Making of Tetris Game in Java

Here in this post, i will share a Java program to make a Tetris game. I have already wrote a post on how to make a tetris game in C language. I have discussed the data structures used in the program in that post. The Java program is almost similar to it. The description is available in this post: Making of Tetris Game in C.

The following video shows the java code in action:


The tetris game Java source code is as follows:

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JOptionPane;



public class Tetris extends JFrame{

boolean stopthread=false;
int SIZE_HORIZ=13;
int SIZE_VERTI=35;
Graphics g;
boolean waitnow=false;
int[][] board=new int[SIZE_VERTI][SIZE_HORIZ];
int t_[]={1,0,1,1,1,2,2,1};
int t_90[]={0,1,1,0,1,1,2,1};
int t_180[]={0,1,1,0,1,1,1,2};
int t_270[]={0,1,1,1,1,2,2,1};
int l_[]={0,2,1,0,1,1,1,2};
int l_90[]={0,1,1,1,2,1,2,2};
int l_180[]={1,0,1,1,1,2,2,0};
int l_270[]={0,0,0,1,1,1,2,1};
int s_[]={0,0,0,1,1,0,1,1};
int z_[]={1,1,1,2,2,0,2,1};
int z_90[]={0,1,1,1,1,2,2,2};
int i_ver[]={0,1,1,1,2,1,3,1};
int i_hor[]={1,0,1,1,1,2,1,3};
String scorestr;
Random rand=new Random(System.currentTimeMillis());
Color colors[]={Color.BLUE,Color.GREEN,Color.MAGENTA,Color.RED,Color.YELLOW};
File HighScore;
Color bgcolor=Color.WHITE;
/*
Numbering for blocks:
(values of fallingBlockNumber)
0=T
1=L
2=S
3=Z
4=I
*/
int [] blockarray;
int fallingblockNum;
int fallingBlockVersion=0;
int fallingBlockRow=0;
int fallingBlockCol=0;
int startdelay=200;
int motiondelay;
int scoreInc=5;
int myscore=0;
int tversion;
boolean spawn=true;
int scorespeedctrl=0;
int timehalving=0;
String highscoreholder;
int highscore;
KeyListener kl;

public Tetris()
{
int i,j;
startdelay=250;
scoreInc=5;
myscore=0;
motiondelay=startdelay;
NextBlock();
for(i=0;i<35;i++)
    for(j=0;j<13;j++)
 board[i][j]=0;
setDefaultCloseOperation(EXIT_ON_CLOSE);
setSize(480,500);
setResizable(false);
setLocationRelativeTo(null);
HighScore=new File(System.getProperty("java.io.tmpdir")+File.separator+"TetrisHighScore");
try{
if(HighScore.exists())
    {
    BufferedReader br=new BufferedReader(new FileReader(HighScore));
    highscoreholder=br.readLine();
    highscore=Integer.parseInt(br.readLine());
    br.close();
    }
else
    {
    highscoreholder=null;
    highscore=0;
    }
}catch(Exception ex){}
kl=new KeyListener() {
    @Override
    public void keyTyped(KeyEvent e) {}

    @Override
    public void keyPressed(KeyEvent e) {
    
        if(!spawn&&!waitnow)
 {
            waitnow=true;
            int key=e.getKeyCode();
     if(key==KeyEvent.VK_UP)//up
  {
  if(fallingblockNum==0||fallingblockNum==1)
      tversion=(fallingBlockVersion+1)%4;
  else if(fallingblockNum==4||fallingblockNum==3)
      tversion=(fallingBlockVersion+1)%2;

  if(fallingblockNum!=2&&isDrawable(fallingBlockRow,fallingBlockCol,tversion))
      {
      clearOldBlockVersion(g);
      fallingBlockVersion=tversion;
      blockarray=getFallingBlockArray();
      drawNewBlockVersion(g);
      }
  }
     else if(key==KeyEvent.VK_LEFT)//left
  {
  if(isDrawable(fallingBlockRow,fallingBlockCol-1,fallingBlockVersion))
      {
      clearOldBlockVersion(g);
      fallingBlockCol--;
      drawNewBlockVersion(g);
      }
  }
     else if(key==KeyEvent.VK_RIGHT)//right
  {
  if(isDrawable(fallingBlockRow,fallingBlockCol+1,fallingBlockVersion))
      {
      clearOldBlockVersion(g);
      fallingBlockCol++;
      drawNewBlockVersion(g);
      }
  }
     else if(key==KeyEvent.VK_DOWN)//down
  {
  if(isDrawable(fallingBlockRow+1,fallingBlockCol,fallingBlockVersion))
      {
      clearOldBlockVersion(g);
      fallingBlockRow++;
      drawNewBlockVersion(g);
      }
  }
            waitnow=false;
 }
    }
    @Override
    public void keyReleased(KeyEvent e) {}
};
th.start();
addKeyListener(kl);
}

void NextBlock()
{
fallingblockNum=rand.nextInt(5);
if(fallingblockNum==0||fallingblockNum==1)
 fallingBlockVersion=rand.nextInt(4);
else if(fallingblockNum==4||fallingblockNum==3)
 fallingBlockVersion=rand.nextInt(2);
else
 fallingBlockVersion=0;
fallingBlockRow=0;
fallingBlockCol=5;
blockarray=getFallingBlockArray();
}

int[] getFallingBlockArray()
{
int a=fallingblockNum*10+fallingBlockVersion;
switch(a)
    {
    case 0:return (t_);
    case 1:return (t_90);
    case 2:return (t_180);
    case 3:return (t_270);
    case 10:return (l_);
    case 11:return (l_90);
    case 12:return (l_180);
    case 13:return (l_270);
    case 20:return (s_);
    case 30:return (z_);
    case 31:return (z_90);
    case 40:return (i_hor);
    case 41:return (i_ver);
    }
return (i_ver);
}

boolean isDrawable(int newrow,int newcol,int blockversion)
{
int i,tempversion;
boolean flag=true;
tempversion=fallingBlockVersion;
fallingBlockVersion=blockversion;
blockarray=getFallingBlockArray();
for(i=0;i<8;i+=2)
    {
    if(newrow+blockarray[i]>34||newrow+blockarray[i]<0)
 {
 flag=false;
 break;
 }
    if(newcol+blockarray[i+1]>12||newcol+blockarray[i+1]<0)
 {
 flag=false;
 break;
 }
    if(board[(newrow+blockarray[i])][(newcol+blockarray[i+1])]==2)
 {
 flag=false;
 break;
 }
    }
fallingBlockVersion=tempversion;
blockarray=getFallingBlockArray();
return flag;
}

void clearOldBlockVersion(Graphics g)
{
int i,r,c;
for(i=0;i<8;i+=2)
 {
 r=fallingBlockRow+blockarray[i];
 c=fallingBlockCol+blockarray[i+1];
 board[r][c]=0;
 g.setColor(bgcolor);
        g.fillRect(8+c*13,32+r*13,14,14);
 }
}

void drawNewBlockVersion(Graphics g)
{
int i,r,c;
for(i=0;i<8;i+=2)
 {
 r=fallingBlockRow+blockarray[i];
 c=fallingBlockCol+blockarray[i+1];
 board[r][c]=1;
        g.setColor(colors[fallingblockNum]);
 g.fillRect(8+c*13,32+r*13,13,13);
        g.setColor(Color.BLACK);//cyan,orange
 g.drawRect(8+c*13,32+r*13,13,13);
 }
}
boolean isGameOver(Graphics g)
{
if(isDrawable(0,5,fallingBlockVersion)==false)
    return true;
drawNewBlockVersion(g);
if(isAtBottom())
    return true;
return false;
}

boolean isAtBottom()
{
int i,max=0,ti,tj;
for(i=0;i<8;i+=2)
    if(blockarray[i]>max)
 max=blockarray[i];
if(fallingBlockRow+max>=34)
 return true;
for(i=0;i<8;i+=2)
    {
    ti=blockarray[i]+fallingBlockRow;
    tj=blockarray[i+1]+fallingBlockCol;
    if(board[ti+1][tj]==2)
       return true;
    }
return false;
}

void showScore(Graphics g)
{
int left,top;
left=getWidth()-100;
top=getHeight()/2;
g.setColor(bgcolor);
g.fillRect(left,top,80,70);
g.setColor(Color.RED);
g.setFont(new Font("Arial",Font.BOLD,14));
g.drawString("Score: "+Integer.toString(myscore),left,top+20);
}

void CollapseFilledRow(Graphics g)
{
int i,j,k,sum,copyskipover=0,r;
for(i=34;i>=0;)
    {
    sum=0;//full flag
    for(j=0;j<13;j++)
 sum+=board[i][j];
    if(sum==2*13)//row full
 {
 myscore+=scoreInc;
 copyskipover++;
 }
    if(sum==0)
 break;
    i--;
    if(copyskipover>0)
 {
 for(j=0;j<13;j++)
     {
     r=i+copyskipover;
     board[r][j]=board[i][j];
     if(board[i][j]==0)
  {
                g.setColor(bgcolor);
                g.fillRect(8+j*13,32+r*13,14,14);
  }
     else
  {
                g.setColor(Color.GREEN);
                g.fillRect(8+j*13,32+r*13,13,13);
                g.setColor(Color.BLACK);
                g.drawRect(8+j*13,32+r*13,13,13);
  }
     }
 }
    }
for(k=0;k<copyskipover;k++)
    {
    r=i+k;
    for(j=0;j<13;j++)
 {
 board[r][j]=0;
        g.setColor(bgcolor);
 g.fillRect(8+j*13,32+r*13,14,14);
 }
    }
showScore(g);
}


public static void main(String[] args)
{
Tetris tt=new Tetris();
tt.setVisible(true);
}

void GameOver(Graphics g)
{
stopthread=true;
g.setColor(Color.RED);
g.setFont(new Font("Arial",Font.BOLD,28));
String str="Game Over.";
g.drawString(str,getWidth()/2-10, getHeight()/2);
if(highscore>0)
    str="Highscore : "+highscoreholder+" - "+Integer.toString(highscore);
g.setFont(new Font("Arial",Font.BOLD,16));
g.drawString(str,getWidth()/2-30, getHeight()/2+80);
if(myscore>highscore)
    {
        highscoreholder=JOptionPane.showInputDialog("New high score. Enter your name:");
        highscore=myscore;
        try{
        if(!HighScore.exists())
            HighScore.createNewFile();
        BufferedWriter bw=new BufferedWriter(new FileWriter(HighScore));
        bw.write(highscoreholder);
        bw.newLine();
        bw.write(Integer.toString(highscore));
        bw.close();
        }catch(Exception ee){}
    }
}


Thread th=new Thread()
{
int i;
public void run()

{
try {
    while(!isShowing())
        Thread.sleep(1000);
    } catch (InterruptedException ex) {}
setOpacity(1f);
setBackground(bgcolor);
getContentPane().setBackground(bgcolor);
g=getGraphics();
g.setColor(Color.RED);
g.drawRect(6,30,13*13+6,35*13+6);
showScore(g);
while(!stopthread)
{
g.setColor(Color.RED);
g.drawRect(6,30,13*13+6,35*13+6);
while (waitnow)
    {
    try {
        Thread.sleep(20);
    } catch (InterruptedException ex) {}
    }
waitnow=true;
if(isAtBottom()&&!spawn)
 {
 for(i=0;i<8;i+=2)
     {
     board[fallingBlockRow+blockarray[i]][fallingBlockCol+blockarray[i+1]]=2;
     }
 spawn=true;
 CollapseFilledRow(g);
 }
    if(spawn)
 {
 NextBlock();
 blockarray=getFallingBlockArray();
 spawn=false;
 if(isGameOver(g))
     {
     GameOver(g);
     return;
     }
 }
    else
 {
 timehalving=(timehalving+1)%3;
 if(timehalving==2)
  {
  clearOldBlockVersion(g);
  fallingBlockRow++;
  drawNewBlockVersion(g);
  }
 }
    scorespeedctrl=(scorespeedctrl+1)%140;
    if(scorespeedctrl==0&&motiondelay>0)
 {
 motiondelay-=12;
 scoreInc+=2;
        if(motiondelay<0)
            motiondelay=0;
 }
    waitnow=false;
    try {
        Thread.sleep(motiondelay);
            } catch (InterruptedException ex) {}
}
}
};
}

Java Code to Restart Windows

Are you thinking how to restart Windows from Java. You can restart Windows operating system by executing a system command. You can restart windows by executing the system command shutdown /r /t 1. The following Java code will restart the windows computer when executed.

try {
    Runtime.getRuntime().exec("shutdown /r /t 1");
    } catch (IOException ex) {}

How to Schedule a Shutdown in Java - Java Code to Schedule a Shutdown

In this post, we will see how to schedule a shutdown in a computer using java program. For this, we execute the appropriate shutdown command after identifying the operating system. The following java code will schedule a shutdown after a 10 minute from execution of the code.

String os=System.getProperty("os.name").toLowerCase();
String command;
try
{
if(os.contains("windows"))
    {
    command="shutdown /s /t 600";
    }
else if(os.contains("linux")||os.contains("mac os x"))
    {
    command="shutdown -h +600";
    }
else
    throw new Exception("Unsupported OS");
Runtime.getRuntime().exec(command);
}catch(Exception ex){
JOptionPane.showMessageDialog(null,ex.getMessage());
}

How to Logoff or Sign Out Windows Using Java Code

You can sign out the current user account from windows using java code. Sign out, log out or logoff button can be created using simple lines of code. To logout from Windows operating system using java code, use the following code:

try {
        Runtime.getRuntime().exec("shutdown /l");
    } catch (IOException ex) {}

As the above code shows, we are simply executing a cmd command shutdown /l. It is actually running shutdown.exe with parameter /l to force logoff. If you add above code in the actionPerformed() function of a button, the button becomes a logoff button. The following video will show an example in which a simple button is made into a logout button.

Java Program for Animated Solution for Tower of Hanoi Puzzle Problem

This is a java program to solve towers of hanoi puzzle problem. This java program give solution for tower of hanoi problem with any number of disks. This program gives animated solution for tower of Hanoi problem. It demonstrates solving the tower of hanoi problem using animation in java.

The solution of hanoi problem given by this program is always optimal. You can enter the number of disks first. This lets you solve puzzle of any level of difficulty. The program is as follows.

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.util.Random;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class HanoiAnimated extends JPanel{

static int tower[][];// the three towers' disks as stack
static int top[];//top of the three stacks
static int from,to;//moving 'from' tower number 'to' tower number
static int diskInAir;//number of disk moved (1 to n)
static int n,l,b,u;
static Color colors[]={Color.BLUE,Color.CYAN,Color.GREEN,Color.MAGENTA,Color.ORANGE,Color.PINK,Color.RED,Color.YELLOW};
public HanoiAnimated()
{
    tower=new int[3][10];
    top=new int[3];
}

static void push(int to, int diskno)
//putting disk on tower
{
tower[to-1][++top[to-1]]=diskno;
}

static int pop(int from)
//take topmost disk from tower
{
return(tower[from-1][top[from-1]--]);
}

Color getColor(int disknum)
{
return colors[disknum%8];
}

void drawStill(Graphics g)
{
int j,i,disk;
g.clearRect(0,0,getWidth(),getHeight());
for(j=1;j<=3;j++)
 {
 //draw tower
        g.setColor(Color.GRAY);
 g.fillRoundRect(j*l,u,5,b-u,1,1);
        
 //draw all disks on tower
 for(i=0;i<=top[j-1];i++)
  {
  disk=tower[j-1][i];
                g.setColor(getColor(disk));
  g.fillRect(j*l-15-disk*5,b-(i+1)*10,35+disk*10,10);
  }
 }
}

void drawFrame(Graphics g,int x,int y)
{
try{
drawStill(g);
g.setColor(getColor(diskInAir));
g.fillRect(x-15-diskInAir*5,y-10,35+diskInAir*10,10);
Thread.sleep(60);
}catch(InterruptedException ex){}
}

void animator(Graphics g)
//to show the movement of disk
{
int x,y,dif,sign;
diskInAir=pop(from);
x=from*l;
y=b-(top[from-1]+1)*10;
//taking disk upward from the tower
for(;y>u-20;y-=8)
 drawFrame(g, x, y);

y=u-20;
dif=to*l-x;
sign=dif/Math.abs(dif);
//moving disk towards a target tower
for(;Math.abs(x-to*l)>=24;x+=sign*12)
 drawFrame(g, x, y);
x=to*l;
//placing disk on a target tower
for(;y<b-(top[to-1]+1)*10;y+=8)
 drawFrame(g, x, y);
push(to,diskInAir);
drawStill(g);
}

void moveTopN(Graphics g, int n, int a, int b, int c) throws InterruptedException
//Move top n disk from tower 'a' to tower 'c'
//tower 'b' used for swapping
{
if(n>=1)
 {
 moveTopN(g,n-1,a,c,b);
 drawStill(g);
 Thread.sleep(700);
 from=a;
 to=c;
 //animating the move
 animator(g);
 moveTopN(g,n-1,b,a,c);
 }
}

public static void main(String[] args)
{

int i;
String s=JOptionPane.showInputDialog("Enter number of disks");
n=Integer.parseInt(s);
HanoiAnimated ha=new HanoiAnimated();
//setting all tower empty
for(i=0;i<3;i++)
 top[i]=-1;

//putting all disks on tower 'a'
for(i=n;i>0;i--)
 {
 push(1,i);
 }

JFrame fr=new JFrame();
fr.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
fr.setLayout(new BorderLayout());
fr.setSize(640,360);
fr.add(ha);
ha.setSize(fr.getSize());
fr.setVisible(true);
l=ha.getWidth()/4;
b=ha.getHeight()-50;
u=b-n*12;
//start solving
try{
ha.moveTopN(ha.getGraphics(),n,1,2,3);
}catch(Exception ex){}
}

}

Simplest UDP Chat Program to Run on Single Computer in Java

This is a very simple UDP chat program using java with GUI (Graphical User Interface). The program uses DatagramPackets to send messages over network. Remember, this program only to be run on a single computer (for network simulation purpose). If you want a real UDP program that works on any network, see this post: Chat Room Application using UDP in Java
Program explanation is given below the program.

import java.io.*;
import javax.swing.*;
import java.net.*;
import java.awt.event.*;
import java.util.*;

public class UDPChat extends JFrame implements ActionListener
{
DatagramSocket ds;
JTextField txt;
JButton send;
JTextArea msgs;
int myport;
int destport;
DatagramPacket p;
byte[] b=new byte[200];

public UDPChat()
{
try{
try{
ds=new DatagramSocket(9999);
myport=9999;
p=new DatagramPacket(b,200);
ds.receive(p);
destport=p.getPort();
}catch(Exception ex)
{
ds=new DatagramSocket(0);
myport=ds.getLocalPort();
destport=9999;
String msg="hi";
p=new DatagramPacket(msg.getBytes(),msg.length(),InetAddress.getLocalHost(),destport);
ds.send(p);
}
setLayout(null);
txt=new JTextField();
send=new JButton("Send");
msgs=new JTextArea();
setSize(300,600);
add(txt);
txt.setSize(220,25);
txt.setLocation(5,5);
add(send);
send.setSize(70,25);
send.setLocation(230,5);
add(msgs);
msgs.setSize(300,530);
msgs.setLocation(0,70);
send.addActionListener(this);
receive.start();
}catch(Exception ex){ex.printStackTrace();}
}

public void actionPerformed(ActionEvent ae)
{
try{
DatagramPacket pkt=new DatagramPacket(txt.getText().getBytes(),txt.getText().length(),InetAddress.getLocalHost(),destport);
ds.send(pkt);
}catch(Exception e){e.printStackTrace();}
txt.setText("");
}

Thread receive=new Thread()
{
public void run()
{
try{
while(true)
 {
 DatagramPacket dp=new DatagramPacket(b,200);
 ds.receive(dp);
 msgs.setText(msgs.getText()+"\n"+new String(b));
 }
}catch(Exception exx){exx.printStackTrace();}
}
};

public static void main(String[] args)
{
UDPChat s=new UDPChat();
s.setVisible(true);
}

}

This simple program is enough to run as server or client. When the first user runs the application, it opens a DatagramSocket with port number 9999 (see line 22 in code). Since no Exception will be thrown from this line, it sets myport as 9999. Then waits to receive a DatagramPacket from the other user. When the packet is received, it takes destport from the received packet. Then starts adding GUI elements and then starts the thread to receive messages.

But when the second instance (client) of the program is run, line 22 in code throws a SocketException. Thus execution goes to the catch block. There it opens a DatagramSocket at some random port (ephemeral port assigned by OS). Then, sends a packet to port 9999 in the same PC. This time the first instance (server) is still waiting for a packet as said earlier. It receives this DatagramPacket sent by client a gets its destination port. The client's destport is 9999, as it knew it is already occupied by the SocketException.

The sending of message is carried out when send button is clicked. We use an ActionListener for that. The method public void actionPerformed(ActionEvent ae) is invoked whenever the button is clicked.

Java Program to Display a String in Reverse Order Without Using Loop or Another Array

In this post, we will see a java program to reverse a string without using any loops or temporary array. Such programming questions are often asked in interviews. It is so simple. We make use of recursion to store the string temporarily. Actually, we do not use any loop. We can easily do it without using any loops. Nor we need to declare an extra array. But when we make use of recursion, stack is used internally for calling each instance of the recursive method. Remember, we should not use the reverse method in StringBuilder class or StringBuffer class. The following java program displays the string in reverse order.

import java.util.Scanner;

public class StrRev{
static int len;
static String str;

static void recurse(int i)
{
    if(i<len-1)
        recurse(i+1);
    System.out.print(str.charAt(i));
}

public static void main(String[] arg)
    {
    System.out.println("Enter string");
    Scanner sc = new Scanner(System.in);
    str=sc.nextLine();
    len=str.length();
    recurse(0);
    }
}

In the above java program we just displayed the string in reverse order. we did not reverse the string in memory. In the following java program, we will see how to rearrange the string in reverse order without using any loops or extra arrays.

import java.util.Scanner;

public class StrRev{
static int len;
static char[] str;

static void recurse(int i)
{
    char a=str[i];
    if(i<len-1)
        recurse(i+1);
    str[len-i-1]=a;
}

public static void main(String[] arg)
    {
    System.out.println("Enter string");
    Scanner sc = new Scanner(System.in);
    str=sc.nextLine().toCharArray();
    len=str.length;
    recurse(0);
    System.out.println(String.copyValueOf(str));
    }
}

In the above java program, we stored the input string initially as a character array. Using a recursive method, we rearranged the character array in reverse order. Then we displayed the character array as a string

Java Program to Find Minimum of Two Numbers

Java Program to compare three numbers and display the minimum of two numbers. Here, we will see a simple java program to display the smaller number among two input numbers. This program takes first input as the number of sets of numbers (a set contains two numbers whose minimum is to be displayed). Then it takes numbers as sets of 2 numbers. For every pair of numbers, the java program displays the smaller number. The following java program does it. It makes use of the ternary operator (conditional operator) and Scanner class in java.

import java.util.Scanner;

public class MinofTwo {

public static void main(String[] arg)   
    {
    Scanner sc = new Scanner(System.in);
    int n=sc.nextInt(),a,b;
    while(n>0)
        {
        System.out.println((a=sc.nextInt())<(b=sc.nextInt())?a:b);
        n--;
        }
    }
}

Java Program to Find Minimum of Three

It is the basics of any programming language to compare three numbers and display the minimum of the three. Here, we will see a short java program to display the smallest number among three input numbers. This program takes first input as the number of sets of numbers (a set contains three numbers whose minimum is to be displayed). Then it takes numbers as sets of three numbers. for every three number, the java program displays the smallest of them. The following program does it. It makes use of the ternary operator (conditional operator) and Scanner class.

import java.util.Scanner;

public class MinOfThree {
public static void main(String[] arg)
 
    {
    Scanner sc = new Scanner(System.in);
    int n=sc.nextInt(),a,b,c;
    while(n>0)
        {
        if((a=sc.nextInt())>(b=sc.nextInt()))
            System.out.println(b<(c=sc.nextInt())?b:c);
        else
            System.out.println(a<(c=sc.nextInt())?a:c);
        n--;
        }
    }
}

Java Program to Reverse a String

Here, we will see a java program to reverse a string. The string is read and is displayed in reverse order. Java program to display an input string in reverse order.

import java.util.Scanner;

public class BMI {
public static void main(String[] arg)
    {
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter a string");
    String s=sc.nextLine();
    for(int i=s.length()-1;i>=0;i--)
            System.out.print(s.charAt(i));
    }
}

Body Mass Index Calculator Using Java - Java Application Code

Body Mass Index Calculation is based on your weight and height. Body Mass Index (BMI) = weight/(height*height) where weight is in kilograms and height in meters. The following Java program takes weight and height as input and displays BMI and grades into underweight,Normal, Overweight or obesity.

import java.util.Scanner;

public class BMI {

public static void main(String[] arg)
    {
    Scanner sc=new Scanner(System.in);
    System.out.println("Enter weight in kgs and height in meters");
    float bmi=sc.nextFloat()/(float)Math.pow(sc.nextFloat(),2);
    String s=(bmi<18.5)?"Underweight":(bmi<25)?"Normal weight":(bmi<30)?"Overweight":"Obesity";
    System.out.println("bmi: "+bmi+"   "+s);
    }
}

Calculate your BMI here

Weight (in kg):
Height (in meters):

UnitCon - Universal Unit Converter and Currency Converter for Computers in Java

UnitCon Unit and Currency converter is a free unit converter software for computer. It allows you to

UnitCon Free Unit Conversion Software and Currency converter

convert between units of about 20 different physical quantities. It also has a currency converter functionality which allows conversion between currencies of almost all countries. The unit conversion functionality is completely offline. But for currency conversion, internet connection is required.

The different physical quantities for which unit conversion is available are:

1. Acceleration
2. Angle
3. Angular Acceleration
4. Angular Velocity
5. Area
6. Capacitance
7. Density
8. Digital Storage
9. Distance
10. Electric charge
11. Energy
12. Force
13. Frequency
14. Luminous Intensity
15. Mass
16. Power
17. Solid Angle
18. Speed or Velocity
19. Temperature
20. Time
21. Torque



Free unit converter and currency converter software

22. Volume

And after all, currency conversion is also available. The software is coded in java. You may require to download and install Java Runtime Environment 1.8 or later if you don't have it installed. The application (below 300KB) is available for free download. The source code of the unit converter is also available for free download.

Click here to free download UnitCon unit converter and currency converter application

Click here to download UnitCon unit converter and currency converter java project Source code

Remote Method Invocation (RMI) in Java - Example Program to Calculate Power using RMI

Remote Proceure Call (RPC) is a protocol by which a computer can call a function in another computer connected through network. The caller passes arguments through network and the callee executes the function and returns the output to the caller. In object oriented languages we use RMI (Remote Method Invocation) instead of RPC. Let us see a sample java program which uses RMI. .

Here, we call the computer that hosts the method, a server. And those computers which call this method are called clients. More precisely, RMI server and RMI clients. RMI clients call remote methods in RMI servers.

Our example program is answer to the following question:

Write a java program that uses RMI in which RMI clients can calculate powers of numbers by invoking a remote method in RMI server.

Java Program to Send a File over a Network after Encryption

Here is a simple java program which can be used to send and receive encrypted files over network. The sender encrypts the file before sending and the receiver decrypts the file and displays it. The program is based on a networking lab experiment at my college. So, although we say 'encryption', characters are simply incremented here. The lab question was as follows:

A client wants a file that is with a server in a secure system. The server encrypts the file in such a way that 'A' will be replaced with 'B', 'B' with 'C'... and 'Z' with 'A' and sends to the client. The client should get file and decrypt it to the original one and display the content. Implement it using TCP/IP.

How to Load a Webpage in a Java App

Do you wonder how to load a webpage in your java application? It is simple. You can load a web document whether it is a web document in internet or a web document from a file in the localhost. I know three methods to do it. You can use JTextPane in java swing (javax.swing), JEditorPane in java swing (javax.swing) or WebView in javaFX (javafx.scene.web.WebView). We will discuss each method to display a webpage in a java application.

The three methods we are going to discuss are:

• Using JTextPane
• Using JEditorPane
• Using WebView in JavaFX

Java FileNotFoundException While File is Existing There

Sometimes you my encounter a FileNotFoundException in java while you can see that the specified file is there. This can happen for several reasons.The java code may throw a  FileNotFoundException even when the file exists. This can be for different reasons.

Reason 1

The file does not exist really. Although you can see the file, the path you may have entered may be incorrect. For example, the following code may throw exception in some operating systems:

File f=new File("D:\myfile.txt");

This is because the file separator character is '\' in windows os. since \ is used to display non graphical characters like '\n' you should use its corresponding escape sequence. So "D:\\myfile.txt" would work on windows. If your java application is a cross platform program, you should consider using File.separator instead of '\\' because not all operating systems use '\\' as file separator character. So, the best approach is ;

File f=new File("D:"+File.separator+"myfile.txt");

File.separator is the system-dependent default name-separator character, represented as a string for convenience. This string contains a single character, namely File.separatorChar. If you want to get this character as a Character itself, you can use File.separatorChar. There can be other mistakes in the path.

Reason 2

The name you entered as a file may be actually a directory. So an exception can be thrown in java when you try to read from or write to such File object.

Reason 3

You cannot open it for reading or writing for some other reasons. You can check the status of file by calling the methods like f.exists(), f.canRead(), f.canWrite(), f.isFile() and f.isDirectory(). All these methods return a boolean. I hope these java method names describe well what they are intended for.

How to Get User Related Informations in Java

In java, there are some methods that will help you to know some user related informations. Here also,

System.getProperty() 

method can give some informations about the user like:

User account name
User Working directory
User Home directory

To get the username of current user on the computer just call

System.getProperty("user.name")

It will return the current username as a string.

To get the working directory of current user on the computer call

System.getProperty("user.dir")

It will return the path of working directory of current user as a string.

To get the home directory (user's home folder) of current user on the computer call

System.getProperty("user.home")

It will return the path to home directory (folder) of current user as a string.

How to Get Operating System Information in Java

Using java, you can gather some information about the operating system of the computer system on which the code is running. That us java provides some API classes that will help you to know about the operating system of the computer on which your application program is running. You can use System.getProperty() method for this purpose. There are some system property keys (strings) which can be used to get the respective information. The parameter to the System.getProperty() method is a string, the property key. The following values for the parameter string will give operating system related informations.

"os.arch"
Operating system architecture

"os.name"
Name of the operating system.

"os.version"
Version of Operating system.

Usage examples:

The following java program displays the Operating system related informations using getProperty() method.

System.out.println("Operating system architecture:"+System.getProperty("os.arch") ); System.out.println("Operating system Name:"+System.getProperty("os.name") ); System.out.println("Operating system version:"+System.getProperty("os.version") );

How to Get JRE installation Directory in a Java Application

To get the java runtime environment (JRE) installation directory of a computer, we can use the getProperty() method in System class. The key to be used for System.getProperty() method to get the java JRE installation directory is "java.home". System.getProperty("java.home") will return the java runtime environment(JRE) installation directory path as a string. the following code will display the jre installation directory path:

System.out.println(System.getProperty("java.home"));

How to Get Class Path in a Java Application

Class Path is the Path used to find directories and JAR archives containing class files. Elements of the class path are separated by platform-specific characters. To get class path from a running java application, you may use the getProperty() method in System class. The following java code will show you how to get class path in java.

String classpath=System.getProperty("java.class.path");
System.out.println("Class path="+ classpath);