Understanding Java StringBuffer Capacity: How It Manages Memory Efficiently

When working with strings in Java, you might encounter the StringBuffer class, which is designed for mutable strings. One crucial aspect of StringBuffer is its capacity, which determines how much memory it allocates for storing characters.

The StringBuffer.capacity() method allows you to check the current capacity of a StringBuffer object. By default, this capacity is set to 16 characters. However, as you append more characters, this capacity automatically increases to accommodate the new data. Specifically, when the current capacity is exceeded, StringBuffer grows its capacity by calculating a new size based on the formula (old capacity×2)+2(old capacity×2)+2. For example, if the initial capacity is 16 and you exceed it, the new capacity will be 34.

Understanding how StringBuffer manages its capacity can significantly optimize memory usage and improve performance in applications that involve extensive string manipulations. This article will delve into the details of how StringBuffer maintains its capacity, including examples and best practices for efficient memory management.

newCapacity = string.length() * 2 + 2;

public class StringDemo {
    public static void main(String[] args) {
        stringBufferDemo();
    }

    private static void stringBufferDemo(){
        StringBuilder sbb;
        StringBuffer sb = new StringBuffer();
        System.out.println("Length : " + sb.length());
        System.out.println("Capacity : " + sb.capacity());

        sb.ensureCapacity(17);
        System.out.println("Capacity : " + sb.capacity());

        sb.ensureCapacity(35);
        System.out.println("Capacity : " + sb.capacity());

        sb.ensureCapacity(71);
        System.out.println("Capacity : " + sb.capacity());
    }
}

Output:
Length : 0
Capacity : 16
Capacity : 34
Capacity : 70
Capacity : 142

By mastering the nuances of StringBuffer, you can ensure that your Java applications run smoothly and efficiently, avoiding common pitfalls associated with string handling.