我正在自学java,在使用类遇到代码时遇到了这个错误
Exception in thread "main" java.util.NoSuchElementException
at java.util.Scanner.throwFor(Unknown Source)
at java.util.Scanner.next(Unknown Source)
at java.util.Scanner.nextDouble(Unknown Source)
at StateCalculator.getOperand(StateCalculator.java:29)
at StateCalculator.main(StateCalculator.java:77)
以下是我的代码:
import java.util.Scanner;
public class StateCalculator {
private double currentValue = 0;
//Initialize to 0
public StateCalculator() {
}
public static int displayMenu() {
Scanner keyboard = new Scanner(System.in);
int menuChoice = 0;
do {
System.out.print("Menu\n 1. Add\n 2. Subtract\n 3. Multiply\n 4. Divide\n 5.Clear\n 6. Quit\n What would you like to do?: ");
menuChoice = keyboard.nextInt();
} while(menuChoice < 1 || menuChoice > 6);
keyboard.close();
return menuChoice;
}
public static double getOperand(String prompt) {
Scanner input = new Scanner(System.in);
double operand = 0;
System.out.print(prompt);
operand = input.nextDouble();
input.close();
return operand;
}
public double getCurrentValue() {
return currentValue;
}
public void add(double operand) {
currentValue += operand;
}
public void subtract(double operand) {
currentValue -= operand;
}
public void multiply(double operand) {
currentValue *= operand;
}
public void divide(double operand) {
if(operand == 0) {
currentValue = Double.NaN;
}
else {
currentValue /= operand;
}
}
public void clear() {
currentValue = 0;
}
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
StateCalculator calculator = new StateCalculator();
int option;
double operand;
do{
System.out.println("The current value is " + calculator.currentValue);
option = StateCalculator.displayMenu();
switch(option) {
case 1:
operand = getOperand("What is the second number?: ");
calculator.add(operand);
break;
case 2:
operand = getOperand("What is the second number?: ");
calculator.subtract(operand);
break;
case 3:
operand = getOperand("What is the second number?: ");
calculator.multiply(operand);
break;
case 4:
operand = getOperand("What is the second number?: ");
calculator.divide(operand);
break;
case 5:
calculator.clear();
break;
}
}while(option != 6);
keyboard.close();
}
}
我尝试在eclipse中运行调试功能,并在我尝试设置operand = input.nextDouble时发现问题出现在我的getOperand方法的第29行。但是,我不明白为什么这会成为一个问题。
答案 0 :(得分:5)
关闭keyboard.close();
(您定义的)
keyboard
Scanner keyboard = new Scanner(System.in);
它关闭System.in
,然后你的其他方法无法工作(因为控制台不会重新打开)。您可以在System.in
上拥有多个扫描程序(只要您不关闭它们),或者传递一个(或者,请不要使用全局扫描程序)。
根据javadoc,
当
Scanner
关闭时,如果源实现Closeable
接口,它将关闭其输入源。