可变的抽象字符串创建类

1. Appendable    此接口的实现类可以拼接字符序列或单个字符。不保证线程安全，由实现类实现是否线程安全。使用java.util.Formatter格式化，必须为Appendable实现类2. CharSequence

/** * 字符串存储数据的字节数组，初始化的时候可以指定容量，默认为16 */byte[] value;
/** * 编码 0为Latin1 1为Utf-16 */byte coder;
/** * 当前字符串实际长度 */int count;
private static final byte[] EMPTYVALUE = new byte[0];
/**    *  byte [] 最大长度限制，超过VM限制长度会导致OOM    */private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

1. 返回当前字符串实际长度，countpublic int length() {        return count;}2. 返回字符串容量public int capacity() {        return value.length >> coder;}3. 扩容，字节数组拷贝public void ensureCapacity(int minimumCapacity) {       if (minimumCapacity > 0) {                ensureCapacityInternal(minimumCapacity);      }}
private void ensureCapacityInternal(int minimumCapacity) {       // overflow-conscious code       int oldCapacity = value.length >> coder;       if (minimumCapacity - oldCapacity > 0) {               value = Arrays.copyOf(value,                    newCapacity(minimumCapacity) << coder);       }}private int newCapacity(int minCapacity) {        // overflow-conscious code       int oldCapacity = value.length >> coder;        int newCapacity = (oldCapacity << 1) + 2;        if (newCapacity - minCapacity < 0) {                newCapacity = minCapacity;        }    int SAFE_BOUND = MAX_ARRAY_SIZE >> coder;    return (newCapacity <= 0 || SAFE_BOUND - newCapacity < 0)            ? hugeCapacity(minCapacity)            : newCapacity;}
private int hugeCapacity(int minCapacity) {        int SAFE_BOUND = MAX_ARRAY_SIZE >> coder;       int UNSAFE_BOUND = Integer.MAX_VALUE >> coder;       if (UNSAFE_BOUND - minCapacity < 0) { // overflow                throw new OutOfMemoryError();        }        return (minCapacity > SAFE_BOUND)            ? minCapacity : SAFE_BOUND;}

线程安全的AbstractStringBuilder实现类

1. toString() 使用缓存机制，在第一次调用toString()的时候会将实时生成的字符串拷贝保存在toStringCache中，下次调用直接返回，如有变化，toStringCache置空

非线程安全的AbstractStringBuilder实现类

1. toString() 实时创建字符串拷贝返回