变量即使在为它们分配值之后也为无效(Java)

Question

我正在尝试编写一种将歌曲信息（用户输入）添加到数据库中的方法。用户添加歌曲详细信息（名称，艺术家，文件大小，持续时间）后，应将其保存在第一个空的歌曲插槽中（总计4个歌曲插槽），然后返回菜单界面。

但是，当我尝试添加第二首歌曲时，第一个插槽总是空的，好像用户输入未保存的详细信息一样。我已经尝试踏上调试器，当我输入名称，艺术家等时，它们都保存在Song1对象中。但是，当我回去输入第二首歌时，Song1对象具有值：name = null，Artist = null，Filesize = 0，持续时间= 0。

我已经呆了几个小时了，真的很困惑，任何帮助都会很棒！

songdatabase类：

public class SongDatabase {
Scanner console = new Scanner(System.in);
public void addNewSong() {
    Song song1 = new Song();
    Song song2 = new Song();
    Song song3 = new Song();
    Song song4 = new Song();

if (song1.isEmpty()) {        
    System.out.println("Name of song:");
    song1.setName(console.next());
    System.out.println("Artist:");
    song1.setArtist(console.next());
    System.out.println("File size (MB):");
    song1.setFileSize(console.nextInt());
    System.out.println("Duration (seconds):");
    song1.setDuration(console.nextInt());
    System.out.println("Song successfully added.");
    System.out.println("");
}
else if (song2.isEmpty()) {        
    System.out.println("Name of song:");
    song2.setName(console.next());
    System.out.println("Artist:");
    song2.setArtist(console.next());
    System.out.println("File size (MB):");
    song2.setFileSize(console.nextInt());
    System.out.println("Duration (seconds):");
    song2.setDuration(console.nextInt());
    System.out.println("Song successfully added.");
    System.out.println("");
}
else if (song3.isEmpty()) {        
    System.out.println("Name of song:");
    song3.setName(console.next());
    System.out.println("Artist:");
    song3.setArtist(console.next());
    System.out.println("File size (MB):");
    song3.setFileSize(console.nextInt());
    System.out.println("Duration (seconds):");
    song3.setDuration(console.nextInt());
    System.out.println("Song successfully added.");
    System.out.println("");
}
else if (song4.isEmpty()) {        
    System.out.println("Name of song:");
    song4.setName(console.next());
    System.out.println("Artist:");
    song4.setArtist(console.next());
    System.out.println("File size (MB):");
    song4.setFileSize(console.nextInt());
    System.out.println("Duration (seconds):");
    song4.setDuration(console.nextInt());
    System.out.println("Song successfully added.");
    System.out.println("");
}
else {
    System.out.println("The database is currently full. Please delete a song before adding a new one.");
}

歌曲类：

public class Song {
private String name, artist;
private int fileSize, duration;
public Song(String name, String artist, int fileSize, int duration) {
    name = "";
    artist = "";
    fileSize = 0;
    duration = 0;
}
public Song(){}
public boolean isEmpty() {
    if (this.name == null && this.artist == null && this.fileSize == 0 && this.duration == 0 ) return true;
    else return false;
}
public void setName(String inputName) {
    inputName = name;
}
public String getName() {
    return name;
}
public void setArtist(String inputArtist) {
    artist = inputArtist;
}
public String getArtist() {
    return artist;
}
public void setFileSize(int inputFileSize) {
    if (inputFileSize>0){
        fileSize = inputFileSize;
    }
}
public int getFileSize() {
    return fileSize;
}
public void setDuration(int inputDuration) {
    if (inputDuration>0) {
    duration = inputDuration;
    }
}
public int getDuration() {
    return duration;
}
}

Answer 1

addNewSong()方法的输入时，您可以创建四个Song对象，然后将它们存储在本地变量中。退出方法时，本地变量会丢失，因此每次输入addNewSong()时，您都会有四首全新的歌曲。您可能需要将歌曲存储在课堂字段中，并在hard之前初始化它们（类似于您的console字段）。只需将这四行从addNewSong()方法移出：

Song song1 = new Song();
Song song2 = new Song();
Song song3 = new Song();
Song song4 = new Song();

还建议您考虑使用阵列或List的歌曲。当您添加新功能或增加歌曲数时，您的代码将很快变成一团糟。

Answer 2

您每次调用addNewSong()方法时都在初始化Song1..4。您需要将初始化外部化，可能是构造函数。

顺便说一句，我考虑使用ArrayList<Song>进行更灵活的实现

Answer 3

首先，您需要了解局部变量和范围。其次，如果固定插槽可以更好地使用阵列。

Song[] songs = new Songs[4];
for(int i=0;i<songs.length;i++){
   // Get input for song from user and use constructor
   songs[i] = new Song(name,artist,size,duration);
}

当您有构造函数时，为什么需要单独设置属性？您应该已经使用了构造函数本身。如果插槽不是固定的arrayList。

List<Song> songsList = new ArrayList<Song>();
// Get songs from user and add them to list.