JDK源码阅读--StringBuilder

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence

被final修饰，表示StringBuilder不能被其它类继承。
继承了AbstractStringBuilder抽象类，实现了 Serializable、CharSequence接口。
StringBuilder中的方法没有被Synchronized修饰，所以StringBuilder不是线程安全的。

 /**
     * Constructs a string builder with no characters in it and an
     * initial capacity of 16 characters.
     */
    public StringBuilder() {
        super(16);
    }

    /**
     * Constructs a string builder with no characters in it and an
     * initial capacity specified by the {@code capacity} argument.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *               argument is less than {@code 0}.
     */
    public StringBuilder(int capacity) {
        super(capacity);
    }

    /**
     * Constructs a string builder initialized to the contents of the
     * specified string. The initial capacity of the string builder is
     * {@code 16} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }

StringBuilder的容量默认是16，当容量不够的时候，容量会继续增加16，依次类推。