1 methods and classes. 2 functions in java in java, every program is a “class” i.e. object type...

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Methods and Classes

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Functions in Java

In Java, every program is a “class” i.e. object type there are no functions outside a class

So far, we have only used the main method But we can define as many methods as we

like in a single class

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Example (Only main)class OnlyMain {

public static void main(String[] args) {

String name = new String(“Joe”);int age = 22;

System.out.println(“Name:” + name);System.out.println(“Age:” + age);

}}

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Example (Several Methods)class SeveralMethods {

void printName(){

String name = new String(“Joe”);System.out.println(“Name: “ + name);

}

void printAge(){

int age = 22;System.out.println(“Age: “ + age);

}


printName();printAge();

}}

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Example (Methods With Parameters)class MethodsWithParameters {

void printName(String name){

System.out.println(“Name: “ + name);}

void printAge(int age){

System.out.println(“Age: “ + age);}


printName(“Joe”);printAge(22);

}}

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Example (Where Did main Go?)class NoMainMethod {

void printName(String name){

System.out.println(“Name: “ + name);}

void printAge(int age){

System.out.println(“Age: “ + age);}

void printDetails() {

printName(“Joe”);printAge(22);

}}

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Method Declarations

A method declaration specifies the code that will be executed when the method is called

When a method is invoked, flow of control jumps to the method and executes its code

When complete, flow returns to where it was called

Methods may or may not return a value

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myMethod();

myMethodcompute

Method Control Flow If the called method is in the same class, only the

method name is needed

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doIt

helpMe

helpMe();

obj.doIt();

main

Method Control Flow

The called method is often part of another class or object

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The Dot Operator

The dot operator is used to call the methods of an object

e.g. Scanner sc = new Scanner(System.in);

input = sc.nextInt();

nextInt() is a method defined in the Scanner class

To call nextInt(), you need an object of type Scanner

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Basics

syntax:<method defn> ::= <modifiers> <type> <identifier>

(<parameters>) {<method body>}

Example: char calc (int num1, int num2, String message){…}

everything preceding {…} is the header everything inside {…} is the body

code that’s run when it’s called

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Exampleclass FunctionTest {

public static double negate(double x) {

return -x;}


System.out.println( negate(3.4) + 1 );}

}

Output: -2.4

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Method Header

A closer look at message headers:

char calc (int num1, int num2, String message)

methodname

returntype

parameter list

The parameter list specifies the typeand name of each parameter

The name of a parameter in the methoddeclaration is called a formal parameter

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Modifiers

Modifiers are optional parts of the header These modifiers control access

public – can be called from other classes private – only be called in the declaring class protected – later

The static modifier preceding a method means it can be called without an instance Math.sqrt(9);

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Method Body The message body contains statements:


{ int sum = num1 + num2; char result = message.charAt (sum);

return result;}

The return expressionmust be consistent withthe return type

sum and resultare local data

They are created each time the method is called, and are destroyed when it finishes executing

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The return Statement

The return type of a method indicates the type of value that the method sends back to the calling location

A method that does not return a value has a void return type

return statement specifies the value that will be returned

return expression;

Its expression must conform to the return type

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Parameters The actual parameters in the invocation are copied

into the formal parameters in the method header



return result;}

ch = obj.calc (25, count, "Hello");

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The main Method Revisited

public static void main (String[] args)

public – can be called from anywhere static – it is called without an object void – does not return a value String[] args

args is a variable of type String[]

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Local Data

Local variables can be declared in a method

The scope of a local variable is the body of the method – it can not be referenced outside



return result;}

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This Will Not Work….

{… char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum);

return result; }

int getSum() { return sum; }

…}

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Neither Will Thisclass FunctionTest {

public static boolean method1() {

return method2();}

public static boolean method2() {

return method1();}


if(method1())System.out.println(“Done!”);

}}

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String Methods

String objects have many defined methods: all methods listed in the text

Characters in a string can be accessed by an index indices start at 0 and increment from left to right

Use length() to get the number of characters

Together these methods are useful for iteratively working through the characters in a String

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String Mutability

Once a String is created, neither its value nor length can be changed we say String objects are immutable

String objects have many defined methods: all methods listed in the text

Some methods return new String objects, such as substrings gives the illusion of mutability

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Using String Methodspublic class StringShift

{

public static void main(String[] args)

{

String name = new String("aaron");

String shifted = new String("");

for(int count=0;count<name.length();count++)

shifted = shifted + (char)(name.charAt(count) + 1);

System.out.println(shifted);

}

} Output: bbspo

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Using String Methods (again)public class StringShift

{

public static void main(String[] args)

{

String name = new String("aaron");

String shifted = new String("");

for(int count=0;count<name.length();count++)

shifted = shifted + (char)(name.charAt(count) + ‘A’ – ‘a’);

System.out.println(shifted);

}

} Output: AARON

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The equals Method

Consider:String string1 = new String(“Aaron”);

String string2 = new String(“Aaron”); This doesn’t do what you normally want:if(string1==string2)

System.out.println(“equal!”); Use the equals method:if(string1.equals(string2))

System.out.println(“equal!”);

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Class Libraries

Classes in Java can be used to hold code libraries

Methods defined in one class can be used in another through the dot operator

The Java Standard Class Library is part of any Java development environment includes System, String, Scanner … and many others

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Example: A static Function Libraryclass Quadratic {

static private double discrim(double a, double b, double c) {

return b*b - 4*a*c;}static public double root1(double a, double b, double c) {

return (-b - Math.sqrt(discrim(a,b,c))) / (2*a);}static public double root2(double a, double b, double c) {return (-b + Math.sqrt(discrim(a,b,c))) / (2*a);}

} Note: NO MAIN METHOD!

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Example continued

class QuadTest {


double a=1, b=-4, c=3;System.out.println( Quadratic.root1(a,b,c) );System.out.println( Quadratic.root2(a,b,c) );//System.out.println (Quadratic.discrim(a,b,c);

}}

Output:1.03.0

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Public/Private Modifiers

root1 and root2 can be called because they are public they can be called from any other class

discrim can not be called in QuadTest, because it is private but it is called in Quadratic

Instance variables can also be declared public and private

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Packages

The Java library is organized in packages Some standard packages:

java.lang java.util java.applet

You can also define your own packages (but we won’t worry about that now)

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The Import Declaration

You can use a class in a package by using the full name java.util.Scanner

Or you can import the class, and then just use the name import java.util.Scanner;

Or you can import every class in the package java.util.*;

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The Import Declaration

All of the classes in java.lang are automatically imported that’s why we don’t import String, Math, etc.

Any other packages must be imported

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Two Equivalent Classesclass DateTest1 {


java.util.Date d = new java.util.Date();System.out.println(d);

}}//=====================================================import java.util.Date;

class DateTest2 {


Date d = new Date();System.out.println(d);

}}

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Example: The Math class

Part of the java.lang package Contains various mathematical functions:

square root exponentiation trigonometry

All the methods are static

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Example: The Random Class

The Random class is part of java.util It provides methods that generate

“pseudorandom” numbers Performs complicated calculations based on

a seed value to generate a stream of apparently random values good enough for us “real” randomness is harder to generate

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The Monty Hall Problem

Based on TV game show Given 3 doors, behind one is a million dollars You pick 1 door The host opens one of the other doors to

show it is not the winner, gives you the option to change doors

Question: Should you change?

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Strategy 1: Never Change Your Choicestatic boolean playGame1(){

Random generator = new Random();

int winningdoor = generator.nextInt(3);int chosendoor = generator.nextInt(3);

if(winningdoor==chosendoor)return true;

elsereturn false;

}

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Strategy 2: Always Change

static boolean playGame2(){

Random generator = new Random();

int winningdoor = generator.nextInt(3);int chosendoor = generator.nextInt(3);

int openeddoor = getDistinctValue(winningdoor,chosendoor);

chosendoor = getDistinctValue(chosendoor,openeddoor);

if(winningdoor==chosendoor)return true;

elsereturn false;

}

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A Utility Method (not the nicest…)static int getDistinctValue(int w, int c){

int returnvalue = 4;

for (int value=0; value<3;value++){

if(value!=w && value !=c)returnvalue = value;

}

return returnvalue;}

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Putting it Together (in MontyHall.java)public static void main(String[] args){

int count1=0;int count2=0;

for(int i=0;i<10000;i++)if(playGame1())

count1++;

for(int i=0;i<10000;i++)if(playGame2())

count2++;

System.out.println("No-switch strategy wins: " + count1 + " out of 10000");System.out.println("Switch strategy wins: " + count2 + " out of 10000");

}

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Output

Slight variation over runs Strategy 1 = 3224, Strategy 2 = 6663 Strategy 1 = 3384, Strategy 2 = 6721 Strategy 1 = 3437, Strategy 2 = 6663 ….

Conclusion: Always switching doors is better This can be proved mathematically too… but

with some initial controversy

1 methods and classes. 2 functions in java in java, every program is a “class” i.e. object type...

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