nullpointerexception——为什么在我尝试添加字符串时，Java中的树集会给我一个nullpointerexception？

我正在为一个Java类做一个项目，我似乎无法克服这个NullPointerException。该项目是一个命令行LinkedIn程序。我正在实现的一个方面是向用户配置文件添加技能集的能力。

我有一个LinkedInUser类，我在其中定义了一个树集，以用户输入的字符串形式保存这些技能集。我使用的是TreeSet，因为作业需要对它们进行排序。

我在LinkedInUser类中定义树集：

private Set skillsets = new TreeSet<>();

用户执行的操作在AddSkillsetAction类中定义：

String skillset;
    
System.out.println("Enter a skillset to add to your list:");
skillset = scanner.nextLine();

loggedInUser.addSkillset(skillset);
System.out.println(skillset + " has been added to your skillsets.");

它们输入的字符串被传递给LinkedInUser类中的addSkillSet函数：

public void addSkillset(String skillset) {
    skillsets.add(skillset);
}

我一直收到一条NullPointerException的电话：

skillsets.add(skillset);

我做错了什么？我已经测试了这条线的每一个级别。我甚至用以下代码测试了addSkillset函数中的树集：

if(skillsets == null) {
   System.out.println("The TreeSet is null.")
}

它告诉我树集是空的。我想用以下方法来实例化场景：

private Set skillsets = new TreeSet<>();

实际上会创建一个空树集，而不是指向空位置。为什么是我的套装；“技能集”；还是指向空？我做错了什么？

编辑：以下是完整的课程：

package edu.institution.asn2;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class LinkedInUser extends UserAccount implements Comparable, Serializable {

    private static final long serialVersionUID = 75648957489235739L;
    private String type;
    private List connections = new ArrayList<>(); 
    private Set skillsets = new TreeSet<>();
    
    public LinkedInUser(String username, String password) {
        super(username, password);
    }
    
    @Override
    public void setType(String type) {
        this.type = type;
    }
    
    public String getType() {
        return this.type;
    }
    
    // Add a connection to user's list
    public void addConnection(LinkedInUser user) throws LinkedInException {
        int index = connections.indexOf(user);
        if (index >= 0) {
            throw new LinkedInException("You are already connected with this user.");
        }
        else {
            connections.add(user);
        }
    }
    
    // Remove a connection from the user's connection list
    public void removeConnection(LinkedInUser user) throws LinkedInException {
        int index = connections.indexOf(user);
        if (index < 0) {
            throw new LinkedInException("You are NOT connected to this user.");
        }
        else {
            connections.remove(index);
        }
    }
    
    // Return a copy of the ArrayList of connections
    public List getConnections() {
        ArrayList copy = new ArrayList<>(connections);
        return copy;
    }
    
    // Return the number of connections
    public int getNumberOfConnections() {
        return connections.size();
    }
    
    // Return the skillsets
    public Set getSkillsets(){
        return skillsets;
    }
    
    // Add a skillset
    public void addSkillset(String skillset) {
        skillsets.add(skillset);
    }
    
    // Remove a skillset
    public void removeSkillset (String skillset) {
        if(skillsets.contains(skillset)){
            skillsets.remove(skillset);
        } else {
            System.out.println(skillset + " is not in your skills list.");
        }
    }
    
    // Override the compareTo function
    @Override
    public int compareTo(LinkedInUser user) {
        int i = this.getUsername().compareToIgnoreCase(user.getUsername());
        return i;
    }

}

以及要添加技能集的类：

package edu.institution.actions.asn7;

import java.util.Scanner;

import edu.institution.ApplicationHelper;
import edu.institution.UserRepository;
import edu.institution.actions.MenuAction;
import edu.institution.asn2.LinkedInUser;

public class AddSkillsetAction implements MenuAction {

    @Override
    public boolean process(Scanner scanner, UserRepository userRepository, LinkedInUser loggedInUser) {
        String skillset;
        
        System.out.println("Enter a skillset to add to your list:");
        skillset = scanner.nextLine();
        
        loggedInUser.addSkillset(skillset);
        System.out.println(skillset + " has been added to your skillsets.");
        
        ApplicationHelper.incrementSkillsetCount(skillset);
        
        return true;
    }
}