在处理过程中接收java.lang空指针异常错误

Question

在我的处理项目中，我收到了一个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，但仍然会出现这个错误。如果有人知道我哪里出了问题，我会非常感激的。

 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

您的球是声明的，但从未在这里初始化：

Ball hole, gball;//declare a ball object for the golfball and the hole 
float cDistance = dist(gball.xpos, gball.ypos, hole.xpos, hole.ypos);

==> NPE

您应该首先使用一些值初始化Balls，如下所示：

gball = new Ball();
hole = new Ball();

实际上，您可能希望为Ball设置一个构造函数，该构造函数还可以将一些值设置为xpos / ypos，或者只是设置一些默认值，例如：

class Ball {
   int xpos = 0;
   int ypos = 0;
   //...

Answer 2

Ball#update()调用未声明的Ball#eclipse()，因此为null。您还声明洞和球类型的球，但从来没有初始化到新的球()。我建议您使用像Eclipse或IntelliJ这样的IDE，它们会使这样的小bug更容易清除。另外，下次发布堆栈跟踪。