arraylist不打印第一個元素

Question

我想寫一些java代碼，但我得到一個異常。 我的問題是，我得到一個空指針異常，當我嘗試添加運動員 程序是接受運動員和計算平均得分這裏 是我的代碼

public class AthleteTest { 

     final int MAX_ATHELETE = 200; 
     private int count=0;  
     Athlete[] at = new Athlete[MAX_ATHELETE]; 
     Scanner sc = new Scanner(System.in); 

    public void addAthletes(){ 
      char add = 'Y'; 
      while(add == 'Y'){ 

       System.out.println("name:"); 
       String name = sc.nextLine(); 
       at[count].setName(name); 

       //Get athlete's Id number 
       System.out.println("id :"); 
       int id = sc.nextInt(); 
       at.setId(id); 

       //sc.nextLine(); 

       count++; 


       System.out.println("Would you like to add another athlete? Y/N"); 
       add = Character.toUpperCase(sc.next().charAt(0)); 
       sc.nextLine(); 
      } 
     } 
    } 


    my Athlete class is as follow 

    public class Athlete { 

     private String name; 
     private int id; 

     private double [] grades; 

     public Athlete(){ 
      this.name = null; 
      this.id= 0; 
     } 

     public Student(String name, int id){ 
      this.name = name; 
      this.id= id; 
     } 

     public String getName() { 
      return name; 
     } 

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

     public int getId() { 
      return id; 
     } 

     public void setId(int id) { 
      this.id = id; 
     } 
    } 

Answer 1

你得到，因爲你的例外在實例化它們之前，試圖訪問運動員對象。有了這個初始化：

Athlete[] at = new Athlete[MAX_ATHELETE]; 

你只是創建了一個地方舉行運動員實例。

改變你在循環代碼是這樣的：

System.out.println("name:"); 
String name = sc.nextLine(); 
at[count] = new Athlete(); // Add this line 
at[count].setName(name); 

而且你應該罰款。