CS100J Lecture 26

1
CS100J Lecture 26

• Previous Lecture
• Application of inheritance
• Higher-order" methods
• Abstract classes
• Visibility modifier protected
• This Lecture
• Interfaces
• Comparable
• Reflection
• super
• Reading
• Lewis Loftus, Section 5.5
• Savitch, Appendix 7

2
Interfaces

• Interface - Like an abstract class
• A collection of abstract methods that must be
  defined by any class that implements the
  interface.
• Primitive types do not implement interfaces
• Syntax of interfaces
• interface interface-name
• list-of-method-signatures
• Extended syntax of classes
• class class-name1
• extends class-name2
• implements list-of-interface-names
• . . .

3
Interface example Comparable

• // Linearly ordered objects.
• interface Comparable
• / Compare this object with obj
• - if this before obj, return negative,
• - if this equals obj, return 0,
• - if this after obj, return positive. /
• public int compareTo(Object obj)
• The following predefined classes all implement
  the Comparable interface
• Byte
• Character
• Double
• Float
• Integer
• Long
• Short
• String

4
An implementation of Comparable

• class Color implements Comparable
• private int c
• public Color(String s)
• if (s.equals("red")) c 0
• else if (s.equals("white")) c 1
• else if (s.equals("blue")) c 2
• else throw
• new RuntimeException("bad color")
• public String toString()
• if (c 0) return "red"
• else if (c 1) return "white"
• else return "blue"

5
Recall the sort method

• / Sort A into non-decreasing order. /
• static void sort( int A )
• int m A.length - 1
• for ( int k 0 k lt m k )
• / Given that A0..k-1 is finished,
• finish A0..k. /
• int minLoc // subscript of
• // smallest in Ak..m
• / Set minLoc so AminLoc is
• smallest in Ak..m. /
• minLoc k
• for (int ik1 i lt m i)
• if ( Ai lt AminLoc )
• minLoc i
• / Exchange Ak and AminLoc /

6
A polymorphic sort method

• / Sort A into non-decreasing order. /
• static void sort( Comparable A )
• int m A.length - 1
• for ( int k 0 k lt m k )
• / Given that A0..k-1 is finished,
• finish A0..k. /
• int minLoc // subscript of
• // smallest in Ak..m
• / Set minLoc so AminLoc is
• smallest in Ak..m. /
• minLoc k
• for (int ik1 i lt m i)
• if (Ai.compareTo(AminLoc)lt 0 )
• minLoc i
• / Exchange Ak and AminLoc /

7
Sorting an array of Colors

• / Print the elements of an arbitrary array of
  objects. /
• static void printArray(Object A)
• System.out.println("-------------")
• for (int i0 i lt A.length i)
• System.out.println(Ai " ")
• System.out.println("-------------")
• Color C
• new Color("blue"),
• new Color("white"),
• new Color("red")
• sort(C) printArray(C)
• Output

8
Sorting an array of Integers

• Integer B
• new Integer(3),
• new Integer(1),
• new Integer(2)
• sort(B) printArray(B)
• Output
• -------------
• 1
• 2
• 3
• -------------
• Note two kinds of polymorphism
• Inheritance hierarchy
• In printArray(B), Integer is a subclass of Object

9
Taxonomy of Polygons
10
A taxonomy of Quadrilaterals
Parallelopoid opposite sides parallel
Paralleloid opposite sides equal length
Rhomboid all sides equal length
Rectangle
11
A different taxonomy of Quadrilaterals
Parallelopoid opposite sides parallel
all angles equal
opposite angles equal
Rectangle
12
Interface example Shape

• // Objects with area and perimeter.
• interface Shape
• public double area()
• public double perimeter()

13
A class can implement multiple interfaces

• import java.lang.reflect.
• abstract class Parallelopoid
• implements Shape, Comparable
• // Compare two shapes based on areas.
• public int compareTo(Object obj)
• double d area() -((Shape)obj).area()
• return (d lt 0) ? -1 (d gt 0) ? 1 0
• public String toString()
• return getClass().getName() ""
• " area " (float)area()
• " perimeter " (float)perimeter()

14
class Rhomboid

• abstract class Rhomboid extends Parallelopoid
• double side
• public double perimeter()
• return 4side
• Rhomboid(double side)
• this.side side
• class Square extends Rhomboid
• Square(double side)
• super(side)
• public double area()
• return sideside
• class Rhombus extends Rhomboid

15
class Paralleloid

• abstract class Paralleloid extends Parallelopoid
• double side1, side2
• Paralleloid( double side1, double side2)
• this.side1 side1 this.side2 side2
• public double perimeter()
• return 2(side1 side2)
• class Rectangle extends Paralleloid
• Rectangle(double side1, double side2)
• super(side1, side2)
• public double area()
• return side1side2
• class Parallelogram extends Paralleloid

16
Sorting an array of Parallelopoids

• Parallelopoid A
• new Square(1.0),
• new Rhombus(1.0, Math.PI/4),
• new Rectangle(1.0, 2.0),
• new Parallelogram(1.0, 2.0, Math.PI/4),
• sort(A) printArray(A)
• Output
• -------------
• Rhombus area 0.70710677 perimeter 4.0
• Square area 1.0 perimeter 4.0
• Parallelogram area 1.4142135 perimeter 6.0
• Rectangle area 2.0 perimeter 6.0
• -------------