GamePanel.java

/**
* This is the classic Snake Game programmed in Java. The class extends JPanel and implements the ActionListener.
* 
* I added a button to restart the game after losing. 
* This class holds the whole game.
*
* @author  Vivian Gleißner (I followed a Youtube Tutorial--> https://www.youtube.com/watch?v=bI6e6qjJ8JQ)
* @version 1.1
* @since   2023-10-02
*/

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.Random;

public class GamePanel extends JPanel implements  ActionListener{
	
	static final int SCREEN_WIDTH = 600;
	static final int SCREEN_HEIGHT = 600;
	static final int UNIT_SIZE = 25; // size of the squares
	static final int GAME_UNITS = (SCREEN_WIDTH * SCREEN_HEIGHT)/UNIT_SIZE;
	static final int DELAY = 130; // the higher the number, the slower the game
	final int x[] = new int[GAME_UNITS]; // holds all the x-coordinates of the snakes body
	final int y[] = new int [GAME_UNITS]; // holds all the y-coordinates of the snakes body
	int bodyParts = 6; 	// holds the amount of body parts of the snake (the length of the snake)
	int applesEaten; // is zero in the beginning
	int appleX; // random x-coordinates of the apples that can be eaten
	int appleY; // random y-coordinates of the apples that can be eaten
	char direction = 'R'; // Direction if the snake when game is started ('R' for right)
	boolean running = false; 
	Timer timer;
	Random random;

	public GamePanel() {
		random = new Random();
		this.setPreferredSize(new Dimension(SCREEN_WIDTH, SCREEN_HEIGHT));
		this.setBackground(Color.black);
		this.setFocusable(true);
		this.addKeyListener(new MyKeyAdapter());
		startGame();
	}
	/**
	 * Method that starts the game
	 */
	public void startGame() {
		newApple();
		running = true;
		timer = new Timer(DELAY, this);
		timer.start();
	}
	
	public void paintComponent(Graphics g) {
		super.paintComponent(g);
		draw(g);
	}
	
	/**
	 * Method that draws the components on the Panel (Apples, Snake,...)
	 * It also displays the score
	 * 
	 * @param g The graphical components
	 */
	public void draw(Graphics g) {
		
		if(running) {
			// going through the panel to draw a grid to make the units visible (draw lines across x-axis)
			for(int i=0; i<SCREEN_HEIGHT/UNIT_SIZE;i++) {
				g.setColor(Color.darkGray);
				g.drawLine(i*UNIT_SIZE, 0, i*UNIT_SIZE, SCREEN_HEIGHT);
				g.drawLine(0, i*UNIT_SIZE, SCREEN_WIDTH, i*UNIT_SIZE);
			}
			g.setColor(Color.red);
			g.fillOval(appleX, appleY, UNIT_SIZE, UNIT_SIZE);
			
			// drawing the snake
			for(int i=0; i<bodyParts;i++) {
				if(i==0) {
					//then it is the head of the snake
					g.setColor(Color.yellow);
					g.fillOval(x[i], y[i], UNIT_SIZE, UNIT_SIZE);
				}
				else {
					g.setColor(Color.yellow);
					g.fillOval(x[i], y[i], UNIT_SIZE, UNIT_SIZE);
				}
			}
			
			// Shows the current score
			g.setColor(Color.white);
			g.setFont(new Font("", Font.BOLD, 20));
			FontMetrics metrics = getFontMetrics(g.getFont());
			g.drawString("Your score: " + applesEaten, (SCREEN_WIDTH - metrics.stringWidth("Your score: " + applesEaten))/2, g.getFont().getSize());
		} else {
			gameOver(g);
		}	
	}
	
	/**
	 * Creates random positions for the apples
	 */
	public void newApple() {
		appleX = random.nextInt((int)(SCREEN_WIDTH/UNIT_SIZE))*UNIT_SIZE;
		appleY = random.nextInt((int)(SCREEN_HEIGHT/UNIT_SIZE))*UNIT_SIZE;
	}
	
	/**
	 * Moves the snake around and shifts the body parts of the snake 
	 */
	public void move() {
		for(int i=bodyParts;i>0;i--) {
			x[i] = x[i-1];
			y[i] = y[i-1];
		}
		
		//switch changes the direction of the snake, depending on switch case
		switch(direction) {
		case 'U': y[0] = y[0] - UNIT_SIZE;
			break;	
		case 'L': x[0] = x[0] - UNIT_SIZE;
			break;	
		case 'D': y[0] = y[0] + UNIT_SIZE;
			break;	
		case 'R': x[0] = x[0] + UNIT_SIZE;
			break;
		}
	}
	
	/**
	 * Check if the sneaks heads position is the same as the current apple
	 */
	public void checkApple() {
		if((x[0] == appleX) && (y[0] == appleY)){
			bodyParts++;
			applesEaten++;
			newApple();
		}
	}
	
	/**
	 * Method that checks if the snakes head collides with another element 
	 * 
	 * Option 1: Collides with own body
	 * Option 2: Collides with border
	 */
	public void checkCollisions() {
		// Checks if head collides with body
		for(int i=bodyParts; i>0; i--) {
			if((x[0] == x[i]) && (y[0] == y[i])) running = false;	
		}
		
		// Checks if head collides with border
		if(x[0]<0) running=false;
		if(x[0]>SCREEN_WIDTH) running=false;
		if(y[0]<0) running=false;
		if(y[0]>SCREEN_HEIGHT) running=false;
		
		if(!running) timer.stop();	
	}
	
	/**
	 * Display text when game is over
	 * 
	 * @param g The text that gets displayed on the Panel
	 */
	public void gameOver(Graphics g) {
		// Game over text gets displayed
		g.setColor(Color.yellow);
		g.setFont(new Font("Ink Free", Font.BOLD, 75));
		FontMetrics metrics = getFontMetrics(g.getFont());
		g.drawString("Game over!", (SCREEN_WIDTH - metrics.stringWidth("Game over!"))/2, SCREEN_HEIGHT/2);
		
		// also display the score when game is over
		g.setColor(Color.white);
		g.setFont(new Font("Ink Free", Font.BOLD, 20));
		FontMetrics metricsScrore = getFontMetrics(g.getFont());
		g.drawString("Score: " + applesEaten, (SCREEN_WIDTH - metricsScrore.stringWidth("Score: " + applesEaten))/2, SCREEN_HEIGHT/3);
		
		String buttonText = "Do you want to try again?";
	    JButton restartButton = new JButton(buttonText);
	    Font font = new Font("Ink Free", Font.PLAIN, 18);
	    restartButton.setFont(font);
	    restartButton.setBorderPainted(true);
	    restartButton.setBackground(Color.yellow);
	    
	    restartButton.setForeground(Color.black);
	    restartButton.setBounds(150,350,300,50);
	    add(restartButton);
	    
	    restartButton.addActionListener(new ActionListener() {
	        public void actionPerformed(ActionEvent e) {
	            SnakeGame.main(null);
	        }
	    });  
        
	}
	
	@Override
	public void actionPerformed(ActionEvent e) {
		if(running) {
			move();
			checkApple();
			checkCollisions();
		} 
		repaint();	
	}
	
	/**
	 * Listing to the 4 keyboard actions (up, right, down, left)
	 * 
	 * @author viviangleissner
	 */
	public class MyKeyAdapter extends KeyAdapter {
		@Override
		public void keyPressed(KeyEvent e) {
			switch (e.getKeyCode()) {
			case KeyEvent.VK_LEFT:
				if(direction != 'R') direction = 'L';
				break;
			
			case KeyEvent.VK_UP:
				if(direction != 'D') direction = 'U';
				break;
				
			case KeyEvent.VK_DOWN:
				if(direction != 'U') direction = 'D';
				break;
			
			case KeyEvent.VK_RIGHT:
				if(direction != 'L') direction = 'R';
				break;	
			}
		}
	}

}