Metric Converter(Simple Conditions)

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double distance = Double.parseDouble(scanner.nextLine());
        String inputMetric = scanner.nextLine(),
                outputMetric = scanner.nextLine();

        if (inputMetric.equals("m")) {
            distance /= 1000;
        } else if (inputMetric.equals("cm")) {
            distance /= 100000;
        } else if (inputMetric.equals("mm")) {
            distance /= 1000000;
        } else if (inputMetric.equals("mi")) {
            distance /= 0.621371192;
        } else if (inputMetric.equals("in")) {
            distance /= 39370.0787;
        } else if (inputMetric.equals("ft")) {
            distance /= 3280.8399;
        } else if (inputMetric.equals("yd")) {
            distance /= 1093.6133;
        }

        if (outputMetric.equals("m")) {
            distance *= 1000;
        } else if (outputMetric.equals("cm")) {
            distance *= 100000;
        } else if (outputMetric.equals("mm")) {
            distance *= 1000000;
        } else if (outputMetric.equals("mi")) {
            distance *= 0.621371192;
        } else if (outputMetric.equals("in")) {
            distance *= 39370.0787;
        } else if (outputMetric.equals("ft")) {
            distance *= 3280.8399;
        } else if (outputMetric.equals("yd")) {
            distance *= 1093.6133;
        }

        System.out.printf("%f", distance);
    }
}

ИЛИ:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double distance = Double.parseDouble(scanner.nextLine());
        String inputMetric = scanner.nextLine(),
                outputMetric = scanner.nextLine();

        switch (inputMetric) {
            case "m" -> distance /= 1000;
            case "cm" -> distance /= 100000;
            case "mm" -> distance /= 1000000;
            case "mi" -> distance /= 0.621371192;
            case "in" -> distance /= 39370.0787;
            case "ft" -> distance /= 3280.8399;
            case "yd" -> distance /= 1093.6133;
        }

        switch (outputMetric) {
            case "m" -> distance *= 1000;
            case "cm" -> distance *= 100000;
            case "mm" -> distance *= 1000000;
            case "mi" -> distance *= 0.621371192;
            case "in" -> distance *= 39370.0787;
            case "ft" -> distance *= 3280.8399;
            case "yd" -> distance *= 1093.6133;
        }

        System.out.printf("%f", distance);
    }
}

Решение с тернарен оператор:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double distance = Double.parseDouble(scanner.nextLine());
        String inputMetric = scanner.nextLine(),
                outputMetric = scanner.nextLine();

        distance /=
                inputMetric.equals("m") ? 1000 :
                        inputMetric.equals("cm") ? 100000 :
                                inputMetric.equals("mm") ? 1000000 :
                                        inputMetric.equals("mi") ? 0.621371192 :
                                                inputMetric.equals("in") ? 39370.0787 :
                                                        inputMetric.equals("ft") ? 3280.8399 :
                                                                inputMetric.equals("yd") ? 1093.6133 : 1;

        distance *=
                outputMetric.equals("m") ? 1000 :
                        outputMetric.equals("cm") ? 100000 :
                                outputMetric.equals("mm") ? 1000000 :
                                        outputMetric.equals("mi") ? 0.621371192 :
                                                outputMetric.equals("in") ? 39370.0787 :
                                                        outputMetric.equals("ft") ? 3280.8399 :
                                                                outputMetric.equals("yd") ? 1093.6133 : 1;

        System.out.printf("%f", distance);
    }
}

Решение с колекция:

import java.util.Map;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double distance = Double.parseDouble(scanner.nextLine());
        String inputMetric = scanner.nextLine(),
                outputMetric = scanner.nextLine();

        Map<String, Double> metricsToKM = Map.of(
                "km", 1.0,
                "m", 1000.0,
                "cm", 100000.0,
                "mm", 1000000.0,
                "mi", 0.621371192,
                "in", 39370.0787,
                "ft", 3280.8399,
                "yd", 1093.6133
        );

        System.out.printf("%f", distance / metricsToKM.get(inputMetric) * metricsToKM.get(outputMetric));
    }
}