Add String Manipulation tutorial under Learn the Basics for Java

tutorials/learnjavaonline.org/en/String Manipulation.md

@@ -0,0 +1,128 @@
+Tutorial
+--------
+
+In Java, strings (`String`) are immutable objects with many built-in utilities that allow inspecting and operating on text. Once a `String` is created, its content cannot change; methods like `substring`, `replace`, `toUpperCase`, etc., return a new `String` object without altering the original. Some common operations:
+
+**Concatenation**: You can use the `+` operator or the `concat` method. Example:
+
+    String a = "Hello";
+    String b = "World";
+    String c = a + " " + b;              // "Hello World"
+    String d = a.concat(" ").concat(b);  // equivalent
+
+**Length and character access**:
+
+  `length()`: returns the number of characters.
+
+  `charAt(int index)`: returns the `char` at the given position (0-based).
+
+    String s = "Java";
+    int len = s.length();    // 4
+    char ch = s.charAt(2);   // 'v'
+
+**Substring or character search**:
+
+`indexOf(String/char)`, `lastIndexOf(...)`, `contains(...)`, `startsWith(...)`, `endsWith(...)`.
+
+    String s = "abracadabra";
+    int pos = s.indexOf("cada");      // 4
+    boolean has = s.contains("cad");  // true
+
+**Substring extraction**:
+
+`substring(int beginIndex)` and `substring(int beginIndex, int endIndex)`.
+
+    String s2 = "Hello, Java!";
+    String part = s2.substring(7, 11); // "Java"
+
+**Common transformations**:
+
+`toUpperCase()`, `toLowerCase()`, `trim()`, `replace(oldChar, newChar)`, `replaceAll(regex, repl)`.
+
+    String s3 = "  Java  ";
+    String t = s3.trim().toUpperCase(); // "JAVA"
+
+**Building and joining strings in loops**:
+
+For multiple concatenations in loops, using `StringBuilder` improves performance, although for basic exercises `+` is acceptable:
+
+    StringBuilder sb = new StringBuilder();
+    for (char c : "abc".toCharArray()) {
+        sb.append(c).append('-');
+    }
+    String result = sb.toString(); // "a-b-c-"
+
+
+These methods are part of the standard Java SE API.
+
+Exercise
+--------
+
+Implement a function that reverses a given string and prints it. The reversal must be done "manually" using character access, without directly using `StringBuilder.reverse()`.
+
+- Define a `String input` with a test value.
+- Implement a method `reverseString(String s)` that:
+- Obtain the length with `length()`.
+- Iterate over the string from the end to the beginning using `charAt(int)`.
+- Build the reversed string, for example by accumulating in a `StringBuilder` or by concatenating characters in a loop.
+- In `main`, print the result of `reverseString(input)`.
+- Verify that, when passing `"Hello, Java!"`, the result is `"!avaJ ,olleH"`.
+
+Tutorial Code
+-------------
+
+public class Main {
+    /**
+     * Reverses the string s without using StringBuilder.reverse().
+     * @param s Original string.
+     * @return Reversed string.
+     */
+    public static String reverseString(String s) {
+        // Use StringBuilder to accumulate without reverse():
+        StringBuilder sb = new StringBuilder();
+        int n = s.length();
+        for (int i = n - 1; i >= 0; i--) {
+            sb.append(s.charAt(i));
+        }
+        return sb.toString();
+
+        // Alternative (less efficient): directly concatenate:
+        // String result = "";
+        // for (int i = s.length() - 1; i >= 0; i--) {
+        //     result += s.charAt(i);
+        // }
+        // return result;
+    }
+
+    public static void main(String[] args) {
+        // Basic test:
+        String input = "Hello, Java!";
+        System.out.println(reverseString(input));
+
+        // Additional tests:
+        // System.out.println(reverseString("Java Tutorials"));
+    }
+}
+
+Expected Output
+---------------
+
+!avaJ ,olleH
+
+Solution
+--------
+
+public class Main {
+    public static String reverseString(String s) {
+        StringBuilder sb = new StringBuilder();
+        for (int i = s.length() - 1; i >= 0; i--) {
+            sb.append(s.charAt(i));
+        }
+        return sb.toString();
+    }
+
+    public static void main(String[] args) {
+        String input = "Hello, Java!";
+        System.out.println(reverseString(input));
+    }
+}

tutorials/learnjavaonline.org/en/Welcome.md

@@ -14,6 +14,7 @@ Just click on the chapter you wish to begin from, and follow the instructions. G
 - [[Arrays]]
 - [[Loops]]
 - [[Functions]]
+- [[String Manipulation]]
 - [[Objects]]
 - [[Compiling and Running with Arguments]]