refactor: Enhance docs, add more tests in IntegerToEnglish

Author: Hardvan

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

@@ -2,7 +2,28 @@
 
 import java.util.Map;
 
+/**
+ * A utility class to convert integers to their English word representation.
+ *
+ * <p>The class supports conversion of numbers from 0 to 2,147,483,647
+ * (the maximum value of a 32-bit signed integer). It divides the number
+ * into groups of three digits (thousands, millions, billions, etc.) and
+ * translates each group into words.</p>
+ *
+ * <h2>Example Usage</h2>
+ * <pre>
+ *   IntegerToEnglish.integerToEnglishWords(12345);
+ *   // Output: "Twelve Thousand Three Hundred Forty Five"
+ * </pre>
+ *
+ * <p>This class uses two maps:</p>
+ * <ul>
+ *   <li>BASE_NUMBERS_MAP: Holds English words for numbers 0-20, multiples of 10 up to 90, and 100.</li>
+ *   <li>THOUSAND_POWER_MAP: Maps powers of 1000 (e.g., Thousand, Million, Billion).</li>
+ * </ul>
+ */
 public final class IntegerToEnglish {
+
     private static final Map<Integer, String> BASE_NUMBERS_MAP = Map.ofEntries(Map.entry(0, ""), Map.entry(1, "One"), Map.entry(2, "Two"), Map.entry(3, "Three"), Map.entry(4, "Four"), Map.entry(5, "Five"), Map.entry(6, "Six"), Map.entry(7, "Seven"), Map.entry(8, "Eight"), Map.entry(9, "Nine"),
         Map.entry(10, "Ten"), Map.entry(11, "Eleven"), Map.entry(12, "Twelve"), Map.entry(13, "Thirteen"), Map.entry(14, "Fourteen"), Map.entry(15, "Fifteen"), Map.entry(16, "Sixteen"), Map.entry(17, "Seventeen"), Map.entry(18, "Eighteen"), Map.entry(19, "Nineteen"), Map.entry(20, "Twenty"),
         Map.entry(30, "Thirty"), Map.entry(40, "Forty"), Map.entry(50, "Fifty"), Map.entry(60, "Sixty"), Map.entry(70, "Seventy"), Map.entry(80, "Eighty"), Map.entry(90, "Ninety"), Map.entry(100, "Hundred"));
@@ -13,11 +34,13 @@ private IntegerToEnglish() {
     }
 
     /**
-        converts numbers < 1000 to english words
+     * Converts numbers less than 1000 into English words.
+     *
+     * @param number the integer value (0-999) to convert
+     * @return the English word representation of the input number
      */
     private static String convertToWords(int number) {
         int remainder = number % 100;
-
         String result;
 
         if (remainder <= 20) {
@@ -27,29 +50,29 @@ private static String convertToWords(int number) {
         } else {
             int tensDigit = remainder / 10;
             int onesDigit = remainder % 10;
-
-            result = String.format("%s %s", BASE_NUMBERS_MAP.get(tensDigit * 10), BASE_NUMBERS_MAP.get(onesDigit));
+            result = String.format("%s %s", BASE_NUMBERS_MAP.get(tensDigit * 10), BASE_NUMBERS_MAP.get(onesDigit)).trim();
         }
 
         int hundredsDigit = number / 100;
-
         if (hundredsDigit > 0) {
-            result = String.format("%s %s%s", BASE_NUMBERS_MAP.get(hundredsDigit), BASE_NUMBERS_MAP.get(100), result.isEmpty() ? "" : " " + result);
+            result = String.format("%s Hundred%s%s", BASE_NUMBERS_MAP.get(hundredsDigit), result.isEmpty() ? "" : " ", result);
         }
 
-        return result;
+        return result.trim();
     }
 
     /**
-      Only convert groups of three digit if they are non-zero
+     * Converts a non-negative integer to its English word representation.
+     *
+     * @param number the integer to convert (0-2,147,483,647)
+     * @return the English word representation of the input number
      */
     public static String integerToEnglishWords(int number) {
         if (number == 0) {
             return "Zero";
         }
 
         StringBuilder result = new StringBuilder();
-
         int index = 0;
 
         while (number > 0) {
@@ -58,23 +81,21 @@ public static String integerToEnglishWords(int number) {
 
             if (remainder > 0) {
                 String subResult = convertToWords(remainder);
-
                 if (!subResult.isEmpty()) {
+                    if (index > 0) {
+                        subResult += " " + THOUSAND_POWER_MAP.get(index);
+                    }
                     if (!result.isEmpty()) {
-                        result.insert(0, subResult + " " + THOUSAND_POWER_MAP.get(index) + " ");
+                        result.insert(0, subResult + " ");
                     } else {
-                        if (index > 0) {
-                            result = new StringBuilder(subResult + " " + THOUSAND_POWER_MAP.get(index));
-                        } else {
-                            result = new StringBuilder(subResult);
-                        }
+                        result.append(subResult);
                     }
                 }
             }
 
             index++;
         }
 
-        return result.toString();
+        return result.toString().trim();
     }
 }

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

@@ -11,5 +11,36 @@ public void testIntegerToEnglish() {
         assertEquals("Two Billion One Hundred Forty Seven Million Four Hundred Eighty Three Thousand Six Hundred Forty Seven", IntegerToEnglish.integerToEnglishWords(2147483647));
         assertEquals("One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven", IntegerToEnglish.integerToEnglishWords(1234567));
         assertEquals("Twelve Thousand Three Hundred Forty Five", IntegerToEnglish.integerToEnglishWords(12345));
+        assertEquals("One Hundred", IntegerToEnglish.integerToEnglishWords(100));
+        assertEquals("Zero", IntegerToEnglish.integerToEnglishWords(0));
+    }
+
+    @Test
+    public void testSmallNumbers() {
+        assertEquals("Ten", IntegerToEnglish.integerToEnglishWords(10));
+        assertEquals("Nineteen", IntegerToEnglish.integerToEnglishWords(19));
+        assertEquals("Twenty One", IntegerToEnglish.integerToEnglishWords(21));
+        assertEquals("Ninety Nine", IntegerToEnglish.integerToEnglishWords(99));
+    }
+
+    @Test
+    public void testHundreds() {
+        assertEquals("One Hundred One", IntegerToEnglish.integerToEnglishWords(101));
+        assertEquals("Five Hundred Fifty", IntegerToEnglish.integerToEnglishWords(550));
+        assertEquals("Nine Hundred Ninety Nine", IntegerToEnglish.integerToEnglishWords(999));
+    }
+
+    @Test
+    public void testThousands() {
+        assertEquals("One Thousand", IntegerToEnglish.integerToEnglishWords(1000));
+        assertEquals("Ten Thousand One", IntegerToEnglish.integerToEnglishWords(10001));
+        assertEquals("Seventy Six Thousand Five Hundred Forty Three", IntegerToEnglish.integerToEnglishWords(76543));
+    }
+
+    @Test
+    public void testEdgeCases() {
+        assertEquals("One Million", IntegerToEnglish.integerToEnglishWords(1_000_000));
+        assertEquals("One Billion", IntegerToEnglish.integerToEnglishWords(1_000_000_000));
+        assertEquals("Two Thousand", IntegerToEnglish.integerToEnglishWords(2000));
     }
 }