Java Basics
(lectures programs)
Sun OnLine Documentations
Everything is an Object
public class HelloDate {
public static void main(String[] args) {
System.out.println
("Hello CS476/CS576 students, it's: ");
System.out.println
(new Date() );
}
}
à To Compile:
% make
OR
% javac HelloDate.java
à To Run:
% java HelloDate
Controlling Program Flow
ü Operators: precedence &
assignment.
ü Decision: if & switch.
ü Looping: while & for.
Ø Example:
VowelsAndConsonants Run Applet
public class VowelsAndConsonants {
public static void main
(String[] args) {
for (int i
= 0; i < 100; i++) {
char c = (char)(Math.random() * 26 + 'a');
System.out.print(c +
": ");
switch (c) {
case 'a':
case 'e':
case 'i':
case 'o':
case 'u':
System.out.println("vowel"); break;
case 'y':
case 'w':
System.out.println( "Sometimes a
vowel"); break;
default:
System.out.println("consonant");
}}}
}
Initilization & Cleanup
Ø Example 1: ArrayNew Run Applet
public class ArrayNew {
static
Random rand = new Random();
public
static void main(String[] args) {
int[] a;
a = new int[rand.nextInt(20)];
System.out.println("length
of a = " + a.length);
for (int i = 0; i < a.length;
i++)
System.out.println("a["
+ i + "] = " + a[i]);
}
}
Ø Example 2: Garbage
Run Applet
class Chair {
static
boolean f = false;
static
int created = 0;
int i;
Chair() {
i
= ++created;
if
( (i % 10000) == 0)
System.out.print(
".");
}
public
void finalize() {
f = true;
}
}
public class Garbage {
public
static void main(String[] args) {
while (!Chair.f) {
new Chair();
}
System.out.println
(
"\nTotal
Chairs Created = "
+ Chair.created
“);
}
}
The Java IO System
Ø Example: IOStreamDemo.java
public class IOStreamDemo {
public static void
main (String[] args)
throws IOException {
// Print the length and content of the input file
File file = new File(args[0]);
long length = file.length();
System.out.println("file lenght is:
" + length);
BufferedReader inpf = new BufferedReader( new FileReader(file));
String str;
System.out.println
("The content of " + file + ":\n");
while((str = inpf.readLine())!= null)
System.out.println(str);
// Transfer bytes
from in to out
String infile =
args[0];
String outfile = args[1];
InputStream fin
= new FileInputStream(infile);
OutputStream fout = new FileOutputStream(outfile);
byte[] buf = new byte[1024];
int
len;
while ((len = fin.read(buf)) > 0) {
fout.write(buf, 0, len);
}
fin.close();
fout.close();
// Reading
standard input and Append to file.
FileOutputStream
appendedFile = new FileOutputStream
(outfile, true);
BufferedReader stdin = new BufferedReader( new InputStreamReader(System.in));
String st="";
System.out.print("\n\nEnter a line or type
CTRL-D to finish:" + "\n");
while
(true) {
try {
st = stdin.readLine();
}
catch(EOFException e) {
System.err.println("End
of stream");
}
if ( st == null) break;
System.out.println(st);
appendedFile.write(st.getBytes());
appendedFile.write('\n');
}
// Reading input
by lines:
BufferedReader
in = new BufferedReader( new FileReader(outfile));
String s, s2 = new String();
while((s = in.readLine())!= null)
s2 += s + "\n";
in.close();
// Reading from String
try {
BufferedReader
in2 = new BufferedReader(new StringReader(s2));
PrintWriter
out1 = new PrintWriter(
new BufferedWriter(new FileWriter(outfile)));
int
lineCount = 1;
while((s =
in2.readLine()) != null )
out1.println(lineCount++ + ": " + s);
out1.close();
}
catch(EOFException e) {
System.err.println("End of stream");
}
// Print the
content of the output file
BufferedReader inf = new BufferedReader( new FileReader(outfile));
System.out.println
("\n\nThe content of "
+ outfile +
":\n");
while((st = inf.readLine())!= null)
System.out.println(st);
}
}
To
execute:
% java IOStreamDemo InputFile OutFile
Collections of Objects
Ø Vector:
CatsAndDogs Run applet
class Cat {
private int catNumber;
Cat (int i) {
catNumber = i;
}
void print() {
System.out.println("Cat number " + catNumber);
}
}
class Dog {
private int dogNumber;
Dog (int i) {
dogNumber = i;
}
void print() {
System.out.println("Dog number " + dogNumber);
}
}
public class CatsAndDogs {
public static void main (String[] args)
{
ArrayList cats = new ArrayList();
for (int i
= 0; i < 7; i++)
cats.add (new Cat(i));
// Not a problem to add a dog to
cats:
cats.add(new
Dog(7));
Iterator e = cats.iterator();
while (e.hasNext())
((Cat)e.next()).print();
// Dog is detected only at run-time
}
}
Ø Hashtable:
Statistics Run applet
class Counter {
int i =
1;
public String toString() {
return Integer.toString(i);
}
}
public class Statistics {
public static void main (String[] args)
{
HashMap
hm = new HashMap();
for (int i
= 0; i < 1000; i++) {
//
Produce a number between 0 and 9:
Integer r =
new Integer((int)(Math.random() * 10));
if (hm.containsKey(r))
((Counter) hm.get(r)).i++;
else
hm.put (r, new Counter());
}
System.out.println (hm);
for(int i
= 0; i < 10; i++) {
Integer r = new
Integer((int)(i));
if(hm.containsKey(r)){
Counter c = (Counter)hm.get(r);
System.out.println
(r + " -> " + c + "\n");
}
}
}
}
Reusing Classes
Composition: Car Run applet
class Engine {
public void start() {}
public void rev() {}
public void stop() {}
}
class Wheel {
public void inflate(int psi) {
System.out.println("inflated to:" + psi + "\n");
}
}
class Window {
public void rollup() {
System.out.println("Rolled Up\n");
}
public void rolldown() {
System.out.println("Rolled Down\n");
}
}
class Door {
public Window window = new Window();
public void open() {}
public void close() {}
}
public class Car {
public Engine engine = new Engine();
public Wheel[] wheel = new Wheel[4];
public Door left =
new Door(),
right = new Door(); // 2-door
public Car() {
for (int i
= 0; i < 4; i++)
wheel[i] = new Wheel();
}
public static void main (String[] args)
{
Car car = new Car();
car.left.window.rollup();
car.wheel[0].inflate(72);
}
}
Polymorphism
Ø Example: Shapes2 Run applet
class Shape {
void draw() {
System.out.println("Default.draw()");
}
void erase() {
System.out.println ("Default.erase()");
}
}
class Circle extends Shape {
void draw() {
System.out.println ("Circle calling super.draw():->
");
super.draw();
}
}
class Square extends Shape {
void draw() {
System.out.println("Square.draw()");
}
void erase() {
System.out.println("Square.erase()");
}
}
class Triangle extends Shape {
void draw() {
System.out.println("Triangle.draw()");
}
void erase() {
System.out.println("Triangle.erase()");
}
}
public class Shapes2 {
public static Shape randShape() {
switch((int)(Math.random()
* 3)) {
default:
case 0: return new Circle();
case 1: return new Square();
case 2: return new Triangle();
}
}
public static void main(String[] args)
{
Shape[] s = new Shape[9];
// Fill
up the array with shapes:
for (int i =
0; i < s.length; i++)
s[i]
= randShape();
// Make
polymorphic method calls:
for (int i =
0; i < s.length; i++){
s[i].draw();
s[i].erase();
}
}
}
Detecting
Types
Ø Example
1: Shapes Run applet
class Shape {
void draw() {
System.out.println(this + ".draw()");
}
}
class Circle extends Shape {
public String toString() { return
"Circle"; }
}
class Square extends Shape {
public String toString() { return
"Square"; }
}
class Triangle extends Shape {
public String toString() { return
"Triangle"; }
}
public class Shapes {
public static void main (String[] args)
{
List s =
new ArrayList();
s.add (new Circle());
s.add (new Square());
s.add (new Triangle());
Iterator e = s.iterator();
while (e.hasNext())
( (Shape)
e.next() ).draw();
}
}
Ø Example
2: CatsAndDogs2 Run
applet
class Pet {
void print() {
}
}
class Cat2 extends Pet{
private int catNumber;
Cat2 (int i) {
catNumber = i;
}
void print() {
System.out.println("Cat number " + catNumber);
}
}
class Dog2 extends Pet{
private int dogNumber;
Dog2 (int i) {
dogNumber = i;
}
void print() {
System.out.println("Dog number " + dogNumber);
}
}
public class CatsAndDogs2 {
public static void main (String[] args)
{
ArrayList cats = new ArrayList();
for (int i
= 0; i < 7; i++)
cats.add (new Cat2 (i));
// Not a problem to add a dog to
cats:
cats.add(new
Dog2 (7));
Iterator e = cats.iterator();
while (e.hasNext())
((Pet)e.next()).print();
// Cat and Dog are detected only at
run-time
}
}