java.util.NoSuchElementException:在java.util.Scanner.nextLine中找不到行(Scanner.java:1585)

Question

我一遍又一遍地提示用户输入,直到用户输入exit,在这种情况下数据对象将被设置为null并且它将离开循环.在大多数情况下,这段代码可以正常工作,但有时候我会得到一个NoSuchElementException : reader = new Scanner(System.in);. 这往往发生在几个循环后,而不是第一次通过.如果我在while循环的范围内声明读者变量,那么它将在第二个循环中失败.

大多数工作(有时抛出异常)

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(在第二个循环中抛出异常)

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

Scanner在代码的开头分配,而不是每次循环:

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();

你这样做的方式Scanner,你创建的每一个都是在下一个循环迭代中孤立的.他们都没有被关闭.当从所有这些垃圾Scanner对象中消耗一些内部资源时,很可能会出现异常.