Chapter 11
Inheritance and Composition
Chapter Objectives
• Learn about inheritance
• Learn about subclasses and superclasses
• Explore how to override the methods of a
superclass
• Examine how constructors of superclasses
and subclasses work
Chapter Objectives
• Examine abstract classes
• Become aware of interfaces
• Learn about composition
Inheritance
• “is-a” relationship
• Single inheritance
– Subclass is derived from one existing class (superclass)
• Multiple inheritance
– Subclass is derived from more than one superclass
– Not supported by Java
– In Java, a class can only extend the definition of one
class
Inheritance
modifier(s) class ClassName extends ExistingClassName modifier(s)
{
memberList
}
Inheritance:
class Circle Derived from Shape
public class Circle extends Shape
{
.
.
.
}
Inheritance
• Private members of superclass
– private to superclass; they cannot be accessed
directly by subclass
• Subclass can override public methods of the
superclass; redefinition applies only to
object of subclass
Inheritance
• To write a method definition of a subclass
specify a call to the public method of the
superclass
– If subclass overrides public method of
superclass, specify call to public method of
superclass: super.MethodName(parameter list)
– If subclass does not override public method of
superclass, specify call to public method of
superclass: MethodName(parameter list)
UML Diagram: class Rectangle
UML Diagram: class Box
Defining Constructors of the
Subclass
• Call to constructor of superclass
– must be first statement
– specified by: super parameter list
Objects myRectangle and myBox
Rectangle myRectangle = new Rectangle(5, 3);
Box myBox = new Box(6, 5, 4);
Protected Members of a Class
The class Object
• Directly or indirectly becomes the
superclass of every class in Java
• public members of class Object can be
overridden/invoked by object of any class
type
The class Object:
Equivalent Definitions
public class Clock
{
//Declare instance variables as given in Chapter 8
//Definition of instance methods as given in Chapter 8
//...
}
The class Object 603 is, in fact, equivalent to the following:
public class Clock extends Object
{
//Declare instance variables as given in Chapter 8
//Definition of instance methods as given in Chapter 8
//...
}
Some Constructors and Methods
of the class Object
Hierarchy of Java Stream Classes
Objects of Superclasses and
Subclasses
• You cannot automatically make reference variable
of subclass type point to object of its superclass
• Dynamic binding: method executed determined at
execution time, not compile time
• Operator instanceof: determines whether reference
variable that points to object is of particular class
type
• ClassCastException thrown if class cast is not
allowed
Abstract Methods and Classes
• Abstract method: method that has only the
heading with no body
– must be declared abstract
• Abstract class: class that is declared with the
reserved word abstract in its heading
Abstract Class
• Can contain instance variables, constructors,
finalizer, abstract and nonabstract methods
• You cannot instantiate object of abstract class
type; can only declare reference variable
• You can instantiate an object of a subclass of an
abstract class, but only if the subclass gives
definitions of all abstract methods of the
superclass
Abstract Class Example
public abstract class AbstractClassExample
{
protected int x;
public void abstract print();
public void setX(int a)
{
x = a;
}
public AbstractClassExample()
{
x = 0;
}
}
Interfaces
• Definition: class that contains only abstract
methods and/or named constants
• How Java implements multiple inheritance
• To be able to handle a variety of events,
Java allows a class to implement more than
one interface
Some Interface Definitions
Composition
• Another way to relate two classes
• One or more members of a class are objects
of another class type
• “has-a” relation between classes
– E.g. “every person has a date of birth”
Composition Example
Programming Example: Grade
Report
• Components: student, course
• Operations on course
–
–
–
–
–
Set course information
Print course information
Show credit hours
Show course number
Show grade
Components Course and Student
Components Course and Student
Programming Example: Grade
Report
• Operations on student
–
–
–
–
–
–
Set student information
Print student information
Calculate number of credit hours taken
Calculate GPA
Calculate billing amount
Sort the courses according to the course number
Programming Example: Grade
Report
• Main algorithm
–
–
–
–
Declare variables
Open input file
Open output file
Get number of students registered and tuition
rate
– Load students’ data
– Print grade reports
UML diagram of class Student
Sample Output:
Grade Report Program
Sample Output:
After Clicking Next in Grade Report
Program
Chapter Summary
• Inheritance
–
–
–
–
–
Single and multiple
Rules
Uses
Superclasses/subclasses (objects)
Overriding/overloading methods
• The class Object
– Constructors
– Rules
Chapter Summary
•
•
•
•
•
Java Stream Classes
Abstract methods
Abstract classes
Interfaces
Composition