[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/README.md

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+### Relevant Articles:

core-java-modules/core-java-numbers-conversions/pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-conversions.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>core-java-numbers-conversions</artifactId>
+    <name>core-java-numbers-conversions</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung.core-java-modules</groupId>
+        <artifactId>core-java-modules</artifactId>
+        <version>0.0.1-SNAPSHOT</version>
+    </parent>
+
+    <build>
+        <finalName>core-java-numbers-conversions</finalName>
+        <resources>
+            <resource>
+                <directory>src/main/resources</directory>
+                <filtering>true</filtering>
+            </resource>
+        </resources>
+    </build>
+
+</project>

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

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+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));
+    }
+}

core-java-modules/pom.xml

@@ -108,6 +108,7 @@
         <module>core-java-numbers-3</module>
         <module>core-java-numbers-4</module>
         <module>core-java-numbers-5</module>
+        <module>core-java-numbers-conversions</module>
         <module>core-java-optional</module>
         <module>core-java-perf</module>
         <module>core-java-reflection</module>