1. COP 3331 Object Oriented Analysis and Design Chapter 5 – Classes and Inheritance
Jean Muhammad

2. Slides by David A. Gaitros
Overview
Overloading
Extending Classes
Hiding Fields and Class Methods
Slides by David A. Gaitros

3. Slides by David A. Gaitros
Overloading
Overloading refers to the ability to allow different methods or constructors of a class to share the same name. The name is said to be overloaded with multiple implementations. Commonly know as the feature of polymorphism in the object oriented paradigm.
Slides by David A. Gaitros

4. Slides by David A. Gaitros
Overloading
The language can tell if two methods with the same name are different by the “signature”. The signature consists of the name of the method and the data type of the parameters. Note the return type, name of parameters, and final designations of parameters are not part of the signature.
Slides by David A. Gaitros

5. Slides by David A. Gaitros
Overloading
Rules for overloading: Two methods in the same class my have the same name provided that they:
Have different number of parameters, or
Same number of parameters but of different type.
Slides by David A. Gaitros

6. Slides by David A. Gaitros
Overloading
Example:
Public class Point{
protected double x,y;
public Point(){
x=0.0; y=0.0; }
public double distance (Point other){
double dx = this.x – other.x;
double dy = this.y – other.y;
return Math.sqrt(dx * dy + dy * dy);
Slides by David A. Gaitros

7. Slides by David A. Gaitros
Overloading
public double distance (double x, double y) {
double dx = this.x –x;
double dy = this.y – y;
return Math.sqrt(dx * dx + dy * dy); }
public double distance (int x, int y) {
double dx = this.x – (double) x;
double dy = this.y – (double) y;
return Math.sqrt (dx * dx + dy *dy);
public double distance () {
return Math.sqrt (x*x + y *y);
Slides by David A. Gaitros

8. Slides by David A. Gaitros
Overloading
Two situations to overload
When there is a general, nondiscriminative description of the functionality that fits all the overloaded methods.
When all the overloaded methods offer the same functionality, with some of them providing default arguments.
Slides by David A. Gaitros

9. Slides by David A. Gaitros
Extending Classes
Inheritance (example) – When class Hourly_employee inherits from, or extends the class Employee, then Hourly_employee is called a subclass of the class Employee. In Java, each class my only inherit properties from another class.
Slides by David A. Gaitros

10. Slides by David A. Gaitros
Extending Classes
Syntax of the extend clause: [ClassModifiers] class ClassName
[extends SuperClass]
[implements Interace1, Interface2, etc] {
ClassMemberDeclarations }
Slides by David A. Gaitros

11. Slides by David A. Gaitros
Extending Classes
import java.awt.Color;
public class ColoredPoint extends Point {
public Color color;
public ColorPoint (final double x, final double y,
final Color color) {
super (x,y);
this.color = color; }
public ColoredPoint (final double x, final double y) {
this (x,y, Color.black); // default color
public ColoredPoint () {
color = Color.black;
Slides by David A. Gaitros

12. Slides by David A. Gaitros
Extending Classes
Note in the constructor. The invocation of super (…) must be the first statement in the constructor of the extended class
Note: The second constructor of the ColoredPoint class supplies a default value for the color field.
Note: The third constructor has not explicit invocation using super or this is made.
This is also an example of overloading.
Slides by David A. Gaitros

13. Slides by David A. Gaitros
Extending Classes
Subtype
Subtypes are a specialized version of a specific type.
For instance, let us say that you are storing integers for test scores. Since your test scores do not go below 0 or above 100, you could have a specialized subtype of integers that only had a valid range of 0 to 100.
Slides by David A. Gaitros

14. Slides by David A. Gaitros
Extending Classes
Overriding
Overriding is different from overloading. In overriding, methods share the same name, signature, and return type in contrast to overloading.
class A {
public void m1() { … method .. }
public void m1( int i) { … method . .}
// Note class A contains two overaloaded methods.
Slides by David A. Gaitros

15. Slides by David A. Gaitros
Extending Classes
Now assumer we have the two classes:
class B {
public void m2() { } }
class C extends B {
public coid m2 () { . . }
B b = new B();
C c = new C();
b.m2(); // invoke the m2() in class B
c.m2(); // invoke the m2() in class C
Slides by David A. Gaitros

16. Hiding Fields and Class Methods
Hiding refers to the introduction of a field or a class method in a subclass that has the same name as a field or class method in a superclass.
Overriding and hiding are different concepts
Instance methods can only be overriden.
Class methods and fields can only be hiden.
Slides by David A. Gaitros

17. Hiding Fields and Class Methods
Example:
public class A {
int x;
void y() { … method . . }
static void z() { … method . .} }
public class B extends A {
float x; // hiding
void y () { … method . .} // overriding
static int z () { … method . . } // hiding
Slides by David A. Gaitros

18. Hiding Fields and Class Methods
There is a crucial distinction between overriding and hiding.
When an overridden method is invoked, the implementation that will be executed is chosen at run time.
When the hidden method or field is invoked or accessed, the copy that will be used is determined at compile time. Class methods and fields are statically bound.
Slides by David A. Gaitros