chapter 8.2
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TRANSCRIPT
USING CLASSES WITH POLYMORPHISM
Chapter 8.2:
Defining Classes with Inheritance Case Study:
Suppose we want implement a class roster that contains both undergraduate and graduate students.
Each student’s record will contain his or her name, three test scores, and the final course grade.
The formula for determining the course grade is different for graduate students than for undergraduate students.
Undergrads: pass if avg test score >= 70
Grads: pass if avg test score >= 80
Modeling Two Types of Students
There are two ways to design the classes to model undergraduate and graduate students.We can define two unrelated classes, one for
undergraduates and one for graduates.We can model the two kinds of students by
using classes that are related in an inheritance hierarchy.
Two classes are unrelated if they are not connected in an inheritance relationship.
Classes for the Class Roster For the Class Roster sample, we design three
classes: Student UndergraduateStudent GraduateStudent
The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects.
The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects.
Inheritance Hierarchy
Definition of GraduateStudent & UndergraduateStudent classes
class GraduateStudent extends Student {
//constructor not shown
public void computeCourseGrade() { int total = 0; total = test1 + test2 + test3; if (total / 3 >= 80) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }}
class UndergraduateStudent extends Student {
//Constructor not shown
public void computeCourseGrade() { int total = 0;
total = test1 + test2 + test3;
if (total / 3 >= 70) { courseGrade = "Pass"; } else { courseGrade = "No Pass"; } }
}
Declaring a Subclass A subclass inherits data and methods from the
superclass. In the subclass, you can also:Add new dataAdd new methodsOverride the methods of the superclass
○ Modify existing behaviour of parent
Overriding vs. Overloading
public class Test { public static void main(String[] args) { A a = new A(); a.p(10); } } class B { public void p(int i) { } } class A extends B { // This method overrides the method in B public void p(int i) { System.out.println(i); } }
public class Test { public static void main(String[] args) { A a = new A(); a.p(10); } } class B { public void p(int i) { } } class A extends B { // This method overloads the method in B public void p(double i) { System.out.println(i); } }
Inheritance Rules
1. The private members of the superclass are private to the superclass
2. The subclass can access the members of the superclass according to the accessibility rules
3. The subclass can include additional data and/or method members
Inheritance Rules (continued)4. The subclass can override, that is, redefine
the methods of the superclass The overriding method in subclass must have similar
Name Parameter list Return type
5. All members of the superclass are also members of the subclassSimilarly, the methods of the superclass (unless
overridden) are also the methods of the subclassRemember Rule 1 & 2 when accessing a member
of the superclass in the subclass
Inheritance Rules (continued)6. (Using the Keyword super)
The keyword super refers to the direct superclass of a subclass . This keyword can be used in two ways:
To call a superclass constructor
super(); //must be the first statement in subclass’s constructor
To call a superclass method super.methodname();
this is only used if the subclass overrides the superclass method
The Object Class is the Superclass of Every Java Class
INHERITANCE: (Accessibility Modifier) Sometimes , it is called visibility modifier Not all properties can be accessed by sub
class. Super class can control a data accessing
from subclass by giving the type of accessing to the members and methods.
A class can declare the data members or method as a public, private or protected.
If it is not declared, the data or method will be set to default type.
INHERITANCE: Member Accessibility
Accessibility criteria
Modifier Same Class Same Package
Subclass Universe
private Yes No No No
default Yes Yes No No
protected Yes Yes Yes No
public Yes Yes Yes Yes
INHERITANCE: Data Accessibility
Sub class B
public int bprotected int c
Super class
int apublic int b
protected int cprivate int d
Sub class A
int apublic int b
protected int c
Package B
Package A
Refer to the previous slide Super class has 2 subclasses : Subclass A and
Subclass B. Subclass A is defined in same package with
superclass, subclass B is defined outside the package.
There are 4 accessibility data types: public, protected, private and default.
Subclass A can access all properties of superclass except private.
But, subclass B can only access the properties outside the package which are public and protected.
Example: Visibility Modifiers
public class C1 { public int x; protected int y; int z; private int u; protected void m() { } }
public class C2 { C1 o = new C1(); can access o.x; can access o.y; can access o.z; cannot access o.u; can invoke o.m(); }
public class C3 extends C1 { can access x; can access y; can access z; cannot access u; can invoke m(); }
package p1;
public class C4 extends C1 { can access x; can access y; cannot access z; cannot access u; can invoke m(); }
package p2;
public class C5 { C1 o = new C1(); can access o.x; cannot access o.y; cannot access o.z; cannot access o.u; cannot invoke o.m(); }
What’s wrong with the code?How to fix it?
class ClassX{ private int m; public String toString() { return new String("(" + m + ")"); }}public class ClassY extends ClassX{ private int n; public String toString() { return new String("(" + m + " , " + n + ")"); }}
class TestAccesibility{ public static void main(String [] args) { ClassX x = new ClassX; ClassY y = new ClassY; System.out.println("x = " + x); System.out.println("y = " + y); }}
Inheritance and Constructors Unlike members of a superclass, constructors of a
superclass are not inherited by its subclasses. You must define a constructor for a class or use
the default constructor added by the compiler. The statement super();
calls the superclass’s constructor. super(); must be the first statement in the
subclass contructor.
A call to the constructor of the superclass must be in the first statement in the child constructor.
public Box(double l, double w, double h){ super(l,w); height = h;}
Rectangle myRectangle = new Rectangle(5, 3);Box myBox = new Box(6, 5, 4);
Superclass’s Constructor Is Always Invoked A subclass constructor may invoke its superclass’s
constructor. If none is invoked explicitly, the compiler puts super() as the first statement in the constructor. For example, the constructor of class A:
public A(double d) { // some statements
}
is equivalent to
public A(double d) { super(); // some statements
}
public A() {
}
is equivalent to
public A() { super();
}
Example on the Impact of a Superclass without no-arg Constructor Find out the error in the program:
class Fruit { public Fruit(String name) { System.out.println("Fruit constructor is invoked"); }}
public class Apple extends Fruit { public Apple(String name) { System.out.println(“Apple constructor is invoked"); }}