indicating that it has been invoked.

The driver class

Listing 3 contains the definition of the driver class named Poly01.
 

public class Poly01{
  public static void main(
                        String[] args){
    B var = new B();
    var.m();
    var.m(3);
    var.m("String");
  }//end main
}//end class Poly01

Listing 3

Invoke all three overloaded methods

The code in the main method

  • Instantiates a new object of the class named B
  • Successively invokes each of the three overloaded versions of the method named m on the reference to that object.
One version is inherited

Note that the overloaded version of the method named m, defined in the class named A, is inherited into the class named B.  Hence, it can be invoked on a reference to an object instantiated from the class named B.

Two versions defined in class B

The other two versions of the method named m are defined in the class named B.  Thus, they also can be invoked on a reference to an object instantiated from the class named B.

The output

As you would expect, the output produced by sending messages to the object asking it to execute each of the three overloaded versions of the method named m is:

m()
m(int x)
m(String y)

Note that the values of the parameters passed to the methods do not appear in the output.  Rather, the parameters are used solely to make it possible for the compiler to select the correct version of the overloaded method to execute in each case.

This output confirms that each overloaded version of the method is properly selected for execution based on the matching of method parameters to the formal argument list of each method.

Summary

Previous lessons introduced overloading and overriding methods.  This lesson concentrates on the use of method overloading to achieve compile-time polymorphism.

All classes in Java form a hierarchy with a class named Object at the root of the hierarchy.  Thus, every class in Java is a direct or indirect subclass of the class named Object.

If a new class doesn't explicitly extend some other class, it will, by default, automatically extend the class named Object.

The Object class defines default versions of eleven different methods.  These methods are inherited into all other classes, and may be overridden to make their behavior more appropriate to objects instantiated from the new class.

Overloaded methods have the same name and different formal argument lists.  They may or may not have the same return type.

Three of the eleven methods defined in the class named Object are overloaded.  The names of the three overloaded methods are wait.  One version takes no parameters.  A second version takes a single parameter of type long.  The third version takes two parameters, one of type long, and one of type int.

The word polymorphism means something like one name, many forms.  Polymorphism manifests itself in Java in the form of multiple methods having the same name.

Polymorphism manifests itself in three distinct forms in Java:

  • Method overloading
  • Method overriding through inheritance
  • Method overriding through the Java interface
This lesson concentrates on method overloading, sometimes referred to as compile-time polymorphism.  This form of polymorphism is distinguished by the fact that the compiler selects among a group of overloaded methods on the basis of the types and the number of parameters passed to the method when it is invoked.  The selection is made when the program is compiled (rather than being made later when the program is run).

Overloaded methods may all be defined in the same class, or may be defined in different classes as long as those classes have a superclass-subclass relationship in the class hierarchy.

The sample program in this lesson illustrates three overloaded versions of the same method name with two of the versions being defined in a single class, and the other version being defined in the superclass of that class.

What's Next?

The next lesson teaches you about assignment compatibility, type conversion, and casting for both primitive and reference types.  It also teaches you about the relationship between reference types, method invocations, and the location in the class hierarchy where a method is defined.

Complete Program Listing

A complete listing of the program is shown in Listing 4 below.
 
/*File Poly01.java
Copyright 2002, R.G.Baldwin

Program output is:
m()
m(int x)
m(String y)
**************************************/

class A extends Object{
  public void m(){
    System.out.println("m()");
  }//end method m()
}//end class A
//===================================//

class B extends A{
  public void m(int x){
    System.out.println("m(int x)");
  }//end method m(int x)
  //---------------------------------//
  
  public void m(String y){
    System.out.println("m(String y)");
  }//end method m(String y)
}//end class B
//===================================//

public class Poly01{
  public static void main(
                        String[] args){
    B var = new B();
    var.m();
    var.m(3);
    var.m("String");
  }//end main
}//end class Poly01
//===================================//

Listing 4

About the author

Richard Baldwin is a college professor (at Austin Community College in Austin, TX) and private consultant whose primary focus is a combination of Java and XML. In addition to the many platform-independent benefits of Java applications, he believes that a combination of Java and XML will become the primary driving force in the delivery of structured information on the Web.

Richard has participated in numerous consulting projects involving Java, XML, or a combination of the two.  He frequently provides onsite Java and/or XML training at the high-tech companies located in and around Austin, Texas.  He is the author of Baldwin's Java Programming Tutorials, which has gained a worldwide following among experienced and aspiring Java programmers. He has also published articles on Java Programming in Java Pro magazine.

Richard holds an MSEE degree from Southern Methodist University and has many years of experience in the application of computer technology to real-world problems.

baldwin.richard@iname.com






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