java - 这个错误是什么意思 : java. lang.NullPointerException

Question

当我尝试测试运行时，我不断收到第 51 行的空异常: 歌曲[i].title=Recording.setTitle(); 该方法似乎调用得很好，那么为什么没有设置ongs[i].title呢？ 我对导航类(class)还很陌生，所以如果这是一个愚蠢的问题，请原谅我。

import java.util.Scanner;

class Recording {
    String title;
    String artist;
    String time;

     String getTitle(){
        return title;
     }


    String getArtist(){
        return artist;
    }

    String getTime(){
        return time;
    }

     static String setTitle(){
        Scanner title_input = new Scanner(System.in);
        System.out.println("What is the title of your recording?");
        String title_received = title_input.nextLine();
        return title_received;
    }

     String setArtist(){
        Scanner artist_input = new Scanner(System.in);
        System.out.println("Who is the artist of your recording?");
        String artist_received = artist_input.nextLine();
        return artist_received;
    }

     String setTime(){
        Scanner time_input = new Scanner(System.in);
        System.out.println("How long is your recording of your recording?");
        String time_received = time_input.nextLine();
        return time_received;
    }
}
public class RecordingSort {

    public static void main(String[] args) {


            Recording[] songs = new Recording[5];

            for(int i = 0; i<5; ++i){
                songs[i].title=Recording.setTitle();
                System.out.println(songs[i].title);
            }

    }

}

Answer 1

您收到此错误是因为尽管您声明并分配了 5 个 Recording 对象的数组，但您从未实际实例化该数组中的任何对象。

尝试在此处添加一行:

for(int i = 0; i<5; ++i){
                songs[i] = new Recording(); //invoke a valid object constructor here                                  
                songs[i].title=Recording.setTitle();