Java 实例化的对象在包含对象的数组列表中显示为null

我对Java非常陌生，但遇到了以下错误：线程“main”Java.lang.NullPointerException中的异常：无法调用“bussines.Plot$Rectangle.setRectangle（int，int）”，因为“Java.util.ArrayList.get（int）.Rectangle”在bussines.Owner.buyRectangle（Owner.Java:24）在construction.Application.run（Application.Java:17）中为nullat construction.Application.main（Application.java:36）

我将在下面插入相关代码段：

public class Plot
{
    Rectangle rectangle;
    Square square;
    
    public class Rectangle
    {
        public int length;
        public int width;
    }
        
        public Rectangle() 
        {
            length = 0;
            width = 0;
        }
        
        public void setRectangle(int l, int w) 
        {
            length = l;
            width = w;
        }
        
        public int getArea() 
        {
            return length * width;
        }
    }
    
    public class Square
    {
        public int size;
        
        public Square() 
        {
            size = 0;
        }
        
        public void setSquare(int s) 
        {
            size = s;
        }
        
        public int getArea() 
        {
            return size * size;
        }
    }
}

---

调整绘图类。您需要在构造函数中初始化矩形和正方形对象

landPlots.get(0).rectangle.setRectangle(l, w);

否则，由于矩形类为null，您的行将有一个null异常

Buying four plots.
Exception in thread "main" java.lang.NullPointerException
at Owner.buyRectangle(Owner.java:24)
at Application.run(Application.java:16)
at NullExceptionApplication.main(NullExceptionApplication.java:6)

Process finished with exit code 1

我做了测试，现在可以了

public class Plot {
Rectangle rectangle;
Square square;

public Plot() {
    rectangle = new Rectangle();
    square = new Square();
}

public class Rectangle {
    public int length;
    public int width;


    public Rectangle() {
        this.length = 0;
        this.width = 0;
    }

    public void setRectangle(int l, int w) {
        length = l;
        width = w;
    }

    public int getArea() {
        return length * width;
    }
}

public class Square {
    public int size;

    public Square() {
        size = 0;
    }

    public void setSquare(int s) {
        size = s;
    }

    public int getArea() {
        return size * size;
    }
  }
}

---

不是arraylist中的对象本身为空，而是

绘图

对象中的

矩形

和

正方形

字段为空。这里的异常stacktrace保存了关于错误的所有必要信息。谢谢@maloomeister，但我似乎找不到解决方法。。。

Buying four plots.
Exception in thread "main" java.lang.NullPointerException
at Owner.buyRectangle(Owner.java:24)
at Application.run(Application.java:16)
at NullExceptionApplication.main(NullExceptionApplication.java:6)

Process finished with exit code 1

public class Plot {
Rectangle rectangle;
Square square;

public Plot() {
    rectangle = new Rectangle();
    square = new Square();
}

public class Rectangle {
    public int length;
    public int width;


    public Rectangle() {
        this.length = 0;
        this.width = 0;
    }

    public void setRectangle(int l, int w) {
        length = l;
        width = w;
    }

    public int getArea() {
        return length * width;
    }
}

public class Square {
    public int size;

    public Square() {
        size = 0;
    }

    public void setSquare(int s) {
        size = s;
    }

    public int getArea() {
        return size * size;
    }
  }
}