Question

当我第一次运行此代码时，我没有问题但是当用已经存在的文件再次运行它时并在此文件中添加其他数据我看到了这个例外

Exception in thread "main" java.lang.NullPointerException
    at Main.main(Main.java:67)

这是第67行

writer.println(proc[j].ID+"\t"+proc[j].name+"\t"+proc[j].Parent+"\t"+proc[j].state);

这是我的代码

import java.util.Scanner;
    import java.io.*;
    public class Main {
        public static void main(String [] args) throws IOException{
            File pro =new File("process.txt");
            Scanner input = new Scanner (System.in);
            int [] ID_Container = new int[10];
            Pro [] proc = new Pro [10] ;
            int memory =1000;
            System.out.println("Enter ID , Name , Parent, State");
            boolean flag =true;

            int i =0;
            while(flag&&i<10){
                System.out.println("please enter the ID");
                int ID=input.nextInt();

                System.out.println("please enter the name");
                String name=input.next();

                System.out.println("please enter the parent");
                int Parent=input.nextInt();

                System.out.println("please enter the State");
                char state=input.next().charAt(0);

                System.out.println("please enter the size");
                int size=input.nextInt();

                if(ID<0||ID>10||ID_Container[ID-1]==1){
                    for (int j = 0; j < 10; j++) {
                        if(ID_Container[j]==0){
                            ID=j+1;
                            System.out.println("Your Id has changed to "+ ID);
                            break;
                        }
                    }
                }

                if(size<memory){
                    proc[i]=new Pro(ID,name,Parent,state,size);
                    ID_Container[ID-1]=1;
                    memory-=size;
                    System.out.println("Do you want to add another process\n1-yes 0-no");
                    int s =input.nextInt();

                    if(s==0)
                        flag= false;
                    i++;
                }
            }
        PrintWriter writer = new PrintWriter(new FileWriter(pro, true));
        writer.println("ID\tname\tparent\tstate");
            for (int j = 0; j < 10; j++) {
                if(ID_Container[j]==0)
                    continue;
                writer.println(proc[j].ID+"\t"+proc[j].name+"\t"+proc[j].Parent+"\t"+proc[j].state);
            }
        writer.close();
        }
    }

    public class Pro {
    int ID;
    String name;
    int Parent;
    char state;
    int size;
    Pro(int ID , String name , int Parent , char state,int size){
        this.ID=ID;
        this.name=name;
        this.Parent=Parent;
        this.state=state;
        this.size=size;
    }
}

Answer 1

由于此时抛出异常，因此对于至少一个特定索引，数组必须包含null。

使用printwriter编辑文件

1 个答案: