[Capitalize_1st] Capitalize the First Letter of a String in Java (#12885)

core-java-modules/core-java-string-operations-5/src/test/java/com/baeldung/capitalizethefirstletter/CapitalizeTheFirstLetterUnitTest.java

@@ -0,0 +1,45 @@
+package com.baeldung.capitalizethefirstletter;
+
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.Test;
+
+import java.util.regex.Pattern;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class CapitalizeTheFirstLetterUnitTest {
+    private static final String EMPTY_INPUT = "";
+    private static final String EMPTY_EXPECTED = "";
+    private static final String INPUT = "hi there, Nice to Meet You!";
+    private static final String EXPECTED = "Hi there, Nice to Meet You!";
+
+    @Test
+    void givenString_whenCapitalizeUsingSubString_shouldGetExpectedResult() {
+        String output = INPUT.substring(0, 1).toUpperCase() + INPUT.substring(1);
+        assertEquals(EXPECTED, output);
+
+        assertThrows(IndexOutOfBoundsException.class, () -> EMPTY_INPUT.substring(1));
+
+    }
+
+    @Test
+    void givenString_whenCapitalizeUsingRegexReplace_shouldGetExpectedResult() {
+        String output = Pattern.compile("^.").matcher(INPUT).replaceFirst(m -> m.group().toUpperCase());
+        assertEquals(EXPECTED, output);
+
+        String emptyOutput = Pattern.compile("^.").matcher(EMPTY_INPUT).replaceFirst(m -> m.group().toUpperCase());
+        assertEquals(EMPTY_EXPECTED, emptyOutput);
+    }
+
+    @Test
+    void givenString_whenCapitalizeUsingApacheCommons_shouldGetExpectedResult() {
+        String output = StringUtils.capitalize(INPUT);
+        assertEquals(EXPECTED, output);
+
+        String emptyOutput = StringUtils.capitalize(EMPTY_INPUT);
+        assertEquals(EMPTY_EXPECTED, emptyOutput);
+
+        String nullOutput = StringUtils.capitalize(null);
+        assertNull(nullOutput);
+    }
+}