### StringBuilder&StringBuffer > 本文源码基于JDK8 StringBuilder与StringBuffer是两个常用的操作字符串的类。StringBuilder是线程不安全的，而StringBuffer是线程安全的；StringBuilder、StringBuffer都继承自AbstractStringBuilder ``` abstract class AbstractStringBuilder implements Appendable, CharSequence { char[] value; //字符数组 int count; } ``` * Serializable：可以序列化的标志； * CharSequence接口：包含了charAt\(\)、length\(\) 、subSequence\(\)、toString\(\)这几个方法，String类也实现了这个接口； ``` public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence{} ``` ``` public final class StringBuffer extends AbstractStringBuilder implements java.io.Serializable, CharSequence{} ``` ### append方法比较 AbstractStringBuilder抽象类的append方法 ``` public AbstractStringBuilder append(String str) { if (str == null) return appendNull(); int len = str.length(); ensureCapacityInternal(count + len);//确保不会溢出，必要时扩容 str.getChars(0, len, value, count);//将整个str拷贝到value的末尾 count += len; //增加计数 return this; } private void ensureCapacityInternal(int minimumCapacity) { // overflow-conscious code if (minimumCapacity - value.length > 0) //如果会溢出，则扩容 expandCapacity(minimumCapacity); } void expandCapacity(int minimumCapacity) { int newCapacity = value.length * 2 + 2; //新的容量是原容量的2倍+2 if (newCapacity - minimumCapacity < 0) newCapacity = minimumCapacity; if (newCapacity < 0) { if (minimumCapacity < 0) // overflow throw new OutOfMemoryError(); newCapacity = Integer.MAX_VALUE; } value = Arrays.copyOf(value, newCapacity); } ``` 注意：如果str是null,则会调用appendNull\(\)方法。这个方法其实是追加了'n'、'u'、'l'、'l'这几个字符 ``` private AbstractStringBuilder appendNull() { int c = count; ensureCapacityInternal(c + 4); final char[] value = this.value; value[c++] = 'n'; value[c++] = 'u'; value[c++] = 'l'; value[c++] = 'l'; count = c; return this; } ``` StringBuffer ``` @Override public synchronized StringBuffer append(String str) { toStringCache = null; super.append(str); return this; } ``` toStringCache这个变量是用于最近一次toString\(\)方法的缓存，任何时候只要StringBuffer被修改了这个变量会被赋值为null； ``` @Override public synchronized String toString() { if (toStringCache == null) { toStringCache = Arrays.copyOfRange(value, 0, count); } return new String(toStringCache, true); } ``` StringBuilder ``` @Override public StringBuilder append(String str) { super.append(str); return this; } ``` ## 总结 * StringBuilder和StringBuffer都是可变字符串，前者线程不安全，后者线程安全。 * StringBuilder和StringBuffer的大部分方法均调用父类AbstractStringBuilder的实现。其扩容机制首先是把容量变为原来容量的2倍加2。最大容量是Integer.MAX\_VALUE，也就是0x7fffffff。 * StringBuilder和StringBuffer的默认容量都是16，最好预先估计好字符串的大小避免扩容带来的时间消耗 ``` public StringBuilder() { super(16); } public StringBuffer() { super(16); } ``` * StringBuilder是jdk1.5引进的，而StringBuffer在1.0就有了；