Question

您好我有一个名为Rectangle的对象，这是代码：

class Rectangle{
    private double length, breath, area, perimeter;
    private static double largestArea;
    public void setSideDetails(double length, double breath){
        this.length = length;
        this.breath = breath;
    }

    public double getArea(){
        area = length*breath;
        return area;
    }

    public double getPerimeter(){
        perimeter = 2*(length+breath);
        return perimeter;
    }

    public static void setLargestArea(double areas[]){
        for(int i = 0; i < areas.length; i++){
            if(areas[i] > largestArea){
                largestArea = areas[i];
            }
        }
    }

    public static double getLargestArea(){
        return largestArea;
    }

    public void drawRectangle(){
        System.out.print(" ");
        for(int i = 0; i < length; i++){
            System.out.print("_");
        }
        System.out.println();
        for(int i = 0; i < breath; i++){
            System.out.print("|");
            for(int j = 0; j < length; j++){
                System.out.print(" ");
            }
            System.out.println("|");
        }
        System.out.print(" ");
        for(int i = 0; i < length; i++){
            System.out.print("¯");
        }
        System.out.println();
    }
}

以及正在使用它的类：

import java.util.*;
class UsingRectangle{
    public static void main(String args[]){
        Scanner in = new Scanner(System.in);
        in.useDelimiter("\n");

        int number = 0;
        boolean isError = false;
        String units;

        System.out.print("How many rectangles do you want to create? ");
        do{
            try{
                number = in.nextInt();
                number = Math.abs(number);
                isError = false;
            }catch(InputMismatchException e){
                System.out.println("Your input was not accepted. Please try again.");
                in.next();
                isError = true;
            }
        }while(isError);
        do{
            isError = false;
            System.out.print("Please enter the units in mm, cm or m: ");
            units = in.next();
            if(!(units.equalsIgnoreCase("mm"))&&!(units.equalsIgnoreCase("cm"))&&!(units.equalsIgnoreCase("m"))){
                System.out.println("Your input was not accepted. Please try again.");
                isError = true;
            }
        }while(isError);

        Rectangle[] r = new Rectangle[number]; //The actual rectangles are instanceiated here.
        double areas[] = new double[number]; //This will be used to collect the areas to be sent to a static method.

        for(int i = 0; i < r.length; i++){
            double length = 0;
            double breath = 0;
            do{
                isError = false;
                try{
                    System.out.print("Please enter the length of Rectangle "+(i+1)+": ");
                    length = Math.abs(in.nextInt());
                }catch(InputMismatchException e){
                    System.out.println("Your input was not accepted. Please try again.");
                    in.next();
                    isError = true;
                }
            }while(isError);
            do{
                isError = false;
                try{
                    System.out.print("Please enter the breath of Rectangle "+(i+1)+" ");
                    breath = Math.abs(in.nextInt());
                }catch(InputMismatchException e){
                    System.out.println("Your input was not accepted. Please try again.");
                    in.next();
                    isError = true;
                }
            }while(isError);
            r[i].setSideDetails(length, breath);
        }
        for(int i = 0; i < r.length; i++){
            r[i].drawRectangle();
            areas[i] = r[i].getArea();
            System.out.println("Area: "+areas[i]+units+"2   Perimeter: "+r[i].getPerimeter()+units);
        }

        System.out.print("The largest area is:  ");
        Rectangle.setLargestArea(areas);
        System.out.print(Rectangle.getLargestArea());
    }
}

我的问题是：

表示r[i].setSideDetails(length,breath);运行时错误 - java.lang.nullPointerException null。我不知道是什么造成了这个。任何搜索都没有结果，除了如何解决之外，我想知道原因，影响和预防。

由于

顺便说一下，我是一名A级学生，在两年内参加考试，所以如果我是专业人士，请不要表现得像我应该知道的那样。

Answer 1

那是因为您刚刚声明了Rectangle[] r = new Rectangle[number];并且没有初始化此数组中的元素。默认情况下，数组的每个元素都是null，在尝试调用方法时会给你NPE。在调用方法之前初始化数组中的每个元素。

示例：的

Rectangle[] r = new Rectangle[number];
for(int i = 0; i < r.length; i++){
    r[i] = new Rectangle(); // You can use either this or some other constructor you have.
}

Answer 2

您收到NullPointerException，因为引用r[i]指向任何/ null。因此，您必须实例化Rectangle个对象并将其放入数组中。

for(int i = 0; i < r.length; i++)
    r[i] = new Rectangle(); // creating a new Rectangle object here

使用对象方法时的java.lang.nullPointerException

2 个答案: