如何实现向LinkedList插入Ball对象的方法？程序异常求助

尝试从文件读取数据创建并打印包含Ball对象的LinkedList，原本打算通过给head赋值插入第一个节点，但程序输出00，说明对象未正确创建，不清楚问题出在哪。

主类代码 #

class test{
    public static void main(String[] args){
    // 不允许修改main方法
    
    String inputfilename = "input.txt";
    String outputfilename= "output.txt";
    BallLinkedList ballSet = new BallLinkedList("", 0, 0);
    constructLLFromFile(inputfilename, ballSet);
    ballSet.printLL();
    
    System.out.println();

    ballSet.printLL();
   }
static BallLinkedList constructLLFromFile(String theInputFile, BallLinkedList ballSet){
    System.out.println("Constructing the linked list from "+theInputFile);
    BallLinkedList head = null;
    try{
      File file = new File(theInputFile);
      Scanner scan = new Scanner(file);
      int data = 0;
      int index = 0;
      while(scan.hasNext()){
        data = Integer.parseInt(scan.nextLine());
        String color = scan.nextLine();
        long  diamater = Long.parseLong(scan.nextLine());
        int bounce  = Integer.parseInt(scan.nextLine());
        Ball insertee = newBall(color,diamater,bounce);
        ballSet.insert(insertee,index);
        index++;
      }
    }catch(Exception eee){
        
    }
    
    return null;    // 不能修改

  }
}

LinkedList类代码 #

import java.io.FileWriter;

class BallLinkedList{
  private Planet head;
  
  BallLinkedList(){
  }
  
  BallLinkedList(Ball initial){
    head=initial;
  }
  
  BallLinkedList(String c, long d, int b){
    head = new Planet(c, d, b);
  }
  // 插入方法位置
  void insert(Ball insertee, int pos){
    if(head == null ){
      head = insertee;

  }else{
    head = head.next;
    pos++;
  }
  }
// 打印链表
  void printLL(){
   while(head != null ){
    System.out.println(head.getColor());
    System.out.println(head.getDiameter());
    System.out.println(head.getBounce());
    head = head.next;
   }
  }
  
}

Ball类代码 #

class Ball{
  private String color;
  private long diameter;
  private int bounce;
  
  Ball next;
  
  Ball(String c, long d, int b){
    color = n;
    diameter = d;
    bounce = m;
  }
  
  
  String getColor(){
    return color;
  }
  
  long getDiameter(){
    return diameter;
  }
  
  int getBounce(){
    return bounce;
  }
  
  public String toString(){
    return "Color: "+color+
           "\nDiameter: "+diameter+
           "\nBounce: "+bounce;
  }
}

输入文件内容 #

Blue
48
0
Red
121
0
Green
127
1
Yellow
680
2
Black
120
6

1. Ball类构造函数参数错误 #

Ball类构造函数中，将参数赋值给了未定义的变量n和m，修正后：

Ball(String c, long d, int b){
    color = c;
    diameter = d;
    bounce = b;
}

2. BallLinkedList类类型混淆 #

• 链表节点应为Ball类型，将private Planet head;改为private Ball head;
• 构造函数BallLinkedList(String c, long d, int b)中，将new Planet(c, d, b)改为new Ball(c, d, b)

3. 插入方法逻辑完全错误 #

当前insert方法未实现正确的链表插入逻辑，修正后：

void insert(Ball insertee, int pos){
    // 插入头部或空链表
    if(pos == 0 || head == null){
        insertee.next = head;
        head = insertee;
        return;
    }
    Ball current = head;
    // 遍历到目标位置的前一个节点
    for(int i=0; i < pos-1 && current.next != null; i++){
        current = current.next;
    }
    // 插入新节点
    insertee.next = current.next;
    current.next = insertee;
}

4. printLL方法破坏链表结构 #

原方法遍历中直接修改head，导致第二次打印无内容，改用临时变量遍历：

void printLL(){
    Ball current = head;
    while(current != null ){
        System.out.println(current.getColor());
        System.out.println(current.getDiameter());
        System.out.println(current.getBounce());
        current = current.next;
    }
}

5. 文件读取逻辑错误 #

输入文件每组数据为3行，原代码多读取一行整数导致报错，修正后：

while(scan.hasNext()){
    String color = scan.nextLine();
    long diameter = Long.parseLong(scan.nextLine());
    int bounce  = Integer.parseInt(scan.nextLine());
    Ball insertee = new Ball(color,diameter,bounce);
    ballSet.insert(insertee,index);
    index++;
}

6. 方法调用错误 #

将Ball insertee = newBall(color,diamater,bounce);改为Ball insertee = new Ball(color,diamater,bounce);，不存在newBall方法。

7. 异常处理空实现 #

catch块添加异常打印，方便排查问题：

catch(Exception eee){
    eee.printStackTrace();
}

内容的提问来源于stack exchange，提问作者pokemonmater940