我在以下代码中的 MainClass.ob1.noOfVerts 变量上收到 NullPointerException:
import java.awt.*;
import javax.swing.*;
public class drawr extends JPanel{
public void paintComponent(Graphics g){
super.paintComponent(g);
g.setColor(Color.decode("#ffc000"));
g.drawLine(0, 0, getWidth(), getHeight());
for(int cnt=0; cnt/displayObject.dim*2<=**MainClass.ob1.noOfVerts**; cnt+=displayObject.dim*2){
g.drawLine(MainClass.ob1.coords[cnt], MainClass.ob1.coords[cnt+1], MainClass.ob1.coords[cnt+2], MainClass.ob1.coords[cnt+3]);
}
}
该对象在这里实例化:
import java.awt.Color;
import javax.swing.*;
public class MainClass{
public static final int windowWidth = 1280;
public static final int windowHeight = 640;
public static boolean crash = false;
public static displayObject **ob1**, ob2, ob3;
public static void main(String[] args)
throws InterruptedException {
String colorString="#ff0000";
int ob1verts[]={10,10,50,50,30,80};
**displayObject ob1=new displayObject("#ff0000", ob1verts);**
int ob2verts[]={30,50,70,90,130,180,75,30};
displayObject ob2=new displayObject("#00ff00", ob2verts);
int ob3verts[]={10,10,70,50,70,80,110,130,30,30};
displayObject ob3=new displayObject("#0000ff", ob3verts);
使用这个构造函数:
public class displayObject {
// Each object is defined by a color, number of vertices, and dim (2,3,4) coordinate vertex locations
// Use dim to verify that the right number of vertices were sent
public static int dim=2;
public String color;
**public int noOfVerts**;
public int coords [];
public displayObject (String col, int verts[]){
if (verts.length%dim != 0){
System.out.printf ("Crap in!");
return;
}
this.coords=new int[verts.length+2];
color=col;
**noOfVerts=verts.length/dim;**
for (int cnt=0; cnt<verts.length; cnt++){
coords[cnt]=verts[cnt];
}
coords[verts.length]=verts[0]; //make last vertex equal first to close the shape
coords[verts.length+1]=verts[1];
}
}
我创建了一个静态变量,我认为它会通过适当的指针引用对象的特定内存空间。为什么对象引用是空指针?
最佳答案
The object is instantiated here:
public static displayObject **ob1**, ob2, ob3;
不,这不是实例化任何东西。它声明一个变量,但它的默认值是null
。在某些时候,您需要为其分配一个非空值,例如
ob1 = new displayObject(...);
现在你有这一行:
displayObject ob1=new displayObject("#ff0000", ob1verts);
...但这不是同一件事。这就是声明一个完全独立的局部变量,并为其赋予一个值。它恰好具有相同的名称 (ob1
),但它是一个不同的变量。您可能只需将其更改为不是局部变量声明即可:
ob1 = new displayObject("#ff0000", ob1verts);
...其他变量也是如此。
(顺便说一句,我强烈建议避免非私有(private)字段,并开始遵循 Java 命名约定。)
关于java - 对象引用困惑,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/31035503/