Example Java Programs
HelloWorld.java
public class HelloWorld {
// method main(): ALWAYS the APPLICATION entry point
public static void main (String[] args) {
System.out.println ("Hello World!");
}
}
// Print Today's Date
import java.util.*;
public class HelloDate {
public static void main (String[] args) {
System.out.println ("Hello, it's: ");
System.out.println(new Date());
}
}
//example: function call in Java
public class FunctionCall {
public static void funct1 () {
System.out.println ("Inside funct1");
}
public static void main (String[] args) {
int val;
System.out.println ("Inside main");
funct1();
System.out.println ("About to call funct2");
val = funct2(8);
System.out.println ("funct2 returned a value of " + val);
System.out.println ("About to call funct2 again");
val = funct2(-3);
System.out.println ("funct2 returned a value of " + val);
}
public static int funct2 (int param) {
System.out.println ("Inside funct2 with param " + param);
return param * 2;
}
}
//Array in Java
public class ArrayDemo {
public static void main(String[] args) {
int[] anArray; // DECLARE an array of integers
anArray = new int[10]; // CREATE an array of integers
// assign a value to each array element
for (int i = 0; i < anArray.length; i++) {
anArray[i] = i;
}
// print a value from each array element
for (int i = 0; i < anArray.length; i++) {
System.out.print(anArray[i] + " ");
}
System.out.println();
}
}
//class usage in Java
class Point2d {
/* The X and Y coordinates of the point--instance variables */
private double x;
private double y;
private boolean debug; // A trick to help with debugging
public Point2d (double px, double py) { // Constructor
x = px;
y = py;
debug = false; // turn off debugging
}
public Point2d () { // Default constructor
this (0.0, 0.0); // Invokes 2 parameter Point2D constructor
}
// Note that a this() invocation must be the BEGINNING of
// statement body of constructor
public Point2d (Point2d pt) { // Another consructor
x = pt.getX();
y = pt.getY();
// a better method would be to replace the above code with
// this (pt.getX(), pt.getY());
// especially since the above code does not initialize the
// variable debug. This way we are reusing code that is already
// working.
}
public void dprint (String s) {
// print the debugging string only if the "debug"
// data member is true
if (debug)
System.out.println("Debug: " + s);
}
public void setDebug (boolean b) {
debug = b;
}
public void setX(double px) {
dprint ("setX(): Changing value of X from " + x + " to " + px );
x = px;
}
public double getX() {
return x;
}
public void setY(double py) {
dprint ("setY(): Changing value of Y from " + y + " to " + py );
y = py;
}
public double getY() {
return y;
}
public void setXY(double px, double py) {
setX(px);
setY(py);
}
public double distanceFrom (Point2d pt) {
double dx = Math.abs(x - pt.getX());
double dy = Math.abs(y - pt.getY());
// check out the use of dprint()
dprint ("distanceFrom(): deltaX = " + dx);
dprint ("distanceFrom(): deltaY = " + dy);
return Math.sqrt((dx * dx) + (dy * dy));
}
public double distanceFromOrigin () {
return distanceFrom (new Point2d ( ));
}
public String toStringForXY() {
String str = "(" + x + ", " + y;
return str;
}
public String toString() {
String str = toStringForXY() + ")";
return str;
}
}
//read integer from Keyboard
import java.io.*;
import java.util.*;
public class KeyboardIntegerReader {
public static void main (String[] args) throws java.io.IOException {
String s1;
String s2;
int num = 0;
// set up the buffered reader to read from the keyboard
BufferedReader br = new BufferedReader (new InputStreamReader (
System.in));
boolean cont = true;
while (cont)
{
System.out.print ("Enter an integer:");
s1 = br.readLine();
StringTokenizer st = new StringTokenizer (s1);
s2 = "";
while (cont && st.hasMoreTokens())
{
try
{
s2 = st.nextToken();
num = Integer.parseInt(s2);
cont = false;
}
catch (NumberFormatException n)
{
System.out.println("The value in \"" + s2 + "\" is not an
integer");
}
}
}
System.out.println ("You entered the integer: " + num);
}
}
//read data from keyboard
import java.io.*;
import java.util.*;
public class KeyboardReader
{
public static void main (String[] args) throws java.io.IOException
{
String s1;
String s2;
double num1, num2, product;
// set up the buffered reader to read from the keyboard
BufferedReader br = new BufferedReader (new InputStreamReader (
System.in));
System.out.println ("Enter a line of input");
s1 = br.readLine();
System.out.println ("The line has " + s1.length() + " characters");
System.out.println ();
System.out.println ("Breaking the line into tokens we get:");
int numTokens = 0;
StringTokenizer st = new StringTokenizer (s1);
while (st.hasMoreTokens())
{
s2 = st.nextToken();
numTokens++;
System.out.println (" Token " + numTokens + " is: " + s2);
}
}
}
//read from file
import java.io.*;
import java.util.*;
public class MyFileReader
{
public static void main (String[] args) throws java.io.IOException
{
String s1;
String s2;
// set up the buffered reader to read from the keyboard
BufferedReader br = new BufferedReader (new FileReader
("MyFileReader.txt"));
s1 = br.readLine();
System.out.println ("The line is " + s1);
System.out.println ("The line has " + s1.length() + " characters");
System.out.println ();
System.out.println ("Breaking the line into tokens we get:");
int numTokens = 0;
StringTokenizer st = new StringTokenizer (s1);
while (st.hasMoreTokens())
{
s2 = st.nextToken();
numTokens++;
System.out.println (" Token " + numTokens + " is: " + s2);
}
}
}
//exception handling in Java
public class HelloWorldException {
public static void main (String[] args) throws Exception {
System.out.println("Bienvenitos!");
throw new Exception("Generic Exception");
}
}
//exception handling
import java.io.*;
import java.util.*;
/** Causes a compilation error due to an unhandled Exception
*/
public class KeyboardReaderError {
public static void main (String[] args) { // throws java.io.IOException
String s1;
String s2;
double num1, num2, product;
// set up the buffered reader to read from the keyboard
BufferedReader br = new BufferedReader (new InputStreamReader
(System.in));
System.out.println ("Enter a line of input");
/* Following line triggers the error. Error will show the type of
unhandled exception and where the call occurs */
s1 = br.readLine();
System.out.println ("The line has " + s1.length() + " characters");
System.out.println ();
System.out.println ("Breaking the line into tokens we get:");
int numTokens = 0;
StringTokenizer st = new StringTokenizer (s1);
while (st.hasMoreTokens()) {
s2 = st.nextToken();
numTokens++;
System.out.println (" Token " + numTokens + " is: " + s2);
}
}
}
//encounters an error
import java.io.*;
// This program shows a stack track that occurs when java
// encounters a terminal error when running a program.
public class DivBy0
{
public static void funct1 ()
{
System.out.println ("Inside funct1()");
funct2();
}
public static void main (String[] args)
{
int val;
System.out.println ("Inside main()");
funct1();
}
public static void funct2 ()
{
System.out.println ("Inside funct2()");
int i, j, k;
i = 10;
j = 0;
k = i/j;
}
}
