Java 尽管计算错误，但扫描程序表达式仍在继续的While循环

对java来说非常陌生，并且想知道为什么即使来自扫描器的输入是“N”，这个while语句也不会结束，从而将表达式求值为false

import java.util.*;

public class addSongs {

    public static void main (String[]args){

        Scanner songAdd = new Scanner(System.in);
        Scanner addContinue = new Scanner(System.in);
        String ceaseAdd = new String();
        ceaseAdd = "Y";

        while(ceaseAdd != "N")
            System.out.println("Enter track title");
            String newSong = songAdd.nextLine();
            MP3_catalogue.title.add(newSong);
            System.out.println("Enter artist name");
            String newArtist = songAdd.nextLine();
            MP3_catalogue.artist.add(newArtist);
            System.out.println("Enter duration");
            String newDuration = songAdd.nextLine();
            MP3_catalogue.duration.add(newDuration);
            System.out.println("Would you like to add another song? Y/N");
            ceaseAdd = addContinue.nextLine().toUpperCase();
    }

}

---

在java中，不能将字符串与“=”进行比较。 “==”检查它是否是完全相同的对象，而不是它是否具有相同的值。 要检查其文本是否相等，必须使用

string.equals()

在您的示例中，它将如下所示：

public static void main (String[]args){

Scanner songAdd = new Scanner(System.in);
Scanner addContinue = new Scanner(System.in);
String ceaseAdd = new String();
ceaseAdd = "Y";

while(!ceaseAdd.equals("N"))
    System.out.println("Enter track title");
    String newSong = songAdd.nextLine();
    MP3_catalogue.title.add(newSong);
    System.out.println("Enter artist name");
    String newArtist = songAdd.nextLine();
    MP3_catalogue.artist.add(newArtist);
    System.out.println("Enter duration");
    String newDuration = songAdd.nextLine();
    MP3_catalogue.duration.add(newDuration);
    System.out.println("Would you like to add another song? Y/N");
    ceaseAdd = addContinue.nextLine().toUpperCase();
}

while(ceaseAdd != "N")
    System.out.println("Enter track title");
String newSong = songAdd.nextLine();
MP3_catalogue.title.add(newSong);
System.out.println("Enter artist name");

---

除了已经指出的字符串检查。缺少

{}

。正确缩进的代码如下所示：

public static void main (String[]args){

Scanner songAdd = new Scanner(System.in);
Scanner addContinue = new Scanner(System.in);
String ceaseAdd = new String();
ceaseAdd = "Y";

while(!ceaseAdd.equals("N"))
    System.out.println("Enter track title");
    String newSong = songAdd.nextLine();
    MP3_catalogue.title.add(newSong);
    System.out.println("Enter artist name");
    String newArtist = songAdd.nextLine();
    MP3_catalogue.artist.add(newArtist);
    System.out.println("Enter duration");
    String newDuration = songAdd.nextLine();
    MP3_catalogue.duration.add(newDuration);
    System.out.println("Would you like to add another song? Y/N");
    ceaseAdd = addContinue.nextLine().toUpperCase();
}

while(ceaseAdd != "N")
    System.out.println("Enter track title");
String newSong = songAdd.nextLine();
MP3_catalogue.title.add(newSong);
System.out.println("Enter artist name");

如果没有

{}

，则

while

仅适用于第一行。应该是这样的：

while(!ceaseAdd.equals("N")){      
    System.out.println("Enter track title");
    String newSong = songAdd.nextLine();
    MP3_catalogue.title.add(newSong);
    System.out.println("Enter artist name");
    String newArtist = songAdd.nextLine();
    MP3_catalogue.artist.add(newArtist);
    System.out.println("Enter duration");
    String newDuration = songAdd.nextLine();
    MP3_catalogue.duration.add(newDuration);
    System.out.println("Would you like to add another song? Y/N");
    ceaseAdd = addContinue.nextLine().toUpperCase();
}

---

将

String

值与

String

的

equals

方法进行比较，而不是与

=

或

进行比较=。这：应该有助于您理解@rgetman的评论为什么是正确的。您的代码和您对发生的事情的描述不一致。您说即使输入了“N”，它也会运行。使用此代码，您将永远无法为
ceaseAdd
输入任何内容，它将永远打印“输入曲目标题”。