java.util.NoSuchElementException : No line found at java. util.Scanner.nextLine(Scanner.java:1585)

Question

我一遍又一遍地提示用户输入，直到用户键入退出，在这种情况下，数据对象将被设置为空，并且它将离开循环。在大多数情况下，此代码可以工作，但有时我会在行上收到 NoSuchElementException:reader = new Scanner(System.in);。这种情况往往发生在几次循环之后，而不是第一次。如果我在 while 循环的范围内声明 reader 变量，那么它将在第二个循环中失败。

大部分工作正常(有时会抛出异常)

int level = 1;  
Scanner reader;
String selection = null;

while (true) {
    if (data.done) {
        level--;
    }

    data.done = false;
    System.out.println(getSubmenu(level, data));

    reader = new Scanner(System.in);

    if (level <6) {
        selection = reader.nextLine();
    } else {
        level = 4;
    }

    if (validSelection(selection)) {
        level = getLevel(level, selection);
        data = getData(level, data, selection);
    } else {
        System.out.println("Invalid entry");
    }

    if (data == null) {
        System.out.println("Level "+ level + "selection " + selection);
        break; // exit command was typed
    }
}
reader.close();

替代(在第二个循环中引发异常)

int level = 1;  
String selection = null;

while (true) {
    if (data.done) {
        level--;
    }

    data.done = false;
    System.out.println(getSubmenu(level, data));

    Scanner reader = new Scanner(System.in);

    if (level <6) {
        selection = reader.nextLine();
    } else {
        level = 4;
    }

    if (validSelection(selection)) {
        level = getLevel(level, selection);
        data = getData(level, data, selection);
    } else {
        System.out.println("Invalid entry");
    }

    if (data == null) {
        System.out.println("Level "+ level + "selection " + selection);
        break; // exit command was typed
    }
    reader.close();
}

堆栈跟踪

java.util.NoSuchElementException: No line found
at java.util.Scanner.nextLine(Scanner.java:1585)
at org.functional.utils.Menu.run(ServerScripts.java:61)
at org.functional.utils.ServerScripts.main(ServerScripts.java:18)

我错过了什么？

Answer 1

在代码开头分配Scanner，而不是每次循环时分配:

int level = 1;  
Scanner reader = new Scanner(System.in);
String selection = null;

while (true) {
    if (data.done) {
        level--;
    }

    data.done = false;
    System.out.println(getSubmenu(level, data));

    if (level <6) {
        selection = reader.nextLine();
    } else {
        level = 4;
    }

    if (validSelection(selection)) {
        level = getLevel(level, selection);
        data = getData(level, data, selection);
    } else {
        System.out.println("Invalid entry");
    }

    if (data == null) {
        System.out.println("Level "+ level + "selection " + selection);
        break; // exit command was typed
    }
}
reader.close();

按照您的做法，您创建的每个 Scanner 在下一个循环迭代中都会被孤立。他们都没有被关闭。当所有这些垃圾 Scanner 对象消耗一些内部资源时，最有可能出现异常。