StringBuilder源码分析

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

final修饰不可被继承

abstract class AbstractStringBuilder implements Appendable, CharSequence {

char[] value; ... }

底层是char数组，默认长度是16；长度不够会进行扩容，扩大为原来的2倍 + 2