fix: handle Null Dereference in RomanToInteger

Author: vil02

.inferconfig

@@ -1,7 +1,6 @@
 {
     "report-block-list-path-regex": [
         "src/main/java/com/thealgorithms/ciphers/a5/CompositeLFSR.java",
-        "src/main/java/com/thealgorithms/conversions/RomanToInteger.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/GCounter.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/PNCounter.java",
         "src/main/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithm.java",

src/main/java/com/thealgorithms/conversions/RomanToInteger.java

@@ -19,6 +19,10 @@ private RomanToInteger() {
         }
     };
 
+    private static int romanSymbolToInt(final char symbol) {
+        return ROMAN_TO_INT.computeIfAbsent(symbol, c -> { throw new IllegalArgumentException("Unknown Roman symbol: " + c); });
+    }
+
     // Roman Number = Roman Numerals
 
     /**
@@ -39,10 +43,10 @@ public static int romanToInt(String a) {
 
             if (prev != ' ') {
                 // checking current Number greater than previous or not
-                newPrev = ROMAN_TO_INT.get(prev) > newPrev ? ROMAN_TO_INT.get(prev) : newPrev;
+                newPrev = romanSymbolToInt(prev) > newPrev ? romanSymbolToInt(prev) : newPrev;
             }
 
-            int currentNum = ROMAN_TO_INT.get(c);
+            int currentNum = romanSymbolToInt(c);
 
             // if current number greater than prev max previous then add
             if (currentNum >= newPrev) {

src/test/java/com/thealgorithms/conversions/RomanToIntegerTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.conversions;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
 
@@ -11,4 +12,11 @@ public void testRomanToInteger() {
         assertEquals(1994, RomanToInteger.romanToInt("MCMXCIV"));
         assertEquals(58, RomanToInteger.romanToInt("LVIII"));
     }
+
+    @Test
+    void testRomanToIntegerThrows() {
+        assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("Z"));
+        assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("MZI"));
+        assertThrows(IllegalArgumentException.class, () -> RomanToInteger.romanToInt("MMMO"));
+    }
 }