Explain the internal structure and working mechanism of the StringBuilder class in Java.

The StringBuilder class in Java is designed for efficient construction and modification of strings without creating new objects for each operation, unlike String.

Internal structure:

• StringBuilder contains a character array char[] value that stores the current characters.
• Initially, the array has a certain size (e.g., 16 characters), which increases as needed.
• There is a field int count or int length indicating the current length of the string in the array.

Working mechanism:

• When adding characters or strings (for example, using the append method), characters are copied into the value array starting from the count position.
• If the array size is insufficient, the array is expanded: a new array of larger size (usually twice as large) is created, and old characters are copied into it.
• The toString() methods create a new String object based on the contents of the value array.

Example:

StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(' ');
sb.append("World");
System.out.println(sb.toString()); // Outputs "Hello World"

Thus, StringBuilder optimizes string operations by reducing the number of objects created and increasing performance during multiple string modifications.