1.00/1.001
Recitation 06
Abstract Classes/Methods and Interfaces
March 19
th
& 20
th
2012
1
Topics
• Abstract Classes (extends)
• Interfaces (implements)
• Polymorphism
• Problem Set 5
2
Abstract Classes: Content
• Some data members, like any class
• Some methods implemented (concrete)
• Some methods declared, but
unimplemented (abstract)
– We generally know what the method does
– How the method performs may be different
for different objects
3
Abstract Classes: Coding
• Abstract classes cannot be instantiated.
– Instantiate (v.) – use the “new” keyword to create
a new Object (or instance of a class)
– Some methods remain unimplemented.
• Subclasses must implement all abstract
methods, or must also be abstract classes.
• Why make a method abstract?
– Provide some default behaviors in concrete
methods
– Programmer is FORCED to implement methods in
a subclass before any object can be instantiated.
4
abstract Keyword
public abstract class MyClass {
// data members
private int myDataMember;
public MyClass (int md){
// concrete methods have‘bodies’ or definitions
myDataMember = md;
}
public int getData(){
// concrete method
return myDataMember;
}
public abstract int calc(int factor);
// abstract methods omit the „body‟
}
ˆ
ˆ ˆ
5
extends Keyword
public class AnotherClass extends MyClass{
public AnotherClass (int md){
// call constructor from “parent” or super class
super(md);
}
// implement all abstract methods
public int calc(int factor){
return factor * factor;
}
}
ˆ
6
Abstract Classes: Exercise 1 p.1
1) Write an abstract class Shape
– Data members: numSides
– Constructor: initialize numSides
– Concrete method: get method for numSides
– Abstract methods: getArea(), getPerimeter()
2) Write a concrete subclass Rectangle
– Data members: width, height
3) Write a concrete subclass RtTriangle
– Data members: width, height
4) In another class, write a main method to
define a Rectangle and a Triangle.
7
Solution: Shape
1 public abstract class Shape
2 {
3 private int numSides;
4
5 public Shape( int newSides)
6 {numSides = newSides;}
7
8 public int getNumSides()
9 {return numSides;}
10
11 public abstract double getArea();
12 public abstract double getPerimeter();
13 }
8
1) Write an abstract class Shape
– Data members: numSides
– Constructor: initialize numSides
– Concrete method: get method for numSides
– Abstract methods: getArea(), getPerimeter()
2) Write a concrete subclass Rectangle
– Data members: width, height
3) Write a concrete subclass RtTriangle
– Data members: width, height
4) In another class, write a main method to
define a Rectangle and a Triangle.
Abstract Classes: Exercise 1 p.2
9
Abstract Classes: Exercise p.3
1) Write an abstract class Shape
– Data members: numSides
– Constructor: initialize numSides
– Concrete method: get method for numSides
– Abstract methods: getArea(), getPerimeter()
2) Write a concrete subclass Rectangle
– Data members: width, height
3) Write a concrete subclass RtTriangle
– Data members: width, height
4) In another class, write a main method to
define a Rectangle and a Triangle.
10
Abstract Classes: Exercise p.4
1) Write an abstract class Shape
– Data members: numSides
– Constructor: initialize numSides
– Concrete method: get method for numSides
– Abstract methods: getArea(), getPerimeter()
2) Write a concrete subclass Rectangle
– Data members: width, height
3) Write a concrete subclass RtTriangle
– Data members: width, height
4) In another class, write a main method to
define a Rectangle and a Triangle.
11
Interfaces
• “Its like a checklist”: Class that implements an
interface must implement/define all methods
declared in the interface.
• A set of related method declarations.
• All method declarations omit the body.
• Constants may be defined.
• Why use interfaces?
– Define a set of behaviors
– Allow “multiple inheritance” by implementing multiple
interfaces
12
Abstract Classes vs. Interfaces
• Abstract Classes have
– Static and instance data
members
– Concrete and/or abstract
methods
– Single inheritance
(via extends)
– Constructor
• Interfaces have
– Static final data members
(constant)
– All methods abstract
– “Multiple Inheritance”
(via implements)
– No constructor
13
Remember Abstract Class Shape
and Subclass Rectangle?
public abstract class Shape {
private int numSides;
public Shape(int numSides){
this.numSides =
numSides;
}
public double getNumSides()
{
return numSides; }
public abstract double
getArea();
public abstract double
getPerimeter();
}
public class Rectangle extends
Shape {
private double height, width;
public Rectangle(double w,
double h) {
super(4);
this.height = h;
this.width = w;
}
public double getArea() {
return height * width;
}
public double getPerimeter() {
return 2 * (height + width);
}
}
14
Interface: Exercise 2 p.1
1) Write an interface Resizable
– Has a method resize(double x) that
resizes a Shape’s dimensions by factor x
2) Make Rectangle implement Resizable
3) Write a main method to:
- Define a Rectangle (width = 2, height = 3)
- Print the Rectangle‟s area & perimeter
- Resize the Rectangle by factor of 2
- Re-print the Rectangle‟s area & perimeter
15
Interface: Exercise 2 p.2
1) Write an interface Resizable
– Has a method resize(double x) that
resizes a Shape’s dimensions by factor x
2) Make Rectangle implement Resizable
3) Write a main method to:
- Define a Rectangle (width = 2, height = 3)
- Print the Rectangle‟s area & perimeter
- Resize the Rectangle by factor of 2
- Re-print the Rectangle‟s area & perimeter
16
Interface: Exercise 2 p.3
1) Write an interface Resizable
– Has a method resize(double x) that
resizes a Shape’s dimensions by factor x
2) Make Rectangle implement Resizable
3) Write a main method to:
- Define a Rectangle (width = 2, height = 3)
- Print the Rectangle‟s area & perimeter
- Resize the Rectangle by factor of 2
- Re-print the Rectangle‟s area & perimeter
17
instanceof Keyword
• The instanceof operator compares an object to a
specified type.
• You can use it to test if an object is:
- an instance of a class,
- an instance of a subclass,
- or an instance of a class that implements a
particular interface.
Source: http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html
18
public class Animal {
//body hidden
}
public class Cow extends Animal{
//body hidden
}
Here class Lion and Cow extends Animal
instanceof Example
public class Lion extends Animal{
//body hidden
public void roar(){//body hidden}
}
1 public static void main(String[] args) {
2 Animal[] zoo= new Animal[2];
3
4 zoo[0] = new Cow();
5 zoo[1] = new Lion();
6
7 for( int i =0; i<zoo.length; i++){
8 Animal a = zoo[i];
9 if( a instanceof Lion){ //test using instanceof keyword
10 System.out.println(”Animal " + i + “ is a Lion”);
11 Lion l = (Lion) a; //Cast the Object to a Lion
12 l.roar(); //Call a method in the Lion class
13 }
14 }
Prints: Animal 1 is a Lion
19
Polymorphism: Exercise
• Write a main method
– Create a Rectangle and a RtTriangle
– Add them to an ArrayList of *Shapes*
– Iterate through the Shapes in the ArrayList
• If the Shape is Resizable, resize it by a factor of 0.5
• Print out perimeter and area
20
• Write a program to model MBTA vehicles
• Three types of vehicles: Bus, Urban Rail,
and Commuter Rail
• Three kinds of Right-of-Way: Dedicated,
Shared, and Mixed (Hybrid)
Problem Set 5
• This homework tests your knowledge of inheritance. Your
solution must inherit as much as possible from the superclasses
and/or interfaces.
• Be sure to use at least one of EACH of the following in your
solution: abstract class, interface, abstract method, final method.
• Hint: The trick is to determine if the set of Route Types and
ROW Types should be Interfaces or Classes (Inheritance
structure)
- Which Types require “multiple inheritance”?
21
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