new StringBuilder() / builder.append() / insert()

// Creates an empty StringBuilder.
new StringBuilder();

// Creates a StringBuilder with an initial string.
new StringBuilder(String str);

// Appends a value to the end.
builder.append(value);

// Inserts a value at the specified position.
builder.insert(int offset, value);

// Deletes characters in the specified range.
builder.delete(int start, int end);

// Deletes the character at the specified index.
builder.deleteCharAt(int index);

// Efficiently builds a string in a loop.
StringBuilder sb = new StringBuilder();
for (int i = 1; i <= 5; i++) {
    sb.append(i);
    if (i < 5) sb.append(", ");
}
System.out.println(sb.toString()); // Prints "1, 2, 3, 4, 5"

// Method chaining allows multiple appends in sequence.
StringBuilder greeting = new StringBuilder()
        .append("Hello")
        .append(", ")
        .append("Java")
        .append("!");
System.out.println(greeting); // Prints "Hello, Java!"

// insert() inserts a string at a specific position.
StringBuilder sb2 = new StringBuilder("Hello World");
sb2.insert(5, ","); // Inserts a comma at index 5.
System.out.println(sb2); // Prints "Hello, World"

// delete() removes a range of characters.
StringBuilder sb3 = new StringBuilder("Java is great!");
sb3.delete(8, 14); // Removes "great".
System.out.println(sb3); // Prints "Java is !"

// deleteCharAt() removes a single character.
StringBuilder sb4 = new StringBuilder("Jaava");
sb4.deleteCharAt(2); // Removes the extra 'a'.
System.out.println(sb4); // Prints "Java"

StringBuilder is a mutable string class that efficiently handles appending, inserting, and deleting characters by rewriting an internal buffer. Avoid using the + operator on String repeatedly inside a loop, as it creates a new object on every iteration and degrades performance. Use StringBuilder for repeated concatenation.

If you need thread-safe string manipulation in a multithreaded environment, use StringBuffer, which has the same API. Note that StringBuffer synchronizes all its methods, making it slower than StringBuilder in single-threaded code.

For reversing, replacing, and converting, see 'reverse() / replace() / toString()'. For joining strings, see 'String.join()'.