Exercises for the Java and HTML course (Beginners Course)

Common Programming Errors of Beginners

Problem 1.

The basic data types can be modified by access modifiers. For example the keyword public is an access modifier. In Java, these access modifiers describe who can use the method. The keyword public means that any method in any class that has access to an instance of the Error1 class can call the method. The keyword static is another access modifier. Static variables and methods (member functions) in Java are the same as static data members and member functions in C/C++. The original purpose of static in C/C++ was to denote local variables that do not go away when the local scope is exited. In that context, the term static indicates that the variable stays around and is still there when the block is entered again. The combination of the three access modifiers public static final is used for a global constant. This global constant cannot be changed.

The error message is: can't assign a value to a final variable

 
// Error1.java 

public class Error1 
{ 
public static final int a = 92;
public static final int b = -87;
public static void main(String[] args)
{
a = a + b;
System.out.println(a);
}
}

Problem 2.

The left curly bracket { stands for begin block and the right curly bracket } stands for end block. The problem at the next program is that the variable b goes out of scope. The error message is:

undefined variable b

 
// Error2.java

public class Error2
{
     public static void main(String[] args)
     {
     int a = 7;
     {
     int b = 8;
     }
     int result = a + b;
     System.out.println(result);
     }
}

Problem 3.

We recall that Java is case sensitive. To solve the problem with the next program, have a look at the table of keywords. The error message is:

Invalid expression statement

 
// Error3.java

public class Error3
{
    public static void main(String args[])
    {
    int true = 1;
    int false = 0;
    int result = true*false;
    System.out.println("The result is " + result);
    }
}

Problem 4.

In Java the data types on the left and right hand sides of = must be the same. Type conversion must be called explicitly. The two error messages are:

 Incompatible type of declaration. 
Explicit cast needed to convert int to byte.

 
// Error4.java

public class Error4
{
  public static void main(String[] args)
  {
  byte j = -128;
  byte k = 127;
  byte l = j + k;
  System.out.println(l);
  }
}

Problem 5.

What is the problem with the following code ? It compiles and runs, but the output is

numbers are the same. However, the numbers are not the same.

 
// Error5.java

public class Error5 
{
  public static void main(String[] args)
  {
  int i = -1281;
  int j = 1280;
  if(i == j);
  System.out.println("numbers are the same");
  }
}

Problem 6.

The following code gives the error message:

Incompatible type int to boolean. Can't convert int to boolean

What is the problem ? Fix it.

 
// Error6.java

public class Error6 
{
  public static void main(String[] args)
  {
  int i = -1280;
  int j = -1280;

  if(i = j)
  System.out.println("numbers are the same");
  }
}

Problem 7.

The following program compiles and runs, but the output is:

wrong values sum = -1

However the output should only be:

sum = -1.

 
// Error7.java

public class Error7
{
   public static void main(String[] args)
   {
   int n = 2;
   int sum = 0;

   switch(n)
   {
   case 1:
   sum++;
   break;

   case 2:
   sum--;

   default:
   System.out.println("wrong values ");
   break;
   }

   System.out.println("sum = " + sum);

   }
}

Problem 8.

Java clearly distinguishes between basic data types and abstract data types which are objects. The error message is:

no method machting addElement(double) found in Vector

However, there is a method addElement in the Vector class.

 
// Error8.java

import java.util.*;

public class Error8
{
   public static void main(String[] args)
   {
   Vector w = new Vector(4);

   double x = 3.14;
   char c = 'Z';
   int k = -50;

   w.addElement(x);
   w.addElement(c);
   w.addElement(k);

   double y = w.elementAt(0);
   System.out.println(y);

   char d = w.elementAt(1);
   System.out.println(d);

   int m = w.elementAt(2);
   System.out.println(m);
    }
}

Problem 9.

Every basic data type has a wrapper class, for example char -> Character

The error message is: incompatible type of declaration

The question is how to convert basic data types (such as char) to abstract data types (such as Character).

 
// Error9.java 

public class Error9
{
   public static void main(String[] args)
   {
   Character Ch = 'A';
   System.out.println(Ch);

   Character X = new Character('a');
   System.out.println(X);
   char x = X;
   System.out.println(x);
   }
}

Pitfalls for Beginners in Java

Problem 1.

What is the output of the following program? Surprisingly for the beginner it is not 9223372036854775808. Explain why.

 
// Pitt1.java

public class Pitt1
{
   public static void main(String[] args)
   {
   long a = 9223372036854775807L;
   long b = 1L;
   long result = a + b;

   System.out.println(result);
   }
}

Problem 2.

As in C++, the left shift operator is given by << and the right shift operator is given by >> . What is the output of the following program ? Notice that 2 (decimal) in binary is 10b. Furthermore notice that the data type int is signed and 32 bits long. What is the output of the following program ?

 
// Pitt2.java

public class Pitt2
{
  public static void main(String[] args)
  {
  int a = 2;
  int b = a << 30;
  System.out.println(b);
  int c = a << 31;
  System.out.println(c);
  int d = a << 32;
  System.out.println(d);
  }
}

Problem 3.

The following program compiles without any problem. However when we run the program it goes on forever. Explain why.

 // Pitt3.java
public class Pitt3
{
public static int f1(int x)    {   return 2*f2(x);   }
public static int f2(int y)   {   return 3*f3(y);   }
public static int f3(int z)   {   return 4*f1(z);   }

public static void main(String[] args)
{
 int a = 6;
 int result = f3(a);
 System.out.println(result);  // =>
 }
}

Problem 4.

To compare whether strings are equal we use the method equals in the String class. Can we also use == ? What is the output of the program ?

 
// Pitt4.java

public class Pitt4
{
    public static void main(String[] args)
    {
    String s1 = "ABC";
    String s2 = "ABD";
    if(s1 == s2)
    System.out.println(s1);
    else
    System.out.println(s2);

    boolean bool = s1.equals(s2);
    System.out.println(bool);
    }
}

Problem 5.

The output of the following program is that The letter C is the same although C and D differ.

 
// Pitt5.java

public class Pitt5
{
    public static void main(String[] args)
    {
    String s1 = new String("ABC");
    String s2 = new String("ABD");

    for(int i=0;i < 3;i++)
    {
    if (s1.charAt(i) == s2.charAt(i));
    {
    System.out.println("The letter " + s1.charAt(i) + " is the same");
    }
    }
    }
}

Problem 6.

In C++ we can use references or pointers to swap two basic data types, for examples two integers. How can we translate the following C++ program into Java ? The problem is that in Java we cannot pass basic data types by reference.

 
// Pitt6.java

#include <iostream.h>

void swap(int& a,int& b)
{
   int temp = a;
   a = b;  b = temp;
}

void main()
{
   int x = 7;
   int y = -4;
   swap(x,y);
   cout << "x = " << x << endl;  // => 4
   cout << "y = " << y << endl;  // => 7
}

Problem 7.

Surprisingly for C++ programmers the following code compiles and runs under Java. However, there is a big problem with this code. Explain why. Look at the output. Fix the problem.

 
// Pitt7.java

public class Pitt7
{
   public static void main(String args[])
   {
   double x[] = { 2.2, 4.5, 3.1 };
   double y[] = { 4.1, 5.6, -8.9 };

   y = x;

   for(int i=0; i < y.length; i++)
   {
   System.out.println("y[" + i + "] = " + y[i]);
   }

   x[0] = -17.4;
 
   for(int i=0; i < y.length; i++)
   {
   System.out.println("y[" + i + "] = " + y[i]);
   }
   }
}

Programming Assignment

Problem 1.

Write a Java application using two for loops to produce the following pattern of asterisks.

*
**
***
****
*****
******
*******
********

Can this problem be solved with one for loop?

Problem 2.

Write an applet that displays a rectangle and a line. The line should intersect the rectangle at two opposite corners.
Hints. Obviously we use the

 
void paint(Graphics g)

method (function) from the Component class. To draw the rectangle and the line we use the methods

 
drawRect(int,int,int,int)

and

 
drawLine(int,int,int,int)

which are in the Graphics class.

Problem 3.

Read in three integers of data type long from the keyboard. Then add the three numbers. Type convert the sum to data type double and divide by 3.0 (data type double ) to find the arithmetic mean.

Problem 4.

From version 1.1 Java includes a class BigInteger to handle verylong integers. Write a Java program that creates two verylong integers, for example 8912345678 and -7892033234567 and then subtract and divide the two numbers. Convert the first verylong integer into a double . Then display the result on the screen.

Hints. Select the proper constructor. The constructor you should use is


BigInteger(String str)

which translates the decimal String representation of a BigInteger into a BigInteger. The method to subtract the verylong numbers is


BigInteger subtract(BigInteger val)

The method to divide the verylong numbers is


BigInteger divide(BigInteger val)

The method to convert from a BigInteger to a double is


double doubleValue()

Do not forget to import the BigInteger class ( java.math.BigInteger ).

We recall that in Java we cannot overload operators such as +, -, *, /, % as we can in C++.

Problem 5.

Write and run a Java program that enters an 8-digit string for a birthdate from the keyboard. The first two digits in the string are the month of birth, the next two are the day and the remaining four are the year. The Java program should squeeze out these substrings and display it on the screen as follows:

month of birth:
day of birth:
year of birth:

Hint. Use the String class and the method

public String substring(int beginIndex,int endIndex)

Try to add exception handling for the case if the month and day are out of range.

Problem 6.

The following Java program compiles and runs. However, when you try to close the window it will not close. Under MS Windows use Crt+Alt+Del and End Task to kill the process. Under LINUX you have to kill the process. Fix the program so that you can close the window.

// MyFrame.java
//
// A frame is a window that has a title bar, a menu bar, a border,
// a cursor, and an icon image. Frame windows are the standard objects
// used for drawing graphics.
//
// Java uses the keyword super to refer to members of the parent class. 
// super(s) invokes the constructor in the Frame class
//
// public Frame(String title)
// constructs a new, initially invisible Frame object with the
// specified title.

import java.awt.Frame;
import java.awt.Button;

class ButtonFrame extends Frame
{
   Button b;

   ButtonFrame(String s)
   {
   super(s);
   setSize(200,100);
   setLocation(400,50);
   b = new Button("Click me!");
   add(b);
   setVisible(true);
   }
}

class MyFrame
{
   public static void main(String[] args)
   {
   ButtonFrame buttonFrame = new ButtonFrame("MyFrame");
   }
}

Problem 7.

Write a text file counter.dat which contains the integer 0. Then write a Java application that reads this file and increments the integer by one. Then the new integer is written back to the file counter.dat . This means every time we open and close the file the integer number is incremented by 1.

Problem 8.

The following Applet simulates a digital clock. The Applet compiles and runs, but the output is only:

 0 : 0 : 0

Explain why. Fix the problem. To what values are the basic data types initialized ?

// Clock.java
import java.applet.Applet;
import java.awt.*;
import java.util.Calendar;

// The Runnable interface should be implemented by any class
// whose instance are intended to be executed by a thread.

public class Clock extends Applet implements Runnable
{
  Thread thread;
  Font font = new Font("Monospaced", Font.BOLD, 64);
  int hour, minute, second;


  // Applet class: called by the browser or appletviewer
  // to inform this applet that it has been loaded into the system  
  public void init()
  {
  if(thread == null)
  {
  thread = new Thread();
  thread.start();
  }
  }

  // override the run() method from Thread class
  public void run()
  {
  for(;;)  // forever
  {
  Calendar time = Calendar.getInstance();
  hour = time.get(Calendar.HOUR);
  minute = time.get(Calendar.MINUTE);
  second = time.get(Calendar.SECOND);
  repaint(500); // request re-drawing every 500 milliseconds
  try { Thread.sleep(500); } catch(Exception e) { }
  }
  }

  public void paint(Graphics g)
  {
  g.setFont(font);
  String time = String.valueOf(hour)
                + ":" + String.valueOf(minute)
                + ":" + String.valueOf(second);
  g.drawString(time, 50, 50);
  }
}

Projects

Project 1.

Write a class SoccerLeague. The soccer league has 8 teams with the names:

"PinkSocks", "WoundedKnee",
"FlatFoot", "LameDogs",
"NoScore", "UnReal", 
"WetPants", "Grasshoppers".

The names of the teams should be stored in a file teams.dat. The method setupMatches creates all the matches (4 for each weekend) for the 7 weekends. Of course each team plays exactly once against each other team. All the matches should be stored in a data file setup.dat. At each weekend the four matches should be retrieved from the data base and the results filled into the data file setup.dat. Then a method showTable should calculate the points (3 for a win, 1 for a draw, and 0 for a loss) and put into the file teams.dat. Then the ranking table should be displayed with the proper ranking for each row.

Number of the ranking, 
the name of the team, 
number of matches played,
the number of points, 
number of goals scored, 
number of goals received.

If the number of points are the same then the goal difference should be used for the ranking. If this is also the same the team with more goals should be placed higher. For example after 2 weekends the table could be

1 FlatFoot    2 6 9 2 
2 WoundedKnee 2 4 6 3
3 LameDogs    2 4 4 1
...........

Project 2.

Write a Java program with a class Rational to do mathematics with rational numbers. The class Rational should include the method add for adding two rational numbers and a method multiply to multiply two rational numbers. The underlying data type should be long . Can you modify the code so that the data type BigInteger can be used ?

Project 3.

Rewrite the following code using the abstract data type BigInteger. This means int i; int n; int d and the number 1000 must be replaced by BigInteger. The methods in class BigInteger we can use are:

add, equals, mod, compareTo

int compareTo(BigInteger val)

returns -1, 0, 1 as this BigInteger is numerically less than, equal to, or greater than val. Fields in the class BigInteger are

static BigInteger ONE
static BigInteger ZERO

There is not method sqrt in class BigInteger. But somehow we can use

BigInteger pow(int exponent)

// TestPrime.java
public class TestPrime
{
   public static void main(String [] args)
   {
   for(int i=0; i < 1000; i++)
   {
   if(isPrime(i)) 
   System.out.print(i + " ");
   }
   }  // end main

   static boolean isPrime(int n)
   {
   if(n < 2) return false;
   if(n == 2) return true;
   if(n%2 == 0) return false;
   for(int d=3; d < Math.sqrt(n); d += 2)
   if(n%d == 0) return false;
   return true;
   }  // end isPrime
}

Project 4.

Write a Java application that implements the TV show "Who wants to be a millionare''.

Project 5.

Rewrite the program ButtonGame.java on page 100 using the Swing components, i.e. JButton, JPanel, JTextField etc.