Kevin Trujillo Kevin Trujillo - 4 months ago 12
Java Question

Trying to pass around a String but receiving "null"

This is probably a simple error that I am over looking but my problem is that when I am trying to return the string "roman", it just returns null.
This is the main program:

/* CCTPG-22 // Assignment 08: Roman Numerals
* @author Kevin R. Trujillo
* @version 10/28/2015
* Purpose: Converts numbers into roman numerals format and vice-a-versa
* Status:
*/

import java.util.*; // For Scanner

public class RomanMain
{
public static void main(String[] args)
{

Scanner in = new Scanner(System.in);

RomanYear year1 = new RomanYear(1975);
RomanYear year2 = new RomanYear(1988);
RomanYear year3 = new RomanYear(2007);

System.out.println(year1.toString());
System.out.println(year2.toString());
System.out.println(year3.toString());
int diff21 = year2.getYearDecimal() - year1.getYearDecimal();
int diff32 = year3.getYearDecimal() - year2.getYearDecimal();

RomanYear y2MinusY1 = new RomanYear(diff21);
RomanYear y3MinusY2 = new RomanYear(diff32);
System.out.println("Year2 minus Year1 is: " + y2MinusY1.getYearRoman());
System.out.println("Year3 minus Year2 is: " + y3MinusY2.getYearRoman());


}

// Add new methods here


} // No code can be here (outside the class)


and here is the RomanYear class:

/**
* Auto Generated Java Class.
*/
public class RomanYear {

private int decimal ;
private String roman ;

public RomanYear() // default constructor
{
decimal = 0 ;
roman = "" ;
}
public RomanYear(int newYear)
{
decimal = newYear ;
roman = setYearDecimal(decimal);
}

public RomanYear(String newYear )
{
roman = newYear ;
decimal = setYearRoman(roman) ;
}

public int setYearRoman(String roman)
{

String romanNumeral = roman.toUpperCase();
for(int x = 0;x < romanNumeral.length(); x++)
{
char convertToDecimal = roman.charAt(x);
// first step: Easy stuff
switch (convertToDecimal)
{
case 'M': decimal += 1000; break;
case 'D': decimal += 500; break;
case 'C': decimal += 100; break;
case 'L': decimal += 50; break;
case 'X': decimal += 10; break;
case 'V': decimal += 5; break;
case 'I': decimal += 1; break;
}
}
// Now adapt for specials
if (romanNumeral.contains("IV"))
{
decimal-=2;
}
if (romanNumeral.contains("IX"))
{
decimal-=2;
}
if (romanNumeral.contains("XL"))
{
decimal-=20;
}
if (romanNumeral.contains("XC"))
{
decimal-=20;
}
if (romanNumeral.contains("CD"))
{
decimal-=200;
}
if (romanNumeral.contains("CM"))
{
decimal-=200;
}
return decimal ;
}

public String setYearDecimal(int yr)
{
if (decimal > 3999)
{
System.out.println("Decimal Number: " + decimal + " is over 3999. ") ;
System.out.println("Please enter a new number") ;
System.out.println("Program is ending.............") ;
try {
Thread.sleep(2000); //5000 milliseconds is one second.
}
catch(InterruptedException ex) {
Thread.currentThread().interrupt();
}

System.exit(0) ;
}

else
{
int digit;
String roman = "";

// 1000's column
digit = yr/1000;
for (int i = 0; i < digit; i++)
roman = roman + "M";
yr = yr % 1000; // leaves 0 to 999

// 100s column
String [] hunds = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
digit = yr/100;
roman = roman + hunds[digit];
yr = yr % 100; // leaves 0 to 99

// 10s column
String [] tens = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
digit = yr/10;
roman = roman + tens[digit];
yr = yr % 10; // leaves 0 to 9

// Ones column
String [] ones = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
roman = roman + ones[yr];

}
return roman ;
}

public int getYearDecimal()
{
return decimal ;
}

public String getYearRoman()
{
return roman ;
}

public String toString()
{

System.out.print("Decimal: " + decimal + " as Roman Numerals is " ) ;
return roman ;
}

/* ADD YOUR CODE HERE */

}


If someone could just explain what I am doing wrong, that would be much appreciated rather than just posting the "correct way" to do it.

Thank you!

Answer

On the top of the Roman class you create a variable called roman.

 public class RomanYear {

  private int decimal ;
  private String roman ;

In setYearDecimal you create a new variable called roman. That local variable hides the global one.

int digit;
String roman = "";

When you do roman = roman + "M"; you are only updating the local roman variable. The global one remains empty.

All you need to do is remove String from in front of the second variable.