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
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