java - 在方法中返回另一个类的方法。导致 java.lang.NullPointerException

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这个问题已经有答案了:

NullPointerException (Java) [duplicate] (4 个回答)

已关闭 7 年前。

我是java新手，我正在创建通用社交媒体网站。我的用户类能够创建“Post”类。当尝试查看这些帖子时，我收到上述错误。我想在类中创建一个方法，以便用户类并查看它是与其帖子类关联的字符串。

   import java.util.Scanner;
public class User {
  private String password, address;
  private Post myPost;
  private int age;
  public String userName;

public User (String u, String p, String l, int a) {
   userName = u;
   password = p;
   address = l;
   age = a;
}

  public String viewLoginName ( ) {
    return userName;
  }

  public String viewPassword ( ) {
    return password;
  }

  public String viewAddress ( ) {
    return address;
  }

  public int viewAge ( ) {
    return age;
  }

  public Post createPost ( ) {
    Scanner reader = new Scanner ( System.in );
    String message;
    System.out.println("What would you like to share? ");
    message = reader.next ( );
    myPost = new Post(userName + ": " + message);
    return myPost;
  }
  public String viewMyPost()
   { 
     return myPost.viewPost();
  }
//the new method meant to allow users to view other users post
 public String viewUserPost(String user)
     {
      return user.viewMyPost();
     } 
}




//Post Class

import java.util.Scanner;
public class Post {
    private String message, acknowledged = "";
    //Scanner reader = new Scanner ( System.in );
    //message = reader.nextString ( );

    public Post(String m) {
        message = m;
    }
    public String viewPost() {
        return message;
    }

    public String acknowledge() {
        System.out.println("What is your username?");
        Scanner reader = new Scanner(System. in );
        String acknowledger;
        acknowledger = reader.next();
        acknowledged = acknowledged + acknowledger + ',';
        return acknowledged;

    }
}

Answer 1

您通过在方法中重新声明 myPost 变量来隐藏它。这使得类字段为空。不要重新声明它。

public String createPost() {
    Scanner reader = new Scanner(System. in );
    String message;
    System.out.println("What would you like to share? ");
    message = reader.next();

    // Post myPost = new Post(userName + ": " + message);
    myPost = new Post(userName + ": " + message); // note the difference?

    return "";
}

作为侧面建议，我将从您的 User 类中获取所有用户界面代码，因为它不属于该类，并且您可能会因为一遍又一遍地创建 Scanner 对象而面临资源问题。相反，UI 代码属于其自己的类中的其他位置，您应该考虑为 createPost 方法提供一个名为 message 的字符串参数。

此外，我还会考虑摆脱 Post 变量本身及其 getter 方法，而只需让 createPost(String message) 方法在调用时返回 Post 对象。现在它返回一个无用的空字符串，这对您或您的用户没有帮助。

public Post createPost(String message) {
    return new Post(userName + ": " + message); 
}