if...elseif 一次又一次输入值时出错

Question

在这个程序中，我必须在循环中一次又一次地输入温度。当我输入温度时，它会显示正确的显示，但是当我再次输入温度时，程序会自行终止。

代码是：

import java.util.Scanner;

public class CheckTemperature
{
    public static void main(String [] args) 
    {
        final double TEMPERATURE = 102.5;
        double thermostat;

        Scanner keyboard = new Scanner(System.in);
        System.out.println("Enter the substance's temperature: ");
        thermostat = keyboard.nextDouble();

        if(thermostat > TEMPERATURE)
        {
            System.out.print("The temperature is too high. ");
            System.out.print("Turn down the thermostat. ");
            System.out.println("Wait for 5 minutes and check the thermostat again. "); 
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }
        else if(thermostat < TEMPERATURE)
        {
            System.out.println("The temperature is low.");
            System.out.println("Turn up the thermostat.");
            System.out.println("Check for 5 minutes and check again.");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }
        else if(thermostat == TEMPERATURE)
        {
            System.out.println("The temperature is acceptable. Check again in 15 minutes.");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }
        else
        {
            System.out.println("Enter the correct temperature value ");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }
    }
}

Answer 1

您的代码不是为了“重复”任何任务 AKA 迭代而构建的。在您的代码中添加一个循环，使其根据需要重复操作。

这里我们使用while (1==1) 来创建一个无限循环。此外，我们在检查输入 == "X" 时添加了退出条件，这会从循环中触发 System.exit。这有助于保持应用程序的正常退出，而无需强制关闭。

    final double TEMPERATURE = 102.5;
    double thermostat;
    while(true){
           Scanner keyboard = new Scanner(System.in);
           System.out.println("Enter the substance's temperature or press X to exit: ");
           String input = keyboard.next();
           if(input.equals("X")){
              System.exit(0);
           } else{
              thermostat = Double.parseDouble(input);   
              if(thermostat > TEMPERATURE) {
                    System.out.print("The temperature is too high. ");
                    System.out.print("Turn down the thermostat. ");
                    System.out.println("Wait for 5 minutes and check the thermostat again. "); 
                } else if(thermostat < TEMPERATURE) {
                    System.out.println("The temperature is low.");
                    System.out.println("Turn up the thermostat.");
                    System.out.println("Check for 5 minutes and check again.");
                } else if(thermostat == TEMPERATURE) {
                    System.out.println("The temperature is acceptable. Check again in 15 minutes.");
                } else {
                    System.out.println("Enter the correct temperature value ");
                }
           }
    }

Answer 2

这是因为您不在循环中。你在顶部问一次温度。 然后一次if，那么就结束了。

public static void main(String [] args) 
{
    final double TEMPERATURE = 102.5;
    double thermostat;
    Scanner keyboard = new Scanner(System.in);

    while (true) {
        System.out.println("Enter the substance's temperature: ");
        thermostat = keyboard.nextDouble();

        if(thermostat > TEMPERATURE)
        {
            System.out.print("The temperature is too high. ");
            System.out.print("Turn down the thermostat. ");
            System.out.println("Wait for 5 minutes and check the thermostat again. ");
        }
        else if(thermostat < TEMPERATURE)
        {
            System.out.println("The temperature is low.");
            System.out.println("Turn up the thermostat.");
            System.out.println("Check for 5 minutes and check again.");
        }

        else if(thermostat == TEMPERATURE) {

            System.out.println("The temperature is acceptable. Check again in 15 minutes.");
        }

        else{
            System.out.println("Enter the correct temperature value ");
        }
    }
}

小心，while 永远不会停止，但你可以随意改变条件。

Answer 3

如果您想一次又一次地获取温度，则必须使用循环。

while(true){
    if(thermostat > TEMPERATURE)
        {
            System.out.print("The temperature is too high. ");
            System.out.print("Turn down the thermostat. ");
            System.out.println("Wait for 5 minutes and check the thermostat again. "); 
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();

        }
        else if(thermostat < TEMPERATURE)
        {
            System.out.println("The temperature is low.");
            System.out.println("Turn up the thermostat.");
            System.out.println("Check for 5 minutes and check again.");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();

        }

        else if(thermostat == TEMPERATURE) {

            System.out.println("The temperature is acceptable. Check again in 15 minutes.");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }

        else{
            System.out.println("Enter the correct temperature value ");
            System.out.println("Enter the thermostat here: ");
            thermostat = keyboard.nextDouble();
        }
    }

Answer 4

Murtaza，您可以只使用循环，也可以使用任何递归方法并在您的主要方法中调用它。您所说的 DOUBLE CANNOT BE DEREFERENCED 错误是由于没有像 equals 这样的方法() 在 Scanner 类中。此外，您不应将 int 值与 double 类型变量进行比较。再次，要在 if-else 块内进行比较，您应该使用 Double 中存在的 equals() 方法，包装类反对传递一个对象作为它的参数。