CIS3023: Programming Fundamentals for CIS Majors IISummer 2010

Ganesh Viswanathan

Interfaces (Part II)

Course Lecture Slides28 June 2010

“A picture is worth a thousand words. An interface is worth a thousand pictures.” -Ben Shneiderman

Interface

• Language construct that specifies functionality without any hint at implementation.

• An interface contains method specifications but no method implementations.

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Comparable Interface// Interface defined in java.lang package

public interface Comparable { public int compareTo(Object o);

}

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compareTo( )• Compares this object with the specified object for order. • Returns a negative integer, zero or a positive integer when this object is

less than, equal to or greater than the specified object.• Throws: ClassCastException if the specified object's type prevents it

from being compared to this Object.

Declaring Classes to Implement Comparable

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Rectangle -

GeometricObject -

«interface» java.lang.Comparable

+compareTo(o: Object): int

Notation: The interface name and the method names are italicized. The dashed lines and hollow triangles are used to point to the interface.

ComparableRectangle -

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public class ComparableRectangle extends Rectangle implements Comparable {

public ComparableRectangle(double width, double height){ super(width, height);

}

// Implement the compareTo method defined in Comparable public int compareTo(Object o) {

ComparableRectangle cr = (ComparableRectangle)o; if (getArea() > cr.getArea())

return 1;else if(getArea() < cr.getArea()) return -1;else return 0;}

}

Using Arrays.sort()If you have an array of objects of some class C and you

want to use

Arrays.sort(Object[] a)

or Arrays.sort(Object[] a, int fromIndex, int toIndex)

to sort this array, you must make class C implement Comparable interface

The above sort() methods use the compareTo() method to compare two array elements of reference type

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Arrays.Sort()

What if you want to sort this array of objects in order of different attributes of class C at different times?

Solution: Use

Arrays.sort(Object[] a, Comparator c)

Or Arrays.sort(Object[] a, int fromIndex, int toIndex, Comparator c)

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Comparator Interfacepublic interface Comparator {

public int compare(Object o1, Object o2)}

Returns: a negative integer, zero, or a positive integer when the first argument is less than, equal to, or greater than the second.

Throws: ClassCastException - if the arguments' types prevent them from being compared by this Comparator.

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SolutionCreate a class that implements the Comparator

interfaceCode the compare() method to define the ordering

between objects of this class

Pass an instance of this class to the Arrays.Sort() method

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Another scenarioWhat if

you are using a class C which does not implement the Comparable interface

You do not have access to C.java fileYou only have access to the C.class file

And you want to sort an arrays of objects of class C in order of some attribute of the class

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SolutionCreate a class that implements the Comparator

interfaceCode the compare() method to define the ordering

between objects of this class

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Another Advantage of using interfaces as data types

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public class Max { public static Comparable max(? o1, ? o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; }}

ComparableRectangle rectangle1 = new ComparableRectangle(4, 5);ComparableRectangle rectangle2 = new ComparableRectangle(3, 6);System.out.println(Max.max(rectangle1, rectangle2));

Another Advantage of using interfaces as data types

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public class Max { public static Comparable max(Comparable o1, Comparable o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; }}

ComparableRectangle rectangle1 = new ComparableRectangle(4, 5);ComparableRectangle rectangle2 = new ComparableRectangle(3, 6);System.out.println(Max.max(rectangle1, rectangle2));

Interfaces, cont

(a)

// Max.java: Find a maximum object public class Max { /** Return the maximum of two objects */ public static Comparable max(Comparable o1, Comparable o2) { if (o1.compareTo(o2) > 0) return o1; else return o2; } }

(b)

// Max.java: Find a maximum object public class Max { /** Return the maximum of two objects */ public static Object max(Object o1, Object o2) { if (((Comparable)o1).compareTo(o2) > 0) return o1; else return o2; } }

Max method in (a) is more robust.

Get more info!

• Tutorial: http://lkamal.blogspot.com/2008/07/java-sorting-comparator-vs-comparable.html

• Comparable vs Comparator:

http://grdurand.com/static/presentation_four/comparable.html

• Using comparable:http://onjava.com/pub/a/onjava/2003/03/12/java_comp.html

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The Cloneable Interface

It is a Marker Interface i.e.

it does not contain constants or methods.

Defined in the java.lang package as follows:

package java.lang;public interface Cloneable { }

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ExamplesMany classes (e.g., Date and Calendar) in the Java library implement Cloneable. Thus, the instances of these classes can be cloned. For example, the following code

displays

Calendar calendar = new GregorianCalendar(2003, 2, 1);Calendar calendarCopy = (Calendar)calendar.clone();

System.out.println("calendar == calendarCopy is " + (calendar == calendarCopy));

System.out.println("calendar.equals(calendarCopy) is " + calendar.equals(calendarCopy));

calendar == calendarCopy is falsecalendar.equals(calendarCopy) is true  

Implementing Cloneable Interface A class that implements the Cloneable interface must override the clone() method in the Object class.

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public class House implements Cloneable{ private int id; private double area;

public House(int id, double area) {this.id = id;this.area = area;

}

public double getId() { return id; } public double getArea() { return area; }

public Object clone() throws CloneNotSupportedException {

return super.clone(); }}

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public class House implements Cloneable{ private int id; private double area; private java.util.Date whenBuilt;

public House(int id, double area) {this.id = id;this.area = area;whenBuilt = new java.util.Date();

}

public double getId() { return id; } public double getArea() { return area; } public java.util.Date getWhenBuilt(){ return whenBuilt;

}

public Object clone() throws CloneNotSupportedException {

return super.clone(); }

}

Shallow vs. Deep Copy

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house1: House

id

area

whenBuilt

1

whenBuilt: Date date object contents house2 = house1.clone()

1750.50 reference

house2: House

id

area

whenBuilt

1

1750.50 reference

House house1 = new House(1, 1750.50);

House house2 = (House)house1.clone();

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public class House implements Cloneable{ private int id; private double area; private java.util.Date whenBuilt;

// other code

public Object clone() throws CloneNotSupportedException { House house = (House)super.clone();

house.whenBuilt = (java.util.Date)(whenBuilt.clone()); return house;

}}

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public class House implements Cloneable{ private int id; private double area; private String name; public House(int id, double area) {

this.id = id;this.area = area;whenBuilt = new java.util.Date();

}

public double getId() { return id; } public double getArea() { return area; } public java.util.Date getWhenBuilt(){ return whenBuilt;

}

public Object clone() throws CloneNotSupportedException {

return super.clone(); }}

Is the clone() method doing deep copy?

Can the clone() method be written as follows?

class House implements Cloneable { private int numberOfRooms; private int squareFeet;

//...

public Object clone() throws CloneNotSupportedException {

return new House(numberOfRooms, squareFeet); }

}

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What happens when the following code is executed?

public class MultiStoryHouse extends House { private int numberOfStories;

public MultiStoryHouse(int numRooms, int sqFootage, int numStories) {super(numRooms, sqFootage); numberofStories = numStories;

}// no clone method defined

}

public class Driver {public static void main(String[] args) {

MultiStoryHouse tsh = new MultiStoryHouse(6, 3000, 2); MultiStoryHouse other = (MultiStoryHouse)tsh.clone(); //Exception

}}

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