StringBuilder capacity

Key points

• StringBuilder wraps a character array. length() is what you stored, capacity() is what is allocated.
• A fresh builder starts at capacity 16, or at the length of the seed string plus 16.
• When an append does not fit, a bigger array is allocated and the contents copied. The new capacity is old * 2 + 2.
• Pre-size with new StringBuilder(1000) when you roughly know the result and those copies disappear.
• setLength changes the length without shrinking the buffer; trimToSize hands the spare memory back.
• StringBuffer is the synchronised twin. Unless several threads share the buffer, StringBuilder is the right pick.

Example

public class Main {
    public static void main(String[] args) {
        StringBuilder sb = new StringBuilder();
        System.out.println("empty      -> length " + sb.length() + ", capacity " + sb.capacity());

        sb.append("0123456789");
        System.out.println("10 chars   -> length " + sb.length() + ", capacity " + sb.capacity());

        sb.append("abcdefghij");
        System.out.println("20 chars   -> length " + sb.length() + ", capacity " + sb.capacity()
                + "  (old * 2 + 2, array copied once)");

        StringBuilder sized = new StringBuilder(1000);
        System.out.println("pre-sized  -> length " + sized.length() + ", capacity " + sized.capacity());

        sb.setLength(3);
        System.out.println("setLength(3) -> \"" + sb + "\", capacity still " + sb.capacity());

        sb.trimToSize();
        System.out.println("trimToSize   -> capacity " + sb.capacity());

        System.out.println("chained: " + new StringBuilder("stressed").reverse());
        System.out.println("length is what you stored, capacity is what is allocated");
    }
}

Appending is cheap on average; pre-sizing removes the copies entirely.