comp 110: introduction to programming

COMP 110:Introduction to Programming

Tyler JohnsonApr 13, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Program 5 Milestone 1 due Wednesday by 5pm


Questions?


Today in COMP 110

Brief Review

Finish Inheritance

Basic Exception Handling

Programming Demo


Review: Overriding Methods

Person has a jump method, so all subclasses have a jump method

Person

Athlete

HighJumper

Skydiver

ExtremeAthlete

XGamesSkater


Review: Overriding Methods

Each subclass has its own jump functionalitypublic class Person {

public void jump() { System.out.println("Whee!");}

}

public class Athlete extends Person {

public void jump() { System.out.println("I jump really well!");}

}


Review: Type Compatibilities

ExtremeAthlete is an AthleteXGamesSkater is a PersonPerson is not necessarily a Skydiver

Person p = new ExtremeAthlete();Legal

Athlete a = new Athlete();Legal

XGamesSkater xgs = new Person();Illegal


Polymorphism

“many forms”

Enables the substitution of one object for another as long as the objects have the same interface

8


Dynamic Binding

public static void jump3Times(Person p) {p.jump();p.jump();p.jump();

}

public static void main(String[] args) {

XGamesSkater xgs = new XGamesSkater();Athlete ath = new Athlete();jump3Times(xgs);jump3Times(ath);

}


Inheritance

Some final things on inheritanceImplementing the equals method


The Class Object

The Java class Object provides methods that are inherited by every class

For exampleequals, toString

These methods should be overridden with methods appropriate for the classes you create


The equals Method

Every class has a default .equals() methodInherited from the class ObjectReturns whether two objects of the class are “equal” in some senseDoes not necessarily do what you want

You decide what it means for two objects of a class you create to be considered equal by overriding the equals method

Perhaps books are equal if the names and page numbers are equalPerhaps only if the names are equalPut this logic inside .equals() method


The equals Method

Object has an equals methodSubclasses should override it

public boolean equals(Object obj) {return (this == obj);

}

What does this method do?Returns whether this has the same address as objThis is the default behavior for subclasses


The equals Method

First try:

public boolean equals(Student std) {return (this.id == std.id);

}

This is overloading, not overridingWe want to be able to test if two Objects are equal

Student

- id: int

+ getID(): int+ setID(int newID): void


The equals Method

Second try

public boolean equals(Object obj) {Student otherStudent = (Student) obj;return (this.id == otherStudent.id);

}

What does this method do?Typecasts the incoming Object to a StudentReturns whether this has the same id as otherStudent


The equals Method

public boolean equals(Object obj) { Student otherStudent = (Student) obj; return (this.id == otherStudent.id);}

Why do we need to typecast?Object does not have an id, obj.id would not compile

What’s the problem with this method?What if the object passed in is not actually a Student?The typecast will fail and we will get a runtime error


The instanceof Operator

We can test whether an object is of a certain class type:

if(obj instanceof Student) {System.out.println("obj is an instance of the class

Student");}

Syntax:object instanceof Class_Name

Use this operator in the equals method


The equals Method

Third trypublic boolean equals(Object obj) {

if ((obj != null) && (obj instanceof Student)) {

Student otherStudent = (Student)obj;

return (this.id == otherStudent.id); }

return false;}

null is a special constant that can be assigned to a variable of a class type – means that the variable does not refer to anything right now


Basic Exception Handling

Section 9.1 in text


Error Handling

Recall from Program 4

Parse a string of the formoperand1 operator operand2

For example "23.55 + 54.43"

We assumed the input was validWhat if it’s not?


Error Handling

Example of invalid input"g23.55 + 54.43"

Your programs would happily parse away and attempt to call

Double.parseDouble("g23.55");

Result?Your program crashes with a “NumberFormatException”


Exceptions

An exception is an object that signals the occurrence of an unusual (exceptional) event during program execution

Exception handling is a way of detecting and dealing with these unusual cases in principled manner i.e. without a run-time error or program crash


Example

Handling divide by zero exceptionsBasketballScores

int score = keyboard.nextInt();int scoreSum = 0;int numGames = 0;

while(score >= 0) {scoreSum += score;numGames++;score = keyboard.nextInt();

}

double average = scoreSum/numGames;

Possible ArithmeticException: / by zero!


Example

We could do thisint score = keyboard.nextInt();int scoreSum = 0;int numGames = 0;


}

double average = 0;

if(numGames > 0)average = scoreSum/numGames;


Example

Using Exception Handling (try/catch blocks)



}

double average = 0;

try {average = scoreSum/numGames;

}catch(ArithmeticException e) {

System.out.println(e.getMessage());System.out.println("Cannot compute average for 0 games");

}


Example

When numGames != 0int score = keyboard.nextInt();int scoreSum = 0;int numGames = 0;


}

double average = 0;




}


Example

When numGames == 0int score = keyboard.nextInt();int scoreSum = 0;int numGames = 0;


}

double average = 0;




}


The try Block

A try block contains the basic algorithm for when everything goes smoothly

Try blocks will possibly throw an exception

Syntaxtry {

Code_To_Try}

Exampletry {

average = scoreSum/numGames;}


The catch Block

The catch block is used to deal with any exceptions that may occur

This is your error handling code

Syntaxcatch(Exception_Class_Name Catch_Block_Parameter) {

Process_Exception_Of_Type_Exception_Class_Name}Possibly_Other_Catch_Blocks

Examplecatch(ArithmeticException e) {


}


Throwing Exceptions

You can also throw your own exceptionstry {


while(score >= 0) { scoreSum += score; numGames++; score = keyboard.nextInt();}

if(numGames <= 0) throw new Exception("Exception: num games is less than 1");

double average = scoreSum/numGames;}catch(Exception e) { //catches any kind of exception

System.out.println(e.getMessage());}


Throwing Exceptions

Syntax

throw new Exception_Class_Name(Possibly_Some_Arguments);

Example

throw new Exception("Exception: num games is less than 1");

or

Exception exceptObject = new Exception("Illegal character.");throw exceptObject;


Exception Objects

An exception is an object

All exceptions inherit the method getMessage() from the class Exception

Example

catch(ArithmeticException e) {System.out.println(e.getMessage());System.out.println("Cannot compute average for 0 games");

}


Java Exception Handling

A try block contains code that may throw an exception

When an exception is thrown, execution of the try block ends immediately

Depending on the type of exception, the appropriate catch block is chosen and executed

If no exception is thrown, all catch blocks are ignored


Programming Demo

Adding Exception Handling to Program 4

If we call Double.parseDouble() with invalid input such as "g23.55", the method will throw a “NumberFormatException”

Catch this exception and signal to the user there was a problem with the input


Programming Demo

Programming


Wednesday

Basic File I/O

comp 110: introduction to programming

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