String Manipulation

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".