How To Write Conversion Method From Roman Numerals To Arabic

I am a student taking a Java programming class. My assignment is to write a program that converts a roman numeral input as entered by a user and converting it to it's integer value (arabic number). These are the methods that I must have in it:

1. Write a method that takes input from the user and passes it to a conversion method.
2. Write a method that yields the numeric value of each of the letters (conversion method).
3. Write a method that outputs the number the user entered and the converted number.
4. Write a main method to test the 3 methods.

I have written the first method, at least I think. Here is what I did there:

public static String romanInput(String number) {
Scanner numberInput = new Scanner (System.in);
System.out.print("Enter a roman numeral: ");
String userInput = numberInput.next();
return userInput;
}

I returned the userInput and I think that is passing it to the conversion method? Now I am working on this conversion method and to be honest I don't know where to begin. I am told how to convert a string in my assignment by the professor. I am told:

- Look at the first two characters. If the first has a larger value than the second, then simply convert the first.
- Call the conversion method again for the substring starting with the second character.
-Add both values. If the first one has a smaller value than the second, compute the difference and add to it the conversion of the tail.

I am also told to use a single-dimensional array. But, I don't know what I am to use a single dimensional array for? So this is what I wrote so far for this method:

public static int numberConversion(int number) {

int romanConv = 0;
char[] romanNum = {'I', 'V', 'X', 'L', 'C', 'D', 'M'};
char[] arabicNum = {1, 5, 10, 50, 100, 500, 1000};
romanNum = arabicNum;
}

I have written a character array for the roman numerals, and then one for arabic numerals, then I set them equal to each other. I also declared an integer variable set to 0 because I think that is what I will be returning at the end of the method. Now I don't know where to start for the conversion algorithm here. I know this is what I have to do, but I don't know how to do it:

1. Add the numbers together if they are in decreasing value or are equal in value. For example: VI is read as 5 + 1 = 6 XVI is read as 10 + 5 + 1 = 16 XXXVIII is 10 + 10 + 10 + 5 + 1 + 1 + 1 = 38
2. Use subtraction if a number is less than the number that follows it. For example, I is less than V, so when I is in front of V, you subtract its value.
3. For example: IV is 5 1 = 4 IX is 10 1 = 9 XL is 50 10 = 40 MCM is 1,000 + (1,000 - 100) = 1,900

I can't use hashtables or enums because I haven't learned about that yet. I have a feeling I need to use a for loop. I know I haven't done any of the actual programming work but I don't know how to begin writing this conversion method.