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Question

public class Song 
{ 
private String title; 
private String artist; 

public Song (String Title, String Artist) 
{ 
    Title = title; 
    Artist = artist; 
} 
public String toString() 
{ 
    return (title +" "+ artist); 
} 
} 

import java.io.File; 
import java.io.IOException; 
import java.util.Scanner; 

public class compactDisc 
{ 
private Song[] arr = new Song[20]; 
private int numSongs = 0; 
public compactDisc() throws IOException 
{ 
    int i=0; 

    File file = new File ("song.txt"); 
    Scanner inputFile = new Scanner(file); 

    while (inputFile.hasNext()&& i<arr.length) 
    { 
     String Title = inputFile.nextLine(); 
     String Artist = inputFile.nextLine(); 
     arr[i] = new Song(Title,Artist); 
     i++; 
    } 
    numSongs = i; 
} 

public String toString() 
{ 
    String str = ""; 
    for (int j = 0; j < numSongs; j++) 
    { 
     str += arr[j].toString(); 
    } 
     return str; 
} 
} 

import java.io.IOException; 
import java.util.Scanner; 

public class collection 
{ 

public static void main(String[] args) throws IOException 
{ 
    compactDisc cd = new compactDisc(); 
    System.out.println(cd); 
} 
} 

这个程序是显示的歌曲列表和艺术家的时候，我的程序只打印出一堆“空” S光盘。歌曲类是我的打印格式，compactDisc类读取一个包含歌曲和艺术家列表的文件到一个数组中，主程序打印它们。但是，当我运行该程序时，它会打印出null nullnull nullnull nullnull nullnull nullnull null而不是实际列表，并且方向似乎正确。我不知道为什么会发生。

Answer 1

请在您的歌班中查看此处。

private String title; 
private String artist; 

public Song (String Title, String Artist) { 
    Title = title; 
    Artist = artist; 
} 

您正在将未从您的Song类初始化的空值赋值给参数，因此您的类变量永远不会被填充。

应该比较像这样

public class Song { 
    private String title; 
    private String artist; 

    public Song (String title, String artist) { 
     this.title = title; 
     this.artist = artist; 
    } 
} 

同样按照约定类应以大写字母开头和变量应该用小字母开头。

Answer 2

的问题是在这里

public Song (String Title, String Artist) 
{ 
    Title = title; 
    Artist = artist; 
} 

参数是冠军，但你要指定标题（为空）;

更改为：

title = Title; 
artist = Artist; 

Answer 3

的问题是在Song类，如（我）的意见解释说：

public Song (String Title, String Artist) 
{ 
    Title = title; // assigns this.title into local Title 
    Artist = artist; // assigns this.artist into local Artist 
}