Calling a constructor within a constructor in JAVA

Constructor is basically used to initialize variables, but sometimes we have more than one constructor in our class which is also known as Constructor overloading. The object we create is actually the call to constructor and since the name of constructor is same as the name of class so we often call them the object of class.
Now suppose we have a constructor and we want to call another constructor inside it, then this can be done with the help of this keyword.

class abc{
    int i, j;
    abc(){
        i = 10;
    }
    abc(int j){
        this();
        this.j = j;
        //this(); calling this() here will give us error because 
        //   what if we started using i and it had not been initialised
        // thats why we must call this or super in first line while calling constructors
    }
    public static void main(String[] args){
        abc a = new abc(6);
    }
}

this keyword represents object of its own class. It can be used for two purposes, one is two access class member with in the class itself or in more simple language whenever we have to distinguish between a local variable and a class variable we have to use this keyword, like we did in our example 'this.j = j' it means assign the value of local variable j to the class variable j.
Second use of this keyword is to call a constructor within in a constructor.
Note: if you are calling constructor within a constructor than this must come in your first line.
In the above example we are calling a blank constructor within a parametrized constructor, so here first it will call the blank constructor and initializes variable i and then initializes variable j.
Similarly we can call parametrized construction within blank constructor like, this(5) as the first line, but we cannot do this for both the constructors, it will give you compile time error.

Snake Game Code in java

This is the working code of well known game Snake.

//package snake_game;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.GraphicsDevice;
import java.awt.GraphicsEnvironment;
import java.awt.RenderingHints;
import java.awt.Robot;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.InputEvent;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.util.Vector;

import javax.swing.ActionMap;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.KeyStroke;

//Each circle in snake body
class snake_node{
 int centerx, centery, radius, pcenterx, pcentery;
 snake_node(){
  
  radius = 10;
 }
}
//actual gui for game
public class Snake_game extends JFrame {
 snake panel = null;
 JMenuBar menu = null;
 JMenu controls = null;
 JMenuItem play = null;
 JMenuItem restart = null;
 JMenuItem instructions = null;
 boolean firstplay = true;
 Snake_game(){
  this.setResizable(false);
  menu = new JMenuBar();
  play = new JMenuItem("Play");
  controls = new JMenu("Controls");  
  restart = new JMenuItem("Restart");
  instructions = new JMenuItem("Instructions");
  restart.setMnemonic(KeyEvent.VK_R);
  restart.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_R, InputEvent.CTRL_MASK));
  restart.setMnemonic(KeyEvent.VK_I);
  instructions.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_I, InputEvent.CTRL_MASK));
  play.setMnemonic(KeyEvent.VK_P);
  play.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_P, InputEvent.CTRL_MASK));
  controls.add(play);
  controls.add(restart);
  controls.add(instructions);
  menu.add(controls);
  this.setJMenuBar(menu);
  //GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().setFullScreenWindow(this);
  panel = new snake();
  this.setLayout(new BorderLayout());
  this.setMinimumSize(new Dimension(800, 595));
  panel.dim = this.getSize();
  this.add(panel, BorderLayout.CENTER);
  this.setLocationRelativeTo(null);
  this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  play.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent eee){play();}});
  restart.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent ee){restart();}});
  instructions.addActionListener(new ActionListener(){public void actionPerformed(ActionEvent ee){instruct();}});
  this.addKeyListener(new KeyAdapter(){public void keyPressed(KeyEvent e){panel.keyPress(e);}});      
  this.setVisible(true);
  this.pack();
 }
        //function to start thread for starting game
 public void play(){
  if(firstplay){
   panel.t.start();
   firstplay = false;
   play.setEnabled(false);
  }
 }
        //function for restarting game, it will resize the snake to its initial position and initial length
 public void restart(){
  int pos = 0;
  panel.vector.clear();
  snake_node[] snode = new snake_node[5];
  for(int i =0; i < 5; i++){
   snode[i] = new snake_node();
   snode[i].centerx = 200;
   snode[i].centery = 150-pos;
   snode[i].pcentery = 150-pos;
   snode[i].pcenterx = 200;
   panel.vector.add(snode[i]);
   pos = pos+10;
  }
  panel.gameover = false;
  panel.dir = 'D';
  panel.repaint();
 }
        //function to be called on the click of instructions
 public void instruct(){
  JOptionPane.showMessageDialog(this, "1. Ctrl+R - Restart \n2. S - Increase Snake speed \n3. D - Decrease Snake speed \n4. LEFT - Turn left \n5. RIGHT - Turn right \n6. UP - go up \n7. DOWN - go down \n8. Ctrl+I - to view Instructions", "Instructions",1);
 }
 public static void main(String[] ar){
  new Snake_game();// object of the game class
 }
}

//a panel where all the drawings take place and it is embedded into JFrame
class snake extends JPanel implements Runnable{
 Vector<snake_node> vector = null;   //vector which will hold all the beads of snake body
 snake_node[] snode = null;
 int centx = 0, centy = 0;
 int inc = 10, pos = 0;
 char dir = 'D';
 Dimension dim = null;
 Thread t = null;
 boolean b = true, gameover = false;
 snake(){
  vector = new Vector();
  snode = new snake_node[5];
  for(int i =0; i < 5; i++){
   snode[i] = new snake_node();
   snode[i].centerx = 200;
   snode[i].centery = 150-pos;
   snode[i].pcentery = 150-pos;
   snode[i].pcenterx = 200;
   vector.add(snode[i]);
   pos = pos+10;
  }
  t = new Thread(this);
  this.setLayout(null);
  this.setBackground(Color.black);
  this.setDoubleBuffered(true);
  dim = this.getSize();
  this.addKeyListener(new KeyAdapter(){public void keyTyped(KeyEvent e){keyPress(e);}});
  this.setVisible(true);
 }
 int time = 70;
 boolean gamepause = true;
        //setting keyevents 
 void keyPress(KeyEvent e){
  if(e.getKeyCode() == KeyEvent.VK_DOWN){
   if(dir != 'U')
    dir = 'D';
  }
  else if(e.getKeyCode() == KeyEvent.VK_UP){
   if(dir != 'D')
    dir = 'U';
  }
  else if(e.getKeyCode() == KeyEvent.VK_LEFT){
   if(dir != 'R')
    dir = 'L';
  }
  else if(e.getKeyCode() == KeyEvent.VK_RIGHT){
   if(dir != 'L')
    dir = 'R';
  }
  else if(e.getKeyCode() == KeyEvent.VK_ESCAPE)
   System.exit(0);
  else if(e.getKeyCode() == KeyEvent.VK_SPACE){
   if(gamepause){
    t.suspend();
    gamepause = false;
   }
   else{
    t.resume();
    gamepause = true;
   }
  }
  else if(e.getKeyCode() == KeyEvent.VK_S){
   time--;
  }
  else if(e.getKeyCode() == KeyEvent.VK_D){
   time++;
  }
  
 }
 //generating random position of food
 int foodx = 20+(int)(Math.random()*56)*10;
 int foody = 20+(int)(Math.random()*52)*10;
 int score = 0;
 public void paintComponent(Graphics g){
  super.paintComponent(g);
  Graphics2D g2 = (Graphics2D)g;
  score = vector.size()-5;
  g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
  int red = 30+(int)(Math.random()*220);
  int green = 30+(int)(Math.random()*220);
  int blue = 30+(int)(Math.random()*220); 
  g2.setColor(Color.red);
  g2.drawRect(600, 0, 199, 550);
  g2.setColor(Color.orange);
  g2.setFont(new Font("bauhaus 93", 1, 50));
  g2.drawString("Snake", 630, 80);
  g2.setFont(new Font("Arial", 1, 20));
  g2.drawString("Score: "+score,650, 230);
  g2.fillRect(0, 0, dim.width-200, dim.height);
  g2.setColor(Color.black);
  g2.fillRect(10, 10, dim.width-220, dim.height-65);
  g2.setColor(new Color(red, green, blue));
  g2.fillOval(foodx, foody, 10, 10);
  g2.setColor(Color.red);
                //if game over then it will be display on screen
  if(gameover){
   g2.setFont(new Font("Monotype Corsiva", 1, 120));
   g2.drawString("Game Over", 60, 260);
   g2.setFont(new Font("typewriter",1,14));
   g2.drawString("This game is made by", 220, 330);
   g2.drawString("Sumit Kumar", 255, 370);
   g2.setColor(Color.yellow);
   g2.drawString("E-mail: sumit.kumar1310@gmail.com", 170, 410);
   //t.stop();
  }
  else{   
   g2.fillOval(vector.get(0).centerx, vector.get(0).centery, vector.get(0).radius, vector.get(0).radius);
   for(int i=1; i < vector.size(); i++){
    g2.fillOval(vector.get(i).centerx, vector.get(i).centery, vector.get(i).radius, vector.get(i).radius);
    vector.get(i).pcenterx = vector.get(i).centerx;
    vector.get(i).pcentery = vector.get(i).centery;
    vector.get(i).centerx = vector.get(i-1).pcenterx;
    vector.get(i).centery = vector.get(i-1).pcentery;
   }
  }  
 }
        //important function which will see collision of snake with wall or itself
 void checkOutOfBounds(int x, int y){
  if(x < 10 || x > dim.width-220 || y < 10 || y > dim.height-61){
   gameover = true;
   try{Thread.sleep(1000);}catch(Exception e){}
   repaint();
  }
  else if(x==foodx && y==foody){
   snake_node snode = new snake_node();
   snode.centerx = vector.get(vector.size()-1).pcenterx;
   snode.centery = vector.get(vector.size()-1).pcentery;
   vector.add(snode);
   foodx = 20+(int)(Math.random()*56)*10;
   foody = 20+(int)(Math.random()*52)*10;   
  }
  for(int i=1; i < vector.size(); i++){
   if(x == vector.get(i).centerx && y == vector.get(i).centery){
    gameover = true;
    try{Thread.sleep(1000);}catch(Exception e){}
    repaint();
   }
  }
 }
 
 public void run(){
  
  while (true){
   switch(dir){
    case 'L':
     vector.get(0).centerx = (vector.get(0).centerx-inc);
     vector.get(0).pcenterx = vector.get(0).centerx;
     checkOutOfBounds(vector.get(0).centerx, vector.get(0).centery);
     repaint();
     break;
    case 'R':     
     vector.get(0).centerx = (vector.get(0).centerx+inc);
     vector.get(0).pcenterx = vector.get(0).centerx;
     checkOutOfBounds(vector.get(0).centerx, vector.get(0).centery);
     repaint();
     break;
    case 'U':     
     vector.get(0).centery = (vector.get(0).centery-inc);
     vector.get(0).pcentery = vector.get(0).centery;
     checkOutOfBounds(vector.get(0).centerx, vector.get(0).centery);
     repaint();
     break;
    case 'D':     
     vector.get(0).centery = (vector.get(0).centery+inc);
     vector.get(0).pcentery = vector.get(0).centery;
     checkOutOfBounds(vector.get(0).centerx, vector.get(0).centery);
     repaint();
     break;
   }
   try{
    Thread.sleep(time);
   }
   catch(Exception e){}
  }
 }
}

Save this file as Snake_game.java
Compile : javac Snake_game.java
Run : java Snake_game