通过键从 HashMap 获取值

Question

I need to compare the roman letters and get the correct integer out of it. If I'm correct, there should be a way to compare the hashmap key with the arraylist element and if they match, get the associated value from the key.

The return 2020 is there just for test purposes, since I wrote a JUnit test in a different class. It can be ignored for now.

I hope someone could give me a hint, since I wouldn't like to use the solutions from the web, because I need to get better with algorithms.

package com.company;

import java.util.*;

public class Main {
    static HashMap<String, Integer> romanNumbers = new HashMap<String, Integer>();

    static {
        romanNumbers.put("I", 1);
        romanNumbers.put("V", 5);
        romanNumbers.put("X", 10);
        romanNumbers.put("L", 50);
        romanNumbers.put("C", 100);
        romanNumbers.put("D", 500);
        romanNumbers.put("M", 1000);
    }

    public static void main(String[] args) {
        romanToArabic("MMXX");
    }

    static int romanToArabic(String roman) {
        ArrayList romanLetters = new ArrayList();
        roman = roman.toUpperCase();

        for (int i = 0; i < roman.length(); i++) {
            char c = roman.charAt(i);

            romanLetters.add(c);
        }
        // [M, M, X, X]
        System.out.println(romanLetters);

        // iterates over the romanLetters
        for (int i = 0; i < romanLetters.size(); i++) {
            System.out.println(romanLetters.get(i));
        }

        // retrive keys and values
        for (Map.Entry romanNumbersKey : romanNumbers.entrySet()) {
            String key = (String) romanNumbersKey.getKey();
            Object value = romanNumbersKey.getValue();

            System.out.println(key + " " + value);
        }

        return 2020;
    }
}

Answer 1

You could just Map.get each array element.

package com.company;

import java.util.ArrayList;
import java.util.HashMap;

public class Main {

    static HashMap<String, Integer> romanNumbers = new HashMap<String, Integer>();

    static {
        romanNumbers.put("I", 1);
        romanNumbers.put("V", 5);
        romanNumbers.put("X", 10);
        romanNumbers.put("L", 50);
        romanNumbers.put("C", 100);
        romanNumbers.put("D", 500);
        romanNumbers.put("M", 1000);
    }

    public static void main(String[] args) {
        System.out.println(romanToArabic("MMXX"));
    }

    static int romanToArabic(String roman) {
        ArrayList romanLetters = new ArrayList();
        roman = roman.toUpperCase();

        for (int i = 0; i < roman.length(); i++) {
            char c = roman.charAt(i);

            romanLetters.add(c);
        }
        // [M, M, X, X]
        System.out.println(romanLetters);

        // iterates over the romanLetters
        int sum = 0;
        for (int i = 0; i < romanLetters.size(); i++) {
            String key = String.valueOf(romanLetters.get(i));
            if (romanNumbers.containsKey(key)) {
                sum += romanNumbers.get(key);
            }
        }

        return sum;
    }

}

As stated in the comments, this just answers your question of how to get values from an hashmap where keys are the array elements. It is not a solution to calculate the numeric value out of a roman number. For that, you will have to look at the next letter, and join if it is larger, before consulting the map for the final value to sum.

Answer 2

I'd like to suggest a completly different approach using an enum.

public enum RomanNumber {
    I(1), V(5), X(10), L(50), C(100), D(500), M(1000);

    private final int arabic;

    RomanNumber(int arabic) {
        this.arabic = arabic;
    }

    public int getArabicNumber() {
        return arabic;
    }

    // This is obviously broken. IV wouldn't work for example.
    public static int toArabic(String romanLetters) {
        romanLetters = romanLetters.toUpperCase();
        int arabicResult = 0;
        for (int i = 0; i < romanLetters.length(); i++) {
            char romanNumber = romanLetters.charAt(i);
            // valueOf(String) returns the enum based on its name. So a String of "I" returns the RomanNumber I enum.
            int arabicNumber = valueOf(String.valueOf(romanNumber)).getArabicNumber();
            arabicResult = arabicResult + arabicNumber;
        }
        return arabicResult;
    }
}

Can be used like this:

String romanNumber = "MMXX";
System.out.println(RomanNumber.toArabic(romanNumber));

Btw. every enum has an ordinal() method which returns the declaration position inside the enum. (I.ordinal() == 1 or V.ordinal() == 2) I think this could help you with the IV problem aswell. :)