example

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

public class Example{

    //converter class with constructor
    public static class Converter {
        HashMap<String, Integer> conversionMap = new HashMap<String, Integer>();

        // this constructor runs every time this object is instantiated
        public Converter() {

             conversionMap.put("cm", 10);
             conversionMap.put("dm", 100);
             conversionMap.put("m", 1000);
        }

        public int convert(String unit, int number){
                // perform conversion
            if(conversionMap.containsKey(unit)){
                return number * this.conversionMap.get(unit);
            }
            else{
                return -1;
            }
        }
    }


     public static void main(String []args){

        // initialize converter object
        Converter conv = new Converter();

        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 = conv.convert(unit, number); // 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");
        }

     }
}