example

import java.util.HashMap;
import java.util.Scanner;

public class Example{

    public static int convert(String unit, int number, HashMap<String, Integer> hmap){

        // perform conversion
        if(hmap.containsKey(unit)){
            return number * hmap.get(unit);
        }
        else{
            return -1;
        }

    }

     public static void main(String []args){


        // create hashmap with conversion values
        HashMap<String, Integer> conversionMap = new HashMap<String, Integer>();
        conversionMap.put("cm", 10);
        conversionMap.put("dm", 100);
        conversionMap.put("m", 1000);

        Scanner myScanner = new Scanner(System.in);  // Create a Scanner object
        System.out.println("Enter unit: ");

        String unit = myScanner.nextLine();  // Read conversion unit

        System.out.println("Enter number to convert: ");
        int number = myScanner.nextInt(); // Read number to convert

        int result = convert(unit, number, conversionMap); // Convert number to millimeters

        //check that unit exists
        if(result == -1){
            //if unit does not exist, print error message
            System.out.println("Unit not found, please try again..");
        }
        else{
            //if unit exists print result
            System.out.println(number + " " + unit + " = " + result + "mm");
        }

     }
}