13 X 11 Java Lecture 3 CS 1311 Structure 13 X 11

Java Lecture 3

CS 1311

Structure

Types of Java Programs

• Applications– Stand alone– Requires JVM (java.exe, etc.) on your machine

• Applets– JVM built in to web browser runs program– Can be downloaded from network or files can be on

your machine– Susceptible to browser problems!

• Servlets– Like an applet but code runs on server when a

particular web page is browsed

A View of Memory

Class Pool(What you wrote)

Stack (Static Area)(Store stuff here)

Heap (Dynamic Area)(Store stuff here)

class CokeMachine {

int numCans = 3;

void vend() {if(numCans > 0) {

System.out.println("Have a Coke!");numCans = numCans - 1;

}else {

System.out.println("Sorry!");}

void load(int n) {numCans = numCans + 1;System.out.println("Ready to vend!");

public static void main(String args[]) {CokeMachine cc;cc = new CokeMachine();cc.vend();cc.vend();cc.vend();cc.vend();cc.load();cc.vend();

Two Views• External

CokeMachine cc;

cc = new CokeMachine();

cc.vend();

cc.load(5);

class CokeMachine {

int numCans = 3;

void vend() { if(numCans > 0) { System.out.println ("Have a Coke!"); numCans = numCans - 1; } else { System.out.println ("Sorry!"); } }

void load(int n) { numCans = numCans + 1; System.out.println ("Ready to vend!"); }}

javac *.java

java Driver

Could write...class Driver {

public static void main(String args[]) {

CokeMachine cc;

cc.vend();

cc.load(5);

} // main

} // Simulation

What happened?

java Driver

DrivermainC

What happened?

Java searches for(and finds) staticmain method

Drivermain

What happened?

Java searches for(and finds) staticmain method

Drivermain

What happened?

CokeMachine cc;

Drivermain

CokeMachinenumCans vend() load()

What happened?

CokeMachine cc;

Drivermain

main cc (CokeMachine)

What happened?

Drivermain

Obj: CokeMachinenumCans vend() load()

What happened?

cc.vend();

Drivermain

Have a Coke!

What happened?

cc.vend();

Drivermain

Have a Coke!Have a Coke!

What happened?

cc.vend();

Drivermain

Have a Coke!Have a Coke!Have a Coke!

What happened?

cc.vend();

Drivermain

Have a Coke!Have a Coke!Sorry!

As a convenience...

• We don't really need the driver class• We can put a static main in the CokeMachine

• But the operation is essentially the same!

• In fact, you should always put a static main in every class you write

• Use it to test the class for correct operation

• Will Java get confused?

No Way, Jose!

class Amain()

class Bmain()

class Cmain()

class Dmain()

class Emain()

class Fmain()

Because!

class Amain()

class Bmain()

class Cmain()

class Dmain()

class Emain()

class Fmain()

javac *.javajava F

Questions?

Fields

• Sometimes called attributes• In our CokeMachine: numCans• Can be

– Primitives– References

• Accesscc.numCans = 42; // Direct access

cc.vend(); // Via modifier

• Is direct access a "GoodThing"® or a "BadThing"®

Controlling Access

• Access to fields (and methods) can be controlledpublic -- Anyone can access

private -- Access is only from within class

Recall

public static void main(String args[])

Exampleclass CokeMachine

int numCans = 3;

double cash = 0.0;

static final double PRICE = 0.65;

void vend() {

if(numCans > 0) {

numCans--;

cash += PRICE;

System.out.println("Have a coke!");

} else {

System.out.println("Sorry, no Cokes");

Let's Test It!class Tester

public static void main(String args[])

CokeMachine cm; // Make reference

cm = new CokeMachine(); // Make a machine

cm.numCans--; // Steal a Coke

System.out.println("Cash = " + cm.cash);

} Where's my money?!?!?!

Let's add some protectionclass CokeMachine

private int numCans = 3;

private double cash = 0.0;

private static final double PRICE = 0.65;

public void vend() {

if(numCans > 0) {

numCans--;

cash += PRICE;

} else {

Warning about Testingclass CokeMachine

if(numCans > 0) {

numCans--;

cash += PRICE;

} else {

class Tester {

public static void main(String a[]) {

CokeMachine cm = new CokeMachine();

cm.numCans--;

class CokeMachine

if(numCans > 0) {

numCans--;

cash += PRICE;

} else {

CokeMachine cm = new CokeMachine();

cm.numCans--;

Details

• Methods– Module defined in a class which effectively becomes a

message that an object of that class can understand

cc.vend();

– Send a "vend()" message to the object referenced by cc

– Almost all code executed in Java will be executed in methods

Anatomy of a method• <access> <static> <return type>

<name>(<parameter list>) { <method body> }• Examples:

public static void main(String args[]){

System.out.println("Hello World!");}

void vend(){ /* code goes here */ }

private int addEmUp(int a, int b, int c) {return a + b + c;

Parts is parts

• <access>

public

private– There are others we won't discuss

• <static>

staticnothing which implies dynamic or instance

Parts is parts

• <return type>– Methods may optionally return a value– May be any valid type

• Class name• Primitive

void indicates no return value– If you promise to return something Java will expect you

to keep your promise– If you promise to not return anything Java will break

your kneecaps with a Louisville Slugger® if you don't keep your promise

A Promise is a Promisepublic int max(int a, int b)

if(a >= b)

return a;

if(a < b)

return b;

A Promise is a Promisepublic int max(int a, int b)

if(a >= b)

return a;

if(b < a)

return b;

Classname.java:<lineno>: Return required at end of int max(int, int). public int max(int a, int b)

Method Name...

• <name>– Method name follows normal identifier rules– Style note: Start method names with a lower case letter

but capitalize multiword names:enqueue

vendACoke

loadCokes

Parameter List

• <parameter list>– Declare each parameter individually– i.e. int a, float f, CokeMachine cm– Java only supports pass by value (in) parameters.

public void swap(int a, int b) {

/* This method is worthless! */

int t = a;

a = b;

b = t;

Parameter Listpublic static void testMachine(int n, CokeMachine cm)

for(int i=0; i < n; i++)

cm.vend();

n = 0; // Just to be mean

cm = null; // and nasty!

• What is the effect on the outside world?

• Could this be considered (just maybe) a side effect?

Pass by Valueclass Demo {

/* Code for testMachine here*/public static void Main(String args[]) {

int i = 3;CokeMachine cm;cm = new CokeMachine();testMachine(i, cm);

i: cm:3

Object: CokeMachinenumCans = 3

int i = 3;CokeMachine cm;cm = new CokeMachine();testMachine(i, cm);

i: cm:3

testMachine: n: cm:3

int i = 3;CokeMachine cm;cm = new CokeMachine();exerciseMachine(i, cm);

i: cm:3

public static void testMachine

(int n, CokeMachine cm) {

for(int i=0; i < n; i++) {

cm.vend();

i: cm:3

for(int i=0; i < n; i++) {

cm.vend();

i: cm:3

for(int i=0; i < n; i++) {

cm.vend();

i: cm:3

for(int i=0; i < n; i++) {

cm.vend();

i: cm:3

Questions?

Sidebar on Methods

• Factorial (Yes, again)

class FactDemo {

int n = 10;

int i;

int answer = 1;

for(i = 1; i <= n; i++) {

answer *= i;

} // for

System.out.println(n + "! = " + answer);

} // main

} // factDemo

Sidebar on Methods

• Make it a method

public static void fact(int n) {

int i;

int answer = 1;

for(i = 1; i <= n; i++) {

answer *= i;

} // for

System.out.println(n + "! = " + answer);

} // fact

Sidebar on Methods

• Make it smaller and more efficient?

int i, ans;

for(i = 1, ans = 1; i <= n; ans *= i++);

System.out.println(n + "! = " + ans);

} // fact

Sidebar on Methods

• Make it more readable?

int i;

int ans = 1;

for(i = 1; i <= n; i++)

ans *= i;

System.out.println(n + "! = " + ans);

} // fact

Sidebar on Methods

• Make it more useful?

public static int fact(int n) {

int i;

int ans = 1;

for(i = 1; i <= n; i++)

ans *= i;

return ans;

} // fact

Sidebar on Methods

• Make it more useful?

class FactDemo {

int i;

int ans = 1;

for(i = 1; i <= n; i++)

ans *= i;

return ans;

} // fact

System.out.println("10! = " + fact(10));

Sidebar on Methods

• Do it recursively!

class FactDemo {

if(n == 0)

return 1;

return n * fact(n - 1);

} // fact

System.out.println("10! = " + fact(10));

Sidebar on Methods

• Do it recursively!

class FactDemo {

if(n == 0)

return 1;

} // fact

for(int i = 1; i < 18; i++)

System.out.println(i + "! = " + fact(i));

ShakyResults

1! = 1

2! = 2

3! = 6

4! = 24

5! = 120

6! = 720

7! = 5040

8! = 40320

9! = 362880

10! = 3628800

11! = 39916800

12! = 479001600

13! = 1932053504

14! = 1278945280

15! = 2004310016

16! = 2004189184

17! = -288522240

Correct value: n! = 6,227,020,800 > ?

Primitive Data Type Ranges

Type Size Min Default

boolean false

1 false* true*

char '\u0000' (null)16

byte (byte) 08 -128 127

short (short) 016 -32,768 32,767

int 032 -2,147,483,648 2,147,483,647

long 0L64 -9,223,372,036,854,775,808 9,223,372,036,854,775,807

float 0.0F32 Approx ±3.4E+38 with 7 significant digits

double 0.0D64 Approx ±1.7E+308 with 15 significant digits

* Not truly min and max.

Notice!

• Notice how Java gave you a helpful warning message to help keep you spacecraft from plunging into the Sun

Sidebar on Methods

• Fix the problem (forever?)!

class FactDemo {

public static long fact(long n) {

if(n == 0)

return 1;

} // fact

for(int i = 1; i < 18; i++)

System.out.println(i + "! = " + fact(i));

Pop Quizclass Widget

int a, b;

public int sum() {

return a + b;

a = 5;

b = 10;

System.out.println(sum());

Pop Quizclass Widget{int a, b;public int sum() {

return a + b;}public static void main(String a[]) {

Widget w;w = new Widget();w.a = 5;w.b = 10;System.out.println(w.sum());

Questions?

Final Thought

• Just what is a class?

declarations

initializationmodules*

methods

Questions?

13 X 11 Java Lecture 3 CS 1311 Structure 13 X 11

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