java - Java 中出现 NullPointerException 错误

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

我有一个名为 Square 的类，它继承了一个名为 Shape 的类。在我的主要部分，我像这样实例化它:

Point p1 = new Point(3,3); 

Shape s1 = new Square(p1,3); 

编译时出现以下错误:

Exception in thread "main" java.lang.NullPointerException
at Shape.<init>(Shape.java:19)
at Square.<init>(Square.java:18)
at mine.main(mine.java:16)

我有一个名为 Shape 的父类(super class)，其实现如下:

  abstract class Shape implements Comparable<Shape> {
  public Point position;

  public Shape(Point p){
    position.x  = p.x;      //Line 19
    position.y  = p.y;
  }
}

点是以下类:

public final class Point {
  public double x;
  public double y;

  // constructor that sets the values of x and y
  public Point(double num1, double num2)
  {  
    this.x = num1;
    this.y = num2;
  }
}

Square 是以下类:

class Square extends Shape {
  private double side; // side is the side length

  public Square(Point p0, double side)
  {  
    super(p0);               // Line 18
    this.side = side;
  }

Answer 1

您从未在Shape类中初始化position。

你可能想做的事情

public Shape(Point p){
   position = p;
}

或者，至少(如果您想“克隆”Point)

public Shape(Point p){
   position = new Point(p.x, p.y);
}