java - 在处理中收到 java.lang nullpointerException 错误

Question

我在处理项目中收到 java.lang 空指针异常，我认为它与 cDistance 有关，但我不太确定。我已经移动了一些东西，但仍然收到此错误。如果有人知道我哪里出错了，我将不胜感激。

 class Ball {
  int xpos, ypos;
  int ballDiam;
  color myColor;
  boolean visible = true;
  Ball(int tempdiam, color tempColor) {
    ballDiam=tempdiam;
    myColor=tempColor;
  }

  void update() {
    if (visible) {
      fill(myColor);
      ellipse(xpos, ypos, ballDiam, ballDiam);
    }
  }
} 

Ball hole, gball;//declare a ball object for the golfball and the hole 
float cDistance = dist(gball.xpos, gball.ypos, hole.xpos, hole.ypos);
int click;//to keep track of clicks
String msg;
int steps = 20;
int difx, dify;
Boolean moving = false;
void setup() {
  msg=""; 
  click=0;
  size(800, 400); 
  hole= new Ball(50, #000000);//making the 
  gball = new Ball(35, #ffffff);
} 
void draw() { 
  background(#009900);
  println("the click count is "+click);
  //set the hole ball as a golf hole right in the middle of the green
  hole.xpos = width/2;
  hole.ypos = height/2;
  hole.update();
  if (click==0) {
    //when no click has happened make the gball ball follow the mouse, 
    //after the click the ball will stay at the last position 
    gball.xpos=mouseX;
    gball.ypos=mouseY;
    msg="please place the golf ball";
  }
  else if (click==1) {//prompt the user to click again to shoot
    msg="now click again to shoot";
    difx = gball.xpos-hole.xpos;
    dify = gball.ypos-hole.ypos;
  }
  else if (click==2) {
    cDistance = dist(gball.xpos, gball.ypos, hole.xpos, hole.ypos);
    if (cDistance>hole.ballDiam/2) {
      moving = true;
      gball.xpos-=difx/steps;
      gball.ypos-=dify/steps;
      gball.xpos+=5;
    }
    else {
      moving = false;
      gball.visible=false;
      click=3;
    }
  }
  gball.update();
  textSize(20);
  text(msg, 0, height-5);
}

void mouseClicked() {
  if (!moving) {
    click++;
  }
}

堆栈跟踪:

java.lang.RuntimeException: java.lang.NullPointerException
    at processing.core.PApplet.runSketch(PApplet.java:10573)
    at processing.core.PApplet.main(PApplet.java:10377)
Caused by: java.lang.NullPointerException
    at sketch_140421a.<init>(sketch_140421a.java:37)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
    at java.lang.Class.newInstance(Class.java:374)
    at processing.core.PApplet.runSketch(PApplet.java:10571)
    ... 1 more

Answer 1

Ball#update() 调用 Ball#eclipse()，该函数尚未声明，因此为 null。 您还声明了 Hole 和 gball 类型为 Ball，但从未将它们初始化为 new Ball()。 我建议您使用 IDE，例如 Eclipse 或 IntelliJ，它们使诸如此类的小错误更容易消除。另外，下次发布堆栈跟踪。