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;
 }

}