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How StringBuilder Works Internally in Java?

Published in Java Programming 2 mins read

The StringBuilder class in Java provides a mutable sequence of characters, allowing efficient modifications to strings without creating new objects. Let's delve into its internal workings:

Internal Representation

  • The StringBuilder class maintains a character array internally, which stores the string's characters.
  • It also keeps track of the current length of the string and the capacity of the array.

Memory Allocation

  • When a StringBuilder is created, it allocates an initial capacity for the character array.
  • If the string grows beyond this capacity, the array is automatically resized to accommodate more characters. This resizing involves creating a new, larger array and copying the existing characters to it.

Methods and Operations

  • append(): Adds characters or strings to the end of the StringBuilder.
  • insert(): Inserts characters or strings at a specific position within the StringBuilder.
  • delete(): Removes characters from the StringBuilder.
  • replace(): Replaces a portion of the StringBuilder with new characters.
  • toString(): Returns a new String object representing the current contents of the StringBuilder.

Example

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World"); // Appends " World" to the end
sb.insert(6, "beautiful "); // Inserts "beautiful " at index 6
sb.delete(12, 18); // Deletes characters from index 12 to 18
sb.replace(0, 5, "Greetings"); // Replaces "Hello" with "Greetings"
String result = sb.toString(); // Returns a new String "Greetings beautiful World"

Efficiency Advantages

  • Immutability: Strings in Java are immutable, meaning their contents cannot be changed. Every modification creates a new String object, leading to unnecessary object creation and garbage collection.
  • Mutable Behavior: StringBuilder overcomes this limitation by providing mutable behavior, allowing modifications without creating new objects. This improves performance, especially for frequent string manipulations.

Conclusion

StringBuilder provides a powerful and efficient way to manipulate strings in Java by offering mutable behavior and avoiding the overhead of creating new String objects for each modification. Its internal character array and methods allow for flexible and efficient string manipulation.

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