我正在做一个制作俄罗斯方 block 的大学项目，但是我现在陷入困境。 当我只想更改数组的值时，出现空指针异常。应该很容易吧？ 显然不是...
我还不是高级程序员，所以如果有人能帮助我找到不正确的代码，并向我具体解释为什么它是错误的，我将不胜感激。
这是我的代码:
package tetris;
import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.event.KeyEvent;
import engine.*;
public class Tetris extends Game {
private int x = 0, y = 0, maxWidth = 0, bSize = 25;
private int[][] field;
private boolean busy;
private Blok blok = null;
public Tetris() {
title = "Tetris";
height = 600;
width = 400;
delay = 100;
delay2 = 10;
field = new int[20][10];
field[19][7] = 1;
field[18][7] = 2;
field[17][7] = 3;
field[19][2] = 4;
field[19][3] = 5;
}
public static void main(String arg[]) {
GameApplication.start(new Tetris());
}
@Override
/**
* Right Arrow: keycode 39
* Left Arrow: keycode 37
*/
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == 37) {
if (tryMove(-1)) {
x--;
}
}
else if(e.getKeyCode() == 39) {
if (tryMove(1)) {
x++;
}
} else if(e.getKeyCode() == 40) {
if (tryMove(0)) {
y++;
}
}
}
@Override
public void keyReleased(KeyEvent e) {
// TODO Auto-generated method stub
}
@Override
public void keyTyped(KeyEvent e) {
// TODO Auto-generated method stub
}
@Override
public void update() {
if(!busy) {
int random = (int) (Math.random()*7);
//debug mode
random = 0;
///
System.out.println(random);
blok = new Blok(random);
y = 1;
x = 3;
busy = true;
}
if (!tryMove(0)){
field[y][x] = 1;
busy = false;
} else {
y++;
}
scanLine();
}
@Override
public void update2() {
}
@Override
public void draw(Graphics2D g) {
g.fillRect(0, 0, width, height);
g.setColor(Color.LIGHT_GRAY);
g.fillRect(0, 0, 250, 500);
for (int i = 0; i<20; i++){
for (int j = 0; j<10; j++){
if (field[i][j]>0) {
g.setColor(Color.black);
g.fillRect(j*25,i*25,bSize,bSize);
}
}
}
if(busy) {
maxWidth = blok.getWidth();
for (int i = 0; i<3; i++){
g.fillRect(blok.getX(i)*25,blok.getY(i)*25,bSize,bSize);
}
}
}
public boolean tryMove(int a) {
boolean b = true;
if (a == -1){
if (x <= 0 || field[y][x-1] != 0){
b = false;
}
} else if (a == 0){
if (y >= 19 || field[y+1][x] != 0){
b = false;
}
} else if (a == 1){
if (x >= (9-maxWidth) || field[y][x+1] != 0){
b = false;
}
}
return b;
}
public void scanLine(){
for (int i = 0; i<20; i++){
boolean vol = true;
for (int j = 0; j<10; j++){
if (field[i][j]==0) {
vol = false;
}
}
if (vol) {
removeLine(i);
}
}
}
public void removeLine(int a){
for (int i = a; i>0; i--){
for (int j = 0; j<10; j++){
field[i][j] = field[i-1][j];
}
}
}
}
以及制作 block (以及出错的地方)
package tetris;
public class Blok {
private int type;
private int[] x,y;
private int width;
public Blok(int type) {
this.setType(type);
setCoords(type);
x = new int[4];
y = new int[4];
}
public int getType() {
return type;
}
public void setType(int type) {
this.type = type;
}
public void setCoords(int type) {
if (type == 0){
setWidth(3);
setX(0,1); setY(0,0);
setX(1,2); setY(1,0);
setX(2,3); setY(2,0);
setX(3,0); setY(3,0);
//x[0] = 1; y[0] = 0;
//x[1] = 2; y[1] = 0;
//x[2] = 3; y[2] = 0;
//x[3] = 0; y[3] = 0;
} else if (type == 1){
setWidth(2);
x[0] = 1; y[0] = 0;
x[1] = 2; y[1] = 0;
x[2] = 0; y[2] = -1;
x[3] = 0; y[3] = 0;
} else if (type == 2){
setWidth(2);
x[0] = 1; y[0] = 0;
x[1] = 2; y[1] = 0;
x[2] = 2; y[2] = -1;
x[3] = 0; y[3] = 0;
} else if (type == 3){
setWidth(2);
x[0] = 1; y[0] = 0;
x[1] = 2; y[1] = 0;
x[2] = 1; y[2] = -1;
x[3] = 0; y[3] = 0;
} else if (type == 4){
setWidth(2);
x[0] = 1; y[0] = -1;
x[1] = 1; y[1] = 0;
x[2] = 0; y[2] = -1;
x[3] = 0; y[3] = 0;
} else if (type == 5){
setWidth(1);
x[0] = 1; y[0] = 0;
x[1] = 1; y[1] = -1;
x[2] = 2; y[2] = -1;
x[3] = 0; y[3] = 0;
} else if (type == 6){
setWidth(2);
x[0] = 1; y[0] = 0;
x[1] = 1; y[1] = 1;
x[2] = 2; y[2] = 1;
x[3] = 0; y[3] = 0;
}
}
public int getX(int type) {
return x[type];
}
public int getY(int type) {
return y[type];
}
public void setX(int index, int value){
x[index] = value; //This is line 82, goes wrong here.
}
public void setY(int index, int value){
y[index] = value;
}
public int getWidth() {
return width;
}
public void setWidth(int w) {
this.width = w;
}
}
这是错误代码:
Exception in thread "Thread-2" java.lang.NullPointerException
at tetris.Blok.setX(Blok.java:82)
at tetris.Blok.setCoords(Blok.java:26)
at tetris.Blok.<init>(Blok.java:10)
at tetris.Tetris.update(Tetris.java:75)
at engine.Engine.run(Engine.java:18)
最佳答案
在你的构造函数中放入
public Blok(int type) {
this.setType(type);
setCoords(type);
x = new int[4];
y = new int[4];
}
您调用 setCoords 来调用 setX(..) ，而 setX 使用 x ，即 null 然后抛出 NullPointerException。
解决方法是更改顺序。
public Blok(int type) {
this.setType(type);
x = new int[4];
y = new int[4];
setCoords(type);
}
但是在构造函数中调用可重写的方法并不是一个好的做法。了解更多 What's wrong with overridable method calls in constructors? 。解决方法是将 setCoords(..) 设置为 Final 或将您的类设置为 Final 或将该方法设置为 private 或从构造函数中删除并从此类的客户端代码中调用它。
关于Java - NullPointerException，更改数组值，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/21297520/