[impr-charAtIdx] extract char improvement (#18966)

Author: sk1418

core-java-modules/core-java-string-apis-2/src/test/java/com/baeldung/stringapi/StringCharAtUnitTest.java

@@ -1,11 +1,13 @@
 package com.baeldung.stringapi;
 
-import org.junit.jupiter.api.Test;
-
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import org.junit.jupiter.api.Test;
+
 public class StringCharAtUnitTest {
+
     @Test
     public void whenCallCharAt_thenSuccess() {
         String sample = "abcdefg";
@@ -15,15 +17,60 @@ public void whenCallCharAt_thenSuccess() {
     @Test()
     public void whenCharAtNonExist_thenIndexOutOfBoundsExceptionThrown() {
         String sample = "abcdefg";
-        assertThrows(IndexOutOfBoundsException.class, () -> sample.charAt(-1));
-        assertThrows(IndexOutOfBoundsException.class, () -> sample.charAt(sample.length()));
+        assertThrows(StringIndexOutOfBoundsException.class, () -> sample.charAt(-1));
+        assertThrows(StringIndexOutOfBoundsException.class, () -> sample.charAt(sample.length()));
+    }
+
+    @Test
+    public void whenUsingToCharArrayThenIndexing_thenCorrect() {
+        String sample = "abcdefg";
+        char[] chars = sample.toCharArray();
+        assertEquals('d', chars[3]);
+        assertEquals('f', chars[5]);
+    }
+
+    @Test
+    public void whenUsingCodePointAt_thenCorrect() {
+        String sample = "abcdefg";
+        assertEquals('d', sample.codePointAt(3));
+        assertEquals('f', sample.codePointAt(5));
+
+        String emojiString = "😊"; // '😊' is Unicode char: U+1F60A
+        assertEquals(0x1F60A, emojiString.codePointAt(0));
+    }
+
+    @Test
+    public void whenUsingGetChars_thenCorrect() {
+        String sample = "abcdefg";
+        char[] singleTargetChar = new char[1];
+        sample.getChars(3, 3 + 1, singleTargetChar, 0);
+        assertEquals('d', singleTargetChar[0]);
+
+        char[] multiTargetChars = new char[3];
+        sample.getChars(3, 3 + 3, multiTargetChars, 0);
+        assertArrayEquals(new char[] { 'd', 'e', 'f' }, multiTargetChars);
+    }
+
+    char getCharUsingStream(String str, int index) {
+        return str.chars()
+            .mapToObj(ch -> (char) ch)
+            .toArray(Character[]::new)[index];
+    }
+
+    @Test
+    public void whenUsingStreamAPI_thenCorrect() {
+        String sample = "abcdefg";
+        assertEquals('d', getCharUsingStream(sample, 3));
+        assertEquals('f', getCharUsingStream(sample, 5));
+
+        assertThrows(ArrayIndexOutOfBoundsException.class, () -> getCharUsingStream(sample, 100));
     }
 
     @Test
     public void whenCallCharAt_thenReturnString() {
         String sample = "abcdefg";
-        assertEquals("a", Character.toString(sample.charAt(0)));
-        assertEquals("a", String.valueOf(sample.charAt(0)));
+        assertEquals("d", Character.toString(sample.charAt(3)));
+        assertEquals("d", String.valueOf(sample.charAt(3)));
     }
 
 }