java - 修复 Scanner 的 java.util.NoSuchElementException

Question

这个问题已经有答案了:

java.util.NoSuchElementException - Scanner reading user input (6 个回答)

已关闭 4 年前。

如何解决 java.util.NoSuchElementException错误？我调用扫描仪对象 3 次，程序将接受前两次输入。但在第三次尝试时，我立即得到 java.util.NoSuchElementException错误。扫描仪对象的名称是 stdInput .

我尝试仅为引发此错误的实例创建一个新的扫描仪对象，但我仍然收到相同的错误，只是来自代码中的不同行。

/**
 * reddit url:  https://www.reddit.com/r/dailyprogrammer/comments/pjbj8/easy_challenge_2/
 */

import java.util.Scanner;

public class challenge {

    public static void main(String[] args) {

        int choiceNum = 0;
        boolean continueRunningProgram = true;
        String repeatProgram = "";
        Scanner stdInput = new Scanner (System.in);

        do {

            System.out.println("Are you solving for force (1), mass (2), or acceleration (3)?");
            choiceNum = stdInput.nextInt();

            if(isValidChoiceNum(choiceNum) == false) {

                do {

                    System.out.println("The number " + choiceNum + " is an invalid choice.  Please choose again.");
                    choiceNum = stdInput.nextInt();

                } while(isValidChoiceNum(choiceNum) == false);

            }

            switch(choiceNum) {

                case 1:
                    System.out.println("The force is " + solvingForForce());
                    break;

                case 2:
                    System.out.println("The mass is " + solvingForMass());
                    break;

                case 3:
                    System.out.println("The acceleration is " + solvingForAcceleration());
                    break;
            }

            System.out.println("Would you like to solve another problem involving force, mass, and acceleration (Y/N)?");
            repeatProgram = stdInput.next();

            if(isValidChoiceChar(repeatProgram) == false) {

                do {

                    System.out.println("The letter " + repeatProgram + " is an invalid choice.  Please choose again.");
                    repeatProgram = stdInput.next();

                } while(isValidChoiceChar(repeatProgram) == false);

            }

            if(repeatProgram.compareTo("Y") == 0) {

                continueRunningProgram = true;

            } else {

                continueRunningProgram = false;

            }

        } while(continueRunningProgram == true);

        stdInput.close();

    } // end of main method

    public static boolean isValidChoiceNum(int c) {

        if(c < 1 || c > 3 ) {

            return false;

        } else {

            return true;

        }

    }

    public static boolean isValidChoiceChar(String c) {

        if(c.compareTo("Y") == 0 || c.compareTo("N") == 0) {

            return true;

        } else {

            return false;

        }

    }

    public static double solvingForForce() {

        Scanner stdInput2 = new Scanner (System.in);

        System.out.println("Please enter a value for mass.");
        double m = stdInput2.nextDouble();

        System.out.println("Please enter a value for acceleration.");
        double a = stdInput2.nextDouble();

        stdInput2.close();

        return m * a;

    }

    public static double solvingForMass() {

        Scanner stdInput2 = new Scanner (System.in);

        System.out.println("Please enter a value for force.");
        double f = stdInput2.nextDouble();

        System.out.println("Please enter a value for acceleration.");
        double a = stdInput2.nextDouble();

        stdInput2.close();

        return f / a;

    }

    public static double solvingForAcceleration() {

        Scanner stdInput2 = new Scanner (System.in);

        System.out.println("Please enter a value for force.");
        double f = stdInput2.nextDouble();

        System.out.println("Please enter a value for mass.");
        double m = stdInput2.nextDouble();

        stdInput2.close();

        return f * m;

    }

} // end of class

Answer 1

停止关闭链接到 System.in 的 Scanner 的疯狂行为!它将关闭底层流 (System.in)，导致任何其他从该 Stream 读取的尝试抛出异常。

是的，您会收到警告，但忽略该警告是安全的，或者您可以添加

@SuppressWarnings("resource")
Scanner in = new Scanner(System.in);

为了避免它。

请记住，如果您没有打开资源，则不应关闭它。让 JVM 在终止程序时关闭它。

<小时/>

这还包括在 try with resources block 内创建一个 Scanner(System.in):

try(Scanner in = new Scanner(System.in)){
   //Code... 
} 

这将在 block 末尾隐式关闭流。

来自Java tutorials :

The try-with-resources statement ensures that each resource is closed at the end of the statement.

<小时/>

此外，您还有多个从 System.in 读取的 Scanner 对象，这是不好的做法。我会将它们作为参数传递给您的方法:

public static double solvingForMass(Scanner in) {

    System.out.println("Please enter a value for force.");
    double f = in.nextDouble();

    System.out.println("Please enter a value for acceleration.");
    double a = in.nextDouble();

    return f / a;

}

<小时/>

另外请注意，如果您曾经做过以下结构:

if(someBool)
   return true;
else
   return false;

可以简化为

return someBool

因此您的 isValidChoice() 方法可以简化为:

public static boolean isValidChoiceChar(String c) {
     return c.compareTo("Y") == 0 || c.compareTo("N") == 0;
}

尽管请注意，您可以使用 equals() 和 equalsIgnoreCase()方法来比较String`的