[BAEL-6115_Num2Letter] Convert a Number to a Letter in Java (#13638)

* [BAEL-6115_Num2Letter] Convert a Number to a Letter in Java

* [BAEL-6115_Num2Letter] Change the 1st method's return type to char

core-java-modules/core-java-numbers-conversions/src/test/java/com/baeldung/numtoletter/ConvertNumberToLetterUnitTest.java

@@ -0,0 +1,52 @@
+package com.baeldung.numtoletter;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+
+public class ConvertNumberToLetterUnitTest {
+
+    static char numToLetterBySubstr(int i) {
+        String LETTERS = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
+        if (i > 0 && i <= 25) {
+            return LETTERS.substring(i, i + 1).charAt(0);
+        } else {
+            return '?';
+        }
+    }
+
+    static char numToLetterByAsciiCode(int i) {
+        if (i > 0 && i <= 25) {
+            return (char) ('A' + i);
+        } else {
+            return '?';
+        }
+    }
+
+    @Test
+    void givenANumber_whenConvertToLetterUsingSubstring_shouldGetExpectedResult() {
+        char negativeInputResult = numToLetterBySubstr(-7);
+        assertEquals('?', negativeInputResult);
+
+        char tooLargeInputResult = numToLetterBySubstr(42);
+        assertEquals('?', tooLargeInputResult);
+
+        char result = numToLetterBySubstr(10);
+        assertEquals('K', result);
+    }
+
+    @Test
+    void givenANumber_whenConvertToLetterUsingAscii_shouldGetExpectedResult() {
+        char negativeInputResult = numToLetterByAsciiCode(-7);
+        assertEquals('?', negativeInputResult);
+
+        char tooLargeInputResult = numToLetterByAsciiCode(42);
+        assertEquals('?', tooLargeInputResult);
+
+        char charResult = numToLetterByAsciiCode(10);
+        assertEquals('K', charResult);
+
+        assertEquals("K", String.valueOf(charResult));
+    }
+}