所以我遵循了一本免费手册，指导如何在Java中编写Asteroids，它运行得很好。我想将它添加到一个程序中，以便我可以调用它，运行它一段时间，然后让它返回一个int。我正在制作的程序是一个应用程序，我一直在寻找尝试和失败的一段时间，弄清楚如何让它作为一个应用程序运行。我在这里读到的东西说我应该这样做：
public class Main extends JFrame {
public static void main(String[] args) {
AsteroidsGame game = new AsteroidsGame();
JFrame frame = new JFrame();
frame.setLayout(new GridLayout (1,1));
frame.add(game);
frame.setSize(500,500);
game.init();
game.start();
frame.setVisible(true);
}
}
但后来我收到错误
线程“main”中的异常java.lang.NullPointerException
at AsteroidsGame.init（AsteroidsGame.java:49）
在Main.main（Main.java:15）
指向第49行的下方，以及第15行的game.init（）。
g = img.getGraphics();
这是AsteroidsGame的代码，我太累了，不能再想了。我只需要让它作为一个应用程序运行，此时任何其他代码更改都无关紧要。我知道这是很多代码，但我希望这是一个我错过的简单的东西。任何帮助是极大的赞赏。谢谢！
import java.applet.*;
import java.awt.*;
import java.awt.event.*;
import java.util.Random;
public class AsteroidsGame extends Applet implements Runnable, KeyListener {
Thread thread;
Dimension dim;
Image img;
Graphics g;
long endTime, startTime, framePeriod;
Ship ship;
boolean paused; // True if the game is paused. Enter is the pause key
Shot[] shots;
int numShots;
boolean shooting;
Asteroid[] asteroids; // the array of asteroids
int numAsteroids; // the number of asteroids currently in the array
double astRadius, minAstVel, maxAstVel; // values used to create
// asteroids
int astNumHits, astNumSplit;
int level; // the current level number
int money;
double fuel;
Random rand = new Random();
public void init() {
resize(500, 500);
shots = new Shot[41]; // 41 is a shot's life period plus one.
// since at most one shot can be fired per frame,
// there will never be more than 41 shots if each one only
// lives for 40 frames.
numAsteroids = 0;
level = rand.nextInt(25)+1; // will be incremented to 1 when first level is set up
astRadius = 10; // values used to create the asteroids
minAstVel = 2;
maxAstVel = 8;
astNumHits = 3;
astNumSplit = 2;
money = 0;
fuel = 100;
endTime = 0;
startTime = 0;
framePeriod = 25;
addKeyListener(this); // tell it to listen for KeyEvents
dim = getSize();
img = createImage(dim.width, dim.height);
g = img.getGraphics();
thread = new Thread(this);
thread.start();
}
public void setUpNextLevel() { // start new level with one more asteroid
level++;
// create a new, inactive ship centered on the screen
// I like .35 for acceleration, .98 for velocityDecay, and
// .1 for rotationalSpeed. They give the controls a nice feel.
ship = new Ship(250, 250, 0, .35, .98, .1, 12);
numShots = 0; // no shots on the screen at beginning of level
paused = false;
shooting = false;
// create an array large enough to hold the biggest number
// of asteroids possible on this level (plus one because
// the split asteroids are created first, then the original
// one is deleted). The level number is equal to the
// number of asteroids at it's start.
asteroids = new Asteroid[level
* (int) Math.pow(astNumSplit, astNumHits - 1) + 1];
numAsteroids = level;
// create asteroids in random spots on the screen
for (int i = 0; i < numAsteroids; i++)
asteroids[i] = new Asteroid(Math.random() * dim.width,
Math.random() * dim.height, astRadius, minAstVel,
maxAstVel, astNumHits, astNumSplit);
}
public void paint(Graphics gfx) {
g.setColor(Color.black);
g.fillRect(0, 0, 500, 500);
for (int i = 0; i < numShots; i++)
// draw all the shots on the screen
shots[i].draw(g);
for (int i = 0; i < numAsteroids; i++)
asteroids[i].draw(g);
ship.draw(g); // draw the ship
g.setColor(Color.cyan); // Display level number in top left corner
g.drawString("Level " + level, 20, 20);
g.drawString("Money " + money, 80, 20);
g.drawString("Fuel " + fuel, 20, 50);
gfx.drawImage(img, 0, 0, this);
}
public void update(Graphics gfx) {
paint(gfx);
}
public void run() {
for (;;) {
startTime = System.currentTimeMillis();
// start next level when all asteroids are destroyed
if (numAsteroids <= 0)
setUpNextLevel();
if (!paused) {
ship.move(dim.width, dim.height); // move the ship
// move shots and remove dead shots
for (int i = 0; i < numShots; i++) {
shots[i].move(dim.width, dim.height);
// removes shot if it has gone for too long
// without hitting anything
if (shots[i].getLifeLeft() <= 0) {
// shifts all the next shots up one
// space in the array
deleteShot(i);
i--; // move the outer loop back one so
// the shot shifted up is not skipped
}
}
if(ship.accelerating && fuel>0){
fuel -= 1.5;
}
if(ship.accelerating && fuel == 0)
{
ship.accelerating = false;
}
// move asteroids and check for collisions
updateAsteroids();
if (shooting && ship.canShoot()) {
// add a shot on to the array
shots[numShots] = ship.shoot();
numShots++;
}
}
repaint();
try {
endTime = System.currentTimeMillis();
if (framePeriod - (endTime - startTime) > 0)
Thread.sleep(framePeriod - (endTime - startTime));
} catch (InterruptedException e) {
}
}
}
private void deleteShot(int index) {
// delete shot and move all shots after it up in the array
numShots--;
for (int i = index; i < numShots; i++)
shots[i] = shots[i + 1];
shots[numShots] = null;
}
private void deleteAsteroid(int index) {
// delete asteroid and shift ones after it up in the array
numAsteroids--;
for (int i = index; i < numAsteroids; i++)
asteroids[i] = asteroids[i + 1];
asteroids[numAsteroids] = null;
}
private void addAsteroid(Asteroid ast) {
// adds the asteroid passed in to the end of the array
asteroids[numAsteroids] = ast;
numAsteroids++;
}
private void updateAsteroids() {
for (int i = 0; i < numAsteroids; i++) {
// move each asteroid
asteroids[i].move(dim.width, dim.height);
// check for collisions with the ship, restart the
// level if the ship gets hit
if (asteroids[i].shipCollision(ship)) {
money+= Math.random()*1000;
deleteAsteroid(i);
//level--; // restart this level
//numAsteroids = 0;
return;
}
// check for collisions with any of the shots
for (int j = 0; j < numShots; j++) {
if (asteroids[i].shotCollision(shots[j])) {
// if the shot hit an asteroid, delete the shot
deleteShot(j);
// split the asteroid up if needed
if (asteroids[i].getHitsLeft() > 1) {
for (int k = 0; k < asteroids[i].getNumSplit(); k++)
addAsteroid(asteroids[i].createSplitAsteroid(
minAstVel, maxAstVel));
}
// delete the original asteroid
deleteAsteroid(i);
j = numShots; // break out of inner loop - it has
// already been hit, don't need to check
// for collision with other shots
i--; // don't skip asteroid shifted back into
// the deleted asteroid's position
}
}
}
}
public void keyPressed(KeyEvent e) {
if (e.getKeyCode() == KeyEvent.VK_ENTER) {
// These first two lines allow the asteroids to move
// while the player chooses when to enter the game.
// This happens when the player is starting a new life.
if (!ship.isActive() && !paused)
ship.setActive(true);
else {
paused = !paused; // enter is the pause button
if (paused) // grays out the ship if paused
ship.setActive(false);
else
ship.setActive(true);
}
} else if (paused || !ship.isActive()) // if the game is
return; // paused or ship is inactive, do not respond
// to the controls except for enter to unpause
else if (e.getKeyCode() == KeyEvent.VK_UP)
{if(fuel>0){
ship.setAccelerating(true);
//fuel -= 1.5;
}
else{
fuel = 0;
}
}
else if (e.getKeyCode() == KeyEvent.VK_LEFT)
{
ship.setTurningLeft(true);
}
else if (e.getKeyCode() == KeyEvent.VK_RIGHT)
ship.setTurningRight(true);
//else if (e.getKeyCode() == KeyEvent.VK_CONTROL)
//shooting = true; // Start shooting if ctrl is pushed
}
public void keyReleased(KeyEvent e) {
if (e.getKeyCode() == KeyEvent.VK_UP)
ship.setAccelerating(false);
else if (e.getKeyCode() == KeyEvent.VK_LEFT)
ship.setTurningLeft(false);
else if (e.getKeyCode() == KeyEvent.VK_RIGHT)
ship.setTurningRight(false);
//else if (e.getKeyCode() == KeyEvent.VK_CONTROL)
//shooting = false;
}
public void keyTyped(KeyEvent e) {
}
}
答案 0 :(得分：0)
我相信createImage（）将返回null，直到组件实际在屏幕上，我相信在线程启动之后。
而不是：
img = createImage(dim.width, dim.height);
g = img.getGraphics();
thread = new Thread(this);
thread.start();
尝试：
new Thread(new Runnable() {
public void run() {
do {
try {
Thread.sleep(25);
} catch(InterruptedException e) {
System.out.println("wait interrupted");
break;
}
} while(!isDisplayable());
// Now it should be safe to make these calls.
img= createImage(dim.width, dim.height);
g = img.getGraphics();
start();
}
}).start();