实现一个简单的贪吃蛇游戏需要使用Java的图形库，通常可以使用Swing或JavaFX。下面是一个使用Swing库来创建一个简单贪吃蛇游戏的示例代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.util.LinkedList;
import java.util.Random;

public class SnakeGame extends JPanel implements ActionListener {
    private final int CELL_SIZE = 20;
    private final int GRID_WIDTH = 20;
    private final int GRID_HEIGHT = 20;
    private final int GAME_SPEED = 150;

    private LinkedList<Point> snake;
    private Point food;
    private int direction;
    private boolean isMoving;
    private boolean isGameOver;

    public SnakeGame() {
        setPreferredSize(new Dimension(CELL_SIZE * GRID_WIDTH, CELL_SIZE * GRID_HEIGHT));
        setBackground(Color.BLACK);
        setFocusable(true);

        addKeyListener(new KeyAdapter() {
            @Override
            public void keyPressed(KeyEvent e) {
                int key = e.getKeyCode();
                if ((key == KeyEvent.VK_LEFT) && (direction != 1)) direction = 0;
                if ((key == KeyEvent.VK_RIGHT) && (direction != 0)) direction = 1;
                if ((key == KeyEvent.VK_UP) && (direction != 3)) direction = 2;
                if ((key == KeyEvent.VK_DOWN) && (direction != 2)) direction = 3;
            }
        });

        snake = new LinkedList<>();
        generateFood();
        isMoving = true;
        isGameOver = false;

        Timer timer = new Timer(GAME_SPEED, this);
        timer.start();
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        if (isMoving && !isGameOver) {
            move();
            checkCollision();
            repaint();
        }
    }

    private void generateFood() {
        Random rand = new Random();
        int x, y;
        do {
            x = rand.nextInt(GRID_WIDTH);
            y = rand.nextInt(GRID_HEIGHT);
        } while (snake.contains(new Point(x, y));
        food = new Point(x, y);
    }

    private void move() {
        Point head = snake.peekFirst();
        Point newHead;
        switch (direction) {
            case 0:
                newHead = new Point(head.x - 1, head.y);
                break;
            case 1:
                newHead = new Point(head.x + 1, head.y);
                break;
            case 2:
                newHead = new Point(head.x, head.y - 1);
                break;
            case 3:
                newHead = new Point(head.x, head.y + 1);
                break;
            default:
                newHead = head;
        }

        snake.addFirst(newHead);

        if (newHead.equals(food)) {
            generateFood();
        } else {
            snake.removeLast();
        }
    }

    private void checkCollision() {
        if (snake.size() == GRID_WIDTH * GRID_HEIGHT) {
            isMoving = false;
            isGameOver = true;
        }

        Point head = snake.peekFirst();
        if (head.x < 0 || head.x >= GRID_WIDTH || head.y < 0 || head.y >= GRID_HEIGHT) {
            isMoving = false;
            isGameOver = true;
        }

        if (snake.size() > 1 && snake.contains(head)) {
            isMoving = false;
            isGameOver = true;
        }
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (isGameOver) {
            g.setColor(Color.WHITE);
            g.setFont(new Font("SansSerif", Font.BOLD, 30));
            g.drawString("Game Over!", CELL_SIZE * 5, CELL_SIZE * 10);
        } else {
            // 绘制食物
            g.setColor(Color.RED);
            g.fillRect(food.x * CELL_SIZE, food.y * CELL_SIZE, CELL_SIZE, CELL_SIZE);

            // 绘制蛇
            g.setColor(Color.GREEN);
            for (Point p : snake) {
                g.fillRect(p.x * CELL_SIZE, p.y * CELL_SIZE, CELL_SIZE, CELL_SIZE);
            }
        }
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("贪吃蛇游戏");
        SnakeGame game = new SnakeGame();
        frame.add(game);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}

这个示例创建了一个简单的贪吃蛇游戏，使用了Swing库来构建图形用户界面。玩家可以使用方向键控制蛇的移动，目标是吃到食物，躲避碰撞和自身。游戏会在一定条件下结束，显示"Game Over"。你可以根据需要对游戏进行扩展和改进。