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COMP 110: Spring 20091

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Math Class

Wrapper Classes

Writing / Testing Methods


Today in COMP 110

ReviewMath class / Wrapper classes / DecompositionCalling methods

Overloading

Programming Demo


The Math Class

Provides many standard mathematical methodsAll methods are static, no need for an object of the Math class

Call methods of the Math class using class nameMath.absMath.maxMath.minMath.powMath.roundOthers

Predefined constantsMath.PIMath.E


Wrapper Classes

Each primitive type has an associated “Wrapper” class

ByteShortIntegerLongFloatDoubleCharacterBoolean


Writing Methods

Solving a problem using decomposition

Divide into subproblems (pseudocode)Solve each subproblem separately as a methodUse the methods you’ve created to solve the problem


Calling Methods within Methods

It’s possible to call methods within other methods

If calling a method of the same class, no need to specify receiving object

public class Example {

public void method1() { method2(); //no object needed, current object assumed}

public void method2() { //do something}

}


Calling Methods within Methods


public void method1() {System.out.println("method1!");method2(); //no object needed, current object assumed

}

public void method2() {System.out.println("method2!");

}

public static void main(String[] args) {Example example = new Example(); //create object of class

Exampleexample.method1();

}}


Input to Methods

The input to a method is in the form of arguments

public class Account {private double balance;private double limit;

public void addPurchase(double amount) { if(balance + amount <= limit)

balance = balance + amount; //only add if the transaction //is valid}

}

The value is filled in by whomever calls the method


Input to Methods


public void addPurchase(double amount) { if(balance + amount <= limit)

balance = balance + amount; //only add if the transaction is valid}

}

public class AccountTester {

public static void main(String[] args) { Account accnt = new Account(); account.addPurchase(15.); //call addPurchase and w/ 15 for the amount account.addPurchase(20.); //call addPurchase and w/ 20 for the amount}

}

Separate Java Files!

A Driver program (Used for testing)


Input to Methods

NEVER do this


public void addPurchase(double amount) { Scanner keyboard = new Scanner(System.in);

amount = keyboard.nextDouble();

if(balance + amount <= limit) balance = balance + amount; //only add if the transaction is

valid}

}

Overwriting the value that was passed in!


Methods that Return a Value


//a helper method to determine if a transaction is validprivate boolean transactionValid(double amount) { if(balance + amount <= limit)

return true; else

return false;}

public void addPurchase(double amount) { boolean valid = transactionValid(amount); //is the transaction valid?

if(valid) balance = balance + amount; //only add if the transaction is

valid}

}


Method Calls in If-Statements


//a helper method to determine if a transaction is validprivate boolean transactionValid(double amount) { if(balance + amount <= limit)

return true; else

return false;}

public void addPurchase(double amount) {

if(transactionValid(amount)) balance = balance + amount; //only add if the transaction is

valid}

}

Call to a method inside an if-statement


Booleans

There’s no need to write

if(systemsGo == true)System.out.println("Launch");

if(transactionValid(amount) == true)System.out.println("Accepted");

The more concise and equivalent way is

if(systemsGo)System.out.println("Launch");

if(transactionValid(amount))System.out.println("Accepted");


Overloading


Overloading

public class InputOne {

public void readInput() {…

}}

public class InputTwo {

public void readInput() {…

}}

public static void main(String[] args) {

InputOne iOne = new InputOne();

InputTwo iTwo = new InputTwo();

iOne.readInput(); //readInput method of class InputOne

iTwo.readInput(); //readInput method of class InputTwo

}

•Methods in different classes can have the same name


Overloading

Methods in the same class can also have the same name

This is called overloading

Distinguished by the number & types of the parameters


Overloading Example

public class Average {

//average two valuespublic double getAverage(double a, double b) { return (a + b) / 2.;}

//average three valuespublic double getAverage(double a, double b, double c) { return (a + b + c) / 3.;}

}


Overloading

You have already been using overloaded methods

System.out.print(7); //print an integerSystem.out.print('7'); //print a characterSystem.out.print("seven"); //print a stringSystem.out.print(7.0); //print a double


Other Overloading Examples

The Math classdouble Math.max(double a, double b)int Math.max(int a, int b)long Math.max(long a, long b)

Allows the followingint m = Math.max(1,3);double d = Math.max(5.6, 5.7);


Overloading

Any kind of method can be overloaded

Void methodsMethods returning a valueStatic methodsNon-static methodsConstructors


Constructor Overloading

public class Pet {

private String name; private int age; private double weight;

public Pet() { name = “No name yet.”; age = 0; weight = 0; }

public Pet(String initName, int initAge, double initWeight) { name = initName; age = initAge; weight = initWeight; }}

Pet myPet = new Pet();

Pet myPet = new Pet("Fang", 12, 10.);


Method Signatures

A method’s signature consists of Method nameNumber of parametersTypes of parameters

Example

public double getAverage(double a, double b) { … }

SignatureName: getAverageNumParams: 2Param Types:

Param1: doubleParam2: double

Return type is NOT considered part of the signature!


Method Signatures

Java does not allow you to define two methods with the same signature in the same class

Examples//these two are the samefloat getAverage(float a, float b)double getAverage(float a, float b)

//these two are differentfloat getAverage(float a, float b)double getAverage(double a, double b)


Automatic Type Conversion

Recall that automatic type conversion can sometimes occur with method calls

double square(double x) {

return x*x; //square the argument and return it}

We can call this method as followssquare(7.0); //returns 49.0square(7); //also returns 49.0, auto type conversion


The situation gets more complicated with overloading


double square(double x) { return x*x; }

int square(int x) { return x*x; } public static void main(String[] args) { Example e = new Example(); e.square(7.0); e.square(7); }}

Interaction with Overloading

Which method is being called?


Overloading/Type Conversion

Java will always use a method that is an exact match before it attempts type conversion


Exact Overloading Match

public void example(int i, double d, char c) {…}

Are these calls to example an exact match?

example(23, 55, 'c');

example(88, 76.0, ';');

example(4.0, 25, '!');

No. Automatic type conversion used

Yes. No need for Automatic type conversion

No. Automatic type conversion not possible


Ambiguous Method Calls

Java will only perform type conversion if the method call is unambiguous

There is only ONE method for which automatic type conversion can be used to find a match


Ambiguous Method Calls


double sum(int a, double b) { return a + b; }

double sum(double a, int b) { return a + b; }

public static void main(String[] args) { Example e = new Example(); e.sum(7, 7); //error, this method call is ambiguous e.sum(7, 7.0); //this is ok e.sum(7.0, 7); //this is ok }}


In Summary

How Java determines which method you intend to call

Exact Match?

Use the Method

Unambiguous Match using Type

Conversion?

Use the Method

Error

Match Based on Method Name, Num & Types of

Parameters


Use of Overloading

Misuse of overloading can lead to strange bugsUse only with good reason

public Pet(double initWeight) { //constructor for weightweight = initWeight;

}

public Pet(int initAge) { //constructor for ageage = initAge;

}

public static void main(String[] args) {Pet myPet = new Pet(65); //meant to set weight, set age instead

}


Programming Demo

Room Occupancy

Create a class called Room that can be used to record the number of people in the rooms of a building


Room Occupancy

AttributesnumberInRoom – the number of people in a roomtotalNumber – the total number of people in all rooms as a static variable

Methodsdefault constructor – sets number of people in room to 0addOneToRoom – add a person to the roomremoveOneFromRoom – remove a person from the room (don’t go below 0 persons)getNumber – returns the number of people in the roomgetTotal – a static method that returns the total number of people in all roomsvalidRemoval(int num) – returns whether num people can be removed from the room


Programming Demo

Programming


Wednesday

Array Basics

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