在方法中返回另一个 class 的方法。导致 java.lang.NullPointerException

Question

我是 java 的新手，我正在创建通用社交媒体网站。我的用户 class 可以创建 "Post" class。尝试查看这些 post 时，我收到上述错误。我想在 class 中创建一个方法，以便用户 class 并查看它是与其关联的字符串 post class.

   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来隐藏 myPost 变量。这会使 class 字段为空。不要重新申报。

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 "";
}

作为附带建议，我会从您的用户 class 中获取所有用户界面代码，因为它不属于那里，并且您冒着运行通过创建 Scanner 对象而陷入资源问题的风险一遍又一遍。相反，UI 代码属于它自己的其他地方 class，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，而只使用 createPost(String message) 方法 return调用时的 Post 对象。现在它 return 是一个无用的空字符串，对您或您的用户没有帮助。

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

在方法中返回另一个 class 的方法。导致 java.lang.NullPointerException

Returning a method of another class within a method. Causing java.lang.NullPointerException

java

return

class