我在传递 char 而不是 int 后得到奇怪的循环行为

您的方法可能包含：main()

System.out.println("---< Fahrenheit / Celsius Converter >---");
String direction = "";
// Use of the String#matches() method with a small Regular Expression.
// (?i)     Ignore Letter Case
// [FC]     Only allow the character q, or Q
while (!direction.matches("(?i)[q]")) {
    direction = getDirection();
    if (!direction.matches("(?i)[q]")) {
        int temperature = getTemperature(direction);
        printResult(direction, temperature);
    }
    System.out.println();
}

该方法可能如下所示：getDirection()

// let's get direction of conversion from user input
public static String getDirection() {
    String direction = "";
    while (direction.isEmpty()) {
        System.out.println("Enter temperature scale to convert:");
        System.out.println("  F) Fahrenheit");
        System.out.println("  C) Celsius");
        System.out.println("  Q) Quit");
        System.out.print("Your choice: --> ");
        direction = sc.nextLine();
        if (direction.equalsIgnoreCase("q")) {
            System.out.println("\nBye-Bye\n");
            break;
        }
        // Use of the String#matches() method with a small Regular Expression.
        // (?i)     Ignore Letter Case
        // [FC]     Only allow the characters F, f, C, or c (q is handled ahead of time)
        if (!direction.matches("(?i)[FC]")) {
            System.err.println("Invalid Temperature Scale Type (" + direction 
                             + ")! Must be F or C! Try again...\n");
            direction = "";
        }
    } 
    return direction;
}

该方法可能如下所示：getTemperature()

// let's get temperature from user
private static int getTemperature(String direction) {
    String temp = "";
    // 'Ternary Operators' used here
    String directString = (direction.equalsIgnoreCase("f") ? "Fahrenheit" : "Celsius");
    String otherDirectString = (direction.equalsIgnoreCase("f") ? "Celsius" : "Fahrenheit");
    System.out.println();
    System.out.println("Convert a Temperature in " + directString + " to " + otherDirectString + ":");
    do {
        System.out.print("Enter a temperature in " + directString + ": --> ");
        temp = sc.nextLine().trim();
        // Since you're working with integer for temperature...
        // Use of the String#matches() method with a small Regular Expression.
        // ("\\d+")   Must be a string representation of an Integer numerical 
        // value with 1 (or possibly) more digits.
        if (!temp.matches("\\d+")) {
            System.err.println("Invalid Temperature Supplied (" + temp + ")! Try Again..." );
            temp = "";
        }
    } while (temp.isEmpty());
    return Integer.valueOf(temp);
}

之所以改名，是因为这就是它本质上所做的一切。它并没有真正得到任何东西：getResult()printResult()

// let's get result using direction and temperature we got
static void printResult(String direction, int temperature) {
    // 'Ternary Operator' used here
    System.out.println("Converted temperature is : " 
            + (direction.toUpperCase().equals("F") 
                 ? fahrenheitToCelsius(temperature) + "C" 
                     : celsiusToFahrenheit(temperature) + "F"));
}

其他杂项方法保持不变：

// let's convert Fahrenheit to Celsius
public static int fahrenheitToCelsius(int temperatureF) {
    return (temperatureF - 32) * 5 / 9;
}

// let's convert Celsius to Fahrenheit
public static int celsiusToFahrenheit(int temperatureC) {
    return temperatureC * 9 / 5 + 32;
}

阅读代码中的注释。您当然会注意到，使用 String#matches（） 方法进行验证，以及注释中解释的各种小正则表达式。

您还会注意到使用三元运算符来减少 IF/ELSE 语句的显示。