JavaYDL11

Chapter 11 Inheritance and
Polymorphism

Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
1

Motivations
Suppose you will define classes to model circles,
rectangles, and triangles. These classes have many
common features. What is the best way to design
these classes so to avoid redundancy? The answer
is to use inheritance.

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Objectives
 To define a subclass from a superclass through inheritance (§11.2).
 To invoke the superclass’s constructors and methods using the super keyword
(§11.3).
 To override instance methods in the subclass (§11.4).
 To distinguish differences between overriding and overloading (§11.5).
 To explore the toString() method in the Object class (§11.6).
 To discover polymorphism and dynamic binding (§§11.7–11.8).
 To describe casting and explain why explicit downcasting is necessary (§11.9).
 To explore the equals method in the Object class (§11.10).
 To store, retrieve, and manipulate objects in an ArrayList (§11.11).
 To implement a Stack class using ArrayList (§11.12).
 To enable data and methods in a superclass accessible from subclasses using the
protected visibility modifier (§11.13).
 To prevent class extending and method overriding using the final modifier
(§11.14).

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Superclasses and Subclasses
GeometricObject
-color: String The color of the object (default: white).
-filled: boolean Indicates whether the object is filled with a color(default: false).
-dateCreated: java.util.Date The date when the object was created.
+GeometricObject() Creates a GeometricObject.
+GeometricObject(color: String, Creates a GeometricObject with the specified color and fill
ed
filled: boolean) values.

GeometricObject
+getColor(): String Returns the color.
+setColor(color: String): void Sets a new color.
+isFilled(): boolean Returns the filled property.
+setFilled(filled: boolean): void Sets a new filled property.
+getDateCreated(): java.util.Date Returns the dateCreated. CircleFromSimpleGeometricObject
+toString(): String Returns a string representation of this object.

RectangleFromSimpleGeometricObject
Circle Rectangle
-radius: double -width: double
+Circle() -height: double
+Circle(radius: double) +Rectangle()
+Circle(radius: double, color String,
: +Rectangle(width: double, height: double)
filled: boolean) +Rectangle(width: double, height: double
+getRadius(): double
+setRadius(radius: double): void
color: String, filled: boolean)
+getWidth(): double TestCircleRectangle
+getArea(): double +setWidth(width: double): void
+getPerimeter(): double +getHeight(): double
+getDiameter(): double
+printCircle(): void
+setHeight(height: double): void
+getArea(): double Run
+getPerimeter(): double

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4

Are superclass’s Constructor
Inherited?
No. They are not inherited.
They are invoked explicitly or implicitly.
Explicitly using the super keyword.
A constructor is used to construct an instance of a class.
Unlike properties and methods, a superclass's
constructors are not inherited in the subclass. They can
only be invoked from the subclasses' constructors, using
the keyword super. If the keyword super is not explicitly
used, the superclass's no-arg constructor is
automatically invoked.
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Superclass’s Constructor Is Always Invoked
A constructor may invoke an overloaded constructor or its
superclass’s constructor. If none of them is invoked
explicitly, the compiler puts super() as the first statement
in the constructor. For example,

public A() { public A() {
is equivalent to
} super();
}

public A(double d) { public A(double d) {
// some statements is equivalent to
super();
} // some statements
}

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Using the Keyword super
The keyword super refers to the superclass
of the class in which super appears. This
keyword can be used in two ways:
 To call a superclass constructor
 To call a superclass method

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CAUTION

You must use the keyword super to call the
superclass constructor. Invoking a
superclass constructor’s name in a subclass
causes a syntax error. Java requires that the
statement that uses the keyword super
appear first in the constructor.

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Constructor Chaining
Constructing an instance of a class invokes all the superclasses’ constructors
along the inheritance chain. This is called constructor chaining.
public class Faculty extends Employee {
public static void main(String[] args) {
new Faculty();
}

public Faculty() {
System.out.println("(4) Faculty's no-arg constructor is invoked");
}
}

class Employee extends Person {
public Employee() {
this("(2) Invoke Employee’s overloaded constructor");
System.out.println("(3) Employee's no-arg constructor is invoked");
}

public Employee(String s) {
System.out.println(s);
}
}

class Person {
public Person() {
System.out.println("(1) Person's no-arg constructor is invoked");
}
} Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
9

animation
Trace Execution
new Faculty(); 1. Start from the
} main method
public Faculty() {
}
}

public Employee() {
}

}
}

class Person {
public Person() {
}
}
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animation
Trace Execution
new Faculty(); 2. Invoke Faculty
} constructor
public Faculty() {
}
}

public Employee() {
}

}
}

class Person {
public Person() {
}
}
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animation
Trace Execution
new Faculty();
}

public Faculty() {
}
} 3. Invoke Employee’s no-
arg constructor
public Employee() {
}

}
}

class Person {
public Person() {
}
}
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12

animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}
4. Invoke Employee(String)
class Employee extends Person { constructor
public Employee() {
}

}
}

class Person {
public Person() {
}
}
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13

animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}

public Employee() {
}

}
} 5. Invoke Person() constructor
class Person {
public Person() {
}
}
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14

animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}

public Employee() {
}

}
}
6. Execute println
class Person {
public Person() {
}
}
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animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}

public Employee() {
}

}
}
7. Execute println
class Person {
public Person() {
}
}
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16

animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}

public Employee() {
}

}
}
8. Execute println
class Person {
public Person() {
}
}
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17

animation
Trace Execution
new Faculty();
}

public Faculty() {
}
}
9. Execute println
public Employee() {
}

}
}

class Person {
public Person() {
}
}
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18

Example on the Impact of a Superclass
without no-arg Constructor
Find out the errors in the program:
public class Apple extends Fruit {
}

class Fruit {
public Fruit(String name) {
System.out.println("Fruit's constructor is invoked");
}
}

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Defining a Subclass
A subclass inherits from a superclass. You can also:
 Add new properties
 Add new methods
 Override the methods of the superclass

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Calling Superclass Methods
You could rewrite the printCircle() method in the Circle class as
follows:

public void printCircle() {
System.out.println("The circle is created " +
super.getDateCreated() + " and the radius is " + radius);
}

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Overriding Methods in the Superclass
A subclass inherits methods from a superclass. Sometimes it is
necessary for the subclass to modify the implementation of a method
defined in the superclass. This is referred to as method overriding.

public class Circle extends GeometricObject {
// Other methods are omitted

/** Override the toString method defined in GeometricObject */
public String toString() {
return super.toString() + "nradius is " + radius;
}
}

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22

NOTE

An instance method can be overridden only
if it is accessible. Thus a private method
cannot be overridden, because it is not
accessible outside its own class. If a method
defined in a subclass is private in its
superclass, the two methods are completely
unrelated.

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NOTE

Like an instance method, a static method
can be inherited. However, a static method
cannot be overridden. If a static method
defined in the superclass is redefined in a
subclass, the method defined in the
superclass is hidden.

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Overriding vs. Overloading
public class Test { public class Test {
public static void main(String[] args) { public static void main(String[] args) {
A a = new A(); A a = new A();
a.p(10); a.p(10);
a.p(10.0); a.p(10.0);
} }
} }

class B { class B {
public void p(double i) { public void p(double i) {
System.out.println(i * 2); System.out.println(i * 2);
} }
} }

class A extends B { class A extends B {
// This method overrides the method in B // This method overloads the method in B
public void p(double i) { public void p(int i) {
System.out.println(i); System.out.println(i);
} }
} }

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The Object Class and Its Methods
Every class in Java is descended from the
java.lang.Object class. If no inheritance is
specified when a class is defined, the
superclass of the class is Object.

public class Circle { public class Circle extends Object {
... Equivalent
...
} }

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The toString() method in Object
The toString() method returns a string representation of the
object. The default implementation returns a string consisting
of a class name of which the object is an instance, the at sign
(@), and a number representing this object.

Loan loan = new Loan();
System.out.println(loan.toString());

The code displays something like Loan@15037e5 . This
message is not very helpful or informative. Usually you should
override the toString method so that it returns a digestible string
representation of the object.

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Polymorphism, Dynamic Binding and Generic Programming
public class PolymorphismDemo {
m(new GraduateStudent());
Method m takes a parameter
m(new Student()); of the Object type. You can
m(new Person());
m(new Object()); invoke it with any object.
}

public static void m(Object x) { An object of a subtype can be used wherever its
System.out.println(x.toString());
} supertype value is required. This feature is
}
known as polymorphism.
class GraduateStudent extends Student {
}

class Student extends Person { When the method m(Object x) is executed, the
return "Student";
argument x’s toString method is invoked. x
} may be an instance of GraduateStudent,
}
Student, Person, or Object. Classes
class Person extends Object {
GraduateStudent, Student, Person, and Object
return "Person"; have their own implementation of the toString
}
} method. Which implementation is used will be
determined dynamically by the Java Virtual
DynamicBindingDemo Machine at runtime. This capability is known
as dynamic binding.
Run
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Dynamic Binding
Dynamic binding works as follows: Suppose an object o is an
instance of classes C1, C2, ..., Cn-1, and Cn, where C1 is a subclass
of C2, C2 is a subclass of C3, ..., and Cn-1 is a subclass of Cn. That
is, Cn is the most general class, and C1 is the most specific class.
In Java, Cn is the Object class. If o invokes a method p, the JVM
searches the implementation for the method p in C1, C2, ..., Cn-1
and Cn, in this order, until it is found. Once an implementation is
found, the search stops and the first-found implementation is
invoked.

Cn Cn-1 ..... C2 C1

Since o is an instance of C1, o is also an
Object instance of C2, C3, …, Cn-1, and Cn
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Method Matching vs. Binding
Matching a method signature and binding a method
implementation are two issues. The compiler finds a
matching method according to parameter type, number
of parameters, and order of the parameters at
compilation time. A method may be implemented in
several subclasses. The Java Virtual Machine
dynamically binds the implementation of the method at
runtime.

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Generic Programming
public class PolymorphismDemo { Polymorphism allows methods to be used
m(new GraduateStudent()); generically for a wide range of object
m(new Student()); arguments. This is known as generic
m(new Person());
m(new Object()); programming. If a method’s parameter
} type is a superclass (e.g., Object), you
public static void m(Object x) { may pass an object to this method of any
}
System.out.println(x.toString()); of the parameter’s subclasses (e.g.,
} Student or String). When an object (e.g., a
class GraduateStudent extends Student {
Student object or a String object) is used
} in the method, the particular
class Student extends Person {
implementation of the method of the
public String toString() { object that is invoked (e.g., toString) is
return "Student"; determined dynamically.
}
}

class Person extends Object {
return "Person";
}
}

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Casting Objects
You have already used the casting operator to convert variables of
one primitive type to another. Casting can also be used to convert an
object of one class type to another within an inheritance hierarchy. In
the preceding section, the statement
m(new Student());

assigns the object new Student() to a parameter of the Object type.
This statement is equivalent to:

Object o = new Student(); // Implicit casting
m(o);

The statement Object o = new Student(), known as
implicit casting, is legal because an instance of
Student is automatically an instance of Object.
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Why Casting Is Necessary?
Suppose you want to assign the object reference o to a variable of the
Student type using the following statement:

Student b = o;

A compile error would occur. Why does the statement Object o =
new Student() work and the statement Student b = o doesn’t? This is
because a Student object is always an instance of Object, but an
Object is not necessarily an instance of Student. Even though you can
see that o is really a Student object, the compiler is not so clever to
know it. To tell the compiler that o is a Student object, use an explicit
casting. The syntax is similar to the one used for casting among
primitive data types. Enclose the target object type in parentheses and
place it before the object to be cast, as follows:

Student b = (Student)o; // Explicit casting
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Casting from
Superclass to Subclass
Explicit casting must be used when casting an
object from a superclass to a subclass. This type
of casting may not always succeed.
Apple x = (Apple)fruit;

Orange x = (Orange)fruit;

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The instanceof Operator
Use the instanceof operator to test whether an object is an instance
of a class:

Object myObject = new Circle();
... // Some lines of code
/** Perform casting if myObject is an instance of
Circle */
if (myObject instanceof Circle) {
System.out.println("The circle diameter is " +
((Circle)myObject).getDiameter());
...
}

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TIP
To help understand casting, you may also
consider the analogy of fruit, apple, and
orange with the Fruit class as the superclass
for Apple and Orange. An apple is a fruit, so
you can always safely assign an instance of
Apple to a variable for Fruit. However, a
fruit is not necessarily an apple, so you have
to use explicit casting to assign an instance
of Fruit to a variable of Apple.

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Example: Demonstrating
Polymorphism and Casting
This example creates two geometric objects: a
circle, and a rectangle, invokes the
displayGeometricObject method to display the
objects. The displayGeometricObject displays
the area and diameter if the object is a circle, and
displays area if the object is a rectangle.

CastingDemo Run
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The equals Method
The equals() method compares the
contents of two objects. The default implementation of the
equals method in the Object class is as follows:
public boolean equals(Object obj) {
return (this == obj);
}
public boolean equals(Object o) {
For example, the if (o instanceof Circle) {
equals method is return radius == ((Circle)o).radius;
overridden in }
the Circle else
return false;
class. }

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NOTE
The == comparison operator is used for
comparing two primitive data type values or for
determining whether two objects have the same
references. The equals method is intended to
test whether two objects have the same
contents, provided that the method is modified
in the defining class of the objects. The ==
operator is stronger than the equals method, in
that the == operator checks whether the two
reference variables refer to the same object.
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The ArrayList Class
You can create an array to store objects. But the array’s size is fixed
once the array is created. Java provides the ArrayList class that can
be used to store an unlimited number of objects.

java.util.ArrayList<E>
+ArrayList() Creates an empty list.
+add(o: E) : void Appends a new element o at the end of this list.
+add(index: int, o: E) : void Adds a new element o at the specified index in this list.
+clear(): void Removes all the elements from this list.
+contains(o: Object): boolean Returns true if this list contains the element o.
+get(index: int) : E Returns the element from this list at the specified index.
+indexOf(o: Object) : int Returns the index of the first matching element in this list.
+isEmpty(): boolean Returns true if this list contains no elements.
+lastIndexOf(o: Object) : int Returns the index of the last matching element in this list.
+remove(o: Object): boolean Removes the element o from this list.
+size(): int Returns the number of elements in this list.
+remove(index: int) : boolean Removes the element at the specified index.
+set(index: int, o: E) : E Sets the element at the specified index.

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Generic Type
ArrayList is known as a generic class with a generic
type E. You can specify a concrete type to replace E
when creating an ArrayList. For example, the
following statement creates an ArrayList and assigns
its reference to variable cities. This ArrayList object
can be used to store strings.
ArrayList<String> cities = new ArrayList<String>();

TestArrayList Run
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Differences and Similarities between
Arrays and ArrayList
Operation Array ArrayList

Creating an array/ArrayList String[] a = new String[10] ArrayList<String> list = new ArrayList<>();
Accessing an element a[index] list.get(index);
Updating an element a[index] = "London"; list.set(index, "London");
Returning size a.length list.size();
Adding a new element list.add("London");
Inserting a new element list.add(index, "London");
Removing an element list.remove(index);
Removing an element list.remove(Object);
Removing all elements list.clear();

DistinctNumbers Run
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The MyStack Classes
A stack to hold objects.

MyStack

MyStack
-list: ArrayList A list to store elements.
+isEmpty(): boolean Returns true if this stack is empty.
+getSize(): int Returns the number of elements in this stack.
+peek(): Object Returns the top element in this stack.
+pop(): Object Returns and removes the top element in this stack.
+push(o: Object): void Adds a new element to the top of this stack.
+search(o: Object): int Returns the position of the first element in the stack from
the top that matches the specified element.

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The protected Modifier
 The protected modifier can be applied on data
and methods in a class. A protected data or a
protected method in a public class can be accessed
by any class in the same package or its subclasses,
even if the subclasses are in a different package.
 private, default, protected, public
Visibility increases

private, none (if no modifier is used), protected, public

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Accessibility Summary

Modifier Accessed Accessed Accessed Accessed
on members from the from the from a from a different
in a class same class same package subclass package

public

protected -

default - -

private - - -

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Visibility Modifiers
package p1;
public class C1 { public class C2 {
public int x; C1 o = new C1();
protected int y; can access o.x;
int z; can access o.y;
private int u; can access o.z;
cannot access o.u;
protected void m() {
} can invoke o.m();
} }

package p2;

public class C3 public class C4 public class C5 {
extends C1 { extends C1 { C1 o = new C1();
can access x; can access x; can access o.x;
can access y; can access y; cannot access o.y;
can access z; cannot access z; cannot access o.z;
cannot access u; cannot access u; cannot access o.u;
can invoke m(); can invoke m(); cannot invoke o.m();
} } }

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A Subclass Cannot Weaken the Accessibility

A subclass may override a protected
method in its superclass and change its
visibility to public. However, a subclass
cannot weaken the accessibility of a
method defined in the superclass. For
example, if a method is defined as public
in the superclass, it must be defined as
public in the subclass.

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NOTE
The modifiers are used on classes and
class members (data and methods), except
that the final modifier can also be used on
local variables in a method. A final local
variable is a constant inside a method.

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The final Modifier
 The final class cannot be extended:
final class Math {
...
}

 The final variable is a constant:
final static double PI = 3.14159;

 The final method cannot be
overridden by its subclasses.

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JavaYDL11

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