Java hashmap 与自己制作的盒子？

Question

我在运行代码时遇到问题。它旨在从包含数据的文件中实现框的映射。我首先输入盒子的名称和重量，然后在该程序中运行另一个程序，添加盒子之间的关系。

文件示例:

5
a 2
b 1
c 1
d 3
e 2
3
a b
a d
b c

代码:

package myutil;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.HashMap;
import java.util.Scanner;

public class MyReader  {
public MyReader() {

}

public static void addData(String currentFile, HashMap<String, Box> oordnadMap) throws FileNotFoundException{



        File file = new File (currentFile);
        Scanner scanner = new Scanner (file);

        int iterator = Integer.parseInt(scanner.nextLine());
        for(int i = 1; i <= iterator; i++){
            Box temp = new  Box(scanner.next(), Integer.parseInt(scanner.next()));
            System.out.println(temp.getName());
            System.out.println(temp.getSize());
            oordnadMap.put(temp.getName(), temp);

        }
        addRelation(oordnadMap, scanner);



}

public static void addRelation(HashMap<String, Box> oordnadMap, Scanner scanner)throws FileNotFoundException{

    int numerOfRelations = scanner.nextInt();
    System.out.println(numerOfRelations);
    for (int i = 1; i <= numerOfRelations; i++){
        String left =scanner.next();
        String right = scanner.next();
        Box Box1 = oordnadMap.get(left);
        Box Box2 = oordnadMap.get(right);
        System.out.println(Box1);
        System.out.println(Box2);
        Box2.setTop(Box1);
        Box1.setBottom(Box2);
    }



}
public static void main (String[] args){
    HashMap<String, Box> oordnadMap = new HashMap<String,Box> ();
    try {
        String currentFile="boxes1.txt";
        addData(currentFile, oordnadMap );
    } catch (FileNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}
}

我还在另一个类中指定了 Box:

package myutil;

import java.util.ArrayList;

public class Box {

    private String name;
    private int Size;
    ArrayList<Box> top;
    ArrayList<Box> bottom;



    public Box(String name, int Size){
        this.setName(name);
        this.setSize(Size);
    }


    public  void RemoveBox(String name, int Size){

    }

    public void setName(String name) {
        this.name = name;
    }

    public void setSize(int size) {
        Size = size;
    }

    public void setTop (Box temp){
        this.top.add(temp);

    }

    public void setBottom (Box temp){
        this.bottom.add(temp);
    }




    public String getName() {
        return name;
    }
    public int getSize() {
        return Size;
    }


}

当我运行它时，我收到错误:

Exception in thread "main" java.lang.NullPointerException
at myutil.Box.setTop(Box.java:33)
at myutil.MyReader.addRelation(MyReader.java:44)
at myutil.MyReader.addData(MyReader.java:27)
at myutil.MyReader.main(MyReader.java:55)

有什么想法吗？

Answer 1

您没有在 Box 类中初始化 top 或 bottom，因此 Java 为它们提供了默认值 null.

初始化它们。

public Box(String name, int Size){
    this.setName(name);
    this.setSize(Size);
    top = new ArrayList<Box>();
    bottom = new ArrayList<Box>();
}