部分方法没有解释，有疑问或错误的地方，欢迎评论指出
import sun.misc.FloatingDecimal;import java.util.Arrays;ort javtil.Arrays;/*** StringBuilder 抽象类*/
abstract class AbstractStringBuilder implements Appendable, CharSequence {/*** 储存值的数组*/char[] value;/*** 容量大小*/int count;/*** 无参构造*/AbstractStringBuilder() {}/*** 根据传入长度构造 AbstractStringBuilder*/AbstractStringBuilder(int capacity) {value = new char[capacity];}/*** 返回字符串长度 （字符数）*/@Overridepublic int length() {return count;}/*** 返回当前容量*/public int capacity() {return value.length;}/*** 确保容量足够*/public void ensureCapacity(int minimumCapacity) {if (minimumCapacity > 0)ensureCapacityInternal(minimumCapacity);}// 判断传入容量是否超过容量 超过则扩容private void ensureCapacityInternal(int minimumCapacity) {if (minimumCapacity - value.length > 0)expandCapacity(minimumCapacity);}/*** 扩容*/void expandCapacity(int minimumCapacity) {//一次长度为length + 2int newCapacity = value.length * 2 + 2;//如果扩容一次还不足minimumCapacity，则取minimumCapacityif (newCapacity - minimumCapacity < 0)newCapacity = minimumCapacity;//如果newCapacity还小于0 ， 则取Integer所支持的最大值if (newCapacity < 0) {if (minimumCapacity < 0) // overflowthrow new OutOfMemoryError();newCapacity = Integer.MAX_VALUE;}//复制数组的值 到新的数组中，新的数组长度为newCapacityvalue = Arrays.copyOf(value, newCapacity);}/*** 尝试减少value所占用的存储空间。* 如果缓冲区 length 大于保持其当前序列所需的缓冲区字符 count ，* 则调整value的大小以提高空间效率*/public void trimToSize() {if (count < value.length) {value = Arrays.copyOf(value, count);}}//设置value数组新长度public void setLength(int newLength) {if (newLength < 0)throw new StringIndexOutOfBoundsException(newLength);//确保容量ensureCapacityInternal(newLength);if (count < newLength) {// 将指定的 Object 引用分配 给 指定 Object 数组指定范围中的每个元素。// 填充的范围从索引 fromIndex（包括）一直到索引 toIndex（不包括）。//（如果 fromIndex==toIndex，则填充范围为空。）// fill(Object[] a, int fromIndex, int toIndex, Object val)Arrays.fill(value, count, newLength, '\0');// 空字符(NUL) 、 ASCII码值 000}count = newLength;}/*** 返回指定位置的字符串*/@Overridepublic char charAt(int index) {if ((index < 0) || (index >= count))throw new StringIndexOutOfBoundsException(index);return value[index];}//复制数组public void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin){if (srcBegin < 0)throw new StringIndexOutOfBoundsException(srcBegin);if ((srcEnd < 0) || (srcEnd > count))throw new StringIndexOutOfBoundsException(srcEnd);if (srcBegin > srcEnd)throw new StringIndexOutOfBoundsException("srcBegin > srcEnd");System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);}//在指定位置插入charpublic void setCharAt(int index, char ch) {if ((index < 0) || (index >= count))throw new StringIndexOutOfBoundsException(index);value[index] = ch;}//appendpublic AbstractStringBuilder append(Object obj) {return append(String.valueOf(obj));}//append字符串，添加字符串public AbstractStringBuilder append(String str) {if (str == null)//添加空 则添加"null"return appendNull();int len = str.length();//确保容量ensureCapacityInternal(count + len);//复制数组str.getChars(0, len, value, count);//容量同步count += len;return this;}// Documentation in subclasses because of synchro differencepublic AbstractStringBuilder append(StringBuffer sb) {if (sb == null)return appendNull();int len = sb.length();ensureCapacityInternal(count + len);sb.getChars(0, len, value, count);count += len;return this;}/*** 从1.8新增* @since 1.8*/AbstractStringBuilder append(AbstractStringBuilder asb) {if (asb == null)return appendNull();int len = asb.length();ensureCapacityInternal(count + len);asb.getChars(0, len, value, count);count += len;return this;}// Documentation in subclasses because of synchro difference@Overridepublic AbstractStringBuilder append(CharSequence s) {if (s == null)return appendNull();if (s instanceof String)return this.append((String)s);if (s instanceof AbstractStringBuilder)return this.append((AbstractStringBuilder)s);return this.append(s, 0, s.length());}//添加 "null" countprivate 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;}//添加 固定长度字符@Overridepublic AbstractStringBuilder append(CharSequence s, int start, int end) {if (s == null)s = "null";if ((start < 0) || (start > end) || (end > s.length()))throw new IndexOutOfBoundsException("start " + start + ", end " + end + ", s.length() "+ s.length());int len = end - start;ensureCapacityInternal(count + len);for (int i = start, j = count; i < end; i++, j++)//找到指定位置的字符复制到valuevalue[j] = s.charAt(i);count += len;return this;}//添加boolean值public AbstractStringBuilder append(boolean b) {if (b) {ensureCapacityInternal(count + 4);value[count++] = 't';value[count++] = 'r';value[count++] = 'u';value[count++] = 'e';} else {ensureCapacityInternal(count + 5);value[count++] = 'f';value[count++] = 'a';value[count++] = 'l';value[count++] = 's';value[count++] = 'e';}return this;}//添加单个字符@Overridepublic AbstractStringBuilder append(char c) {ensureCapacityInternal(count + 1);value[count++] = c;return this;}//添加整数public AbstractStringBuilder append(int i) {if (i == Integer.MIN_VALUE) {append("-2147483648");return this;}int appendedLength = (i < 0) ? Integer.stringSize(-i) + 1: Integer.stringSize(i);int spaceNeeded = count + appendedLength;ensureCapacityInternal(spaceNeeded);Integer.getChars(i, spaceNeeded, value);count = spaceNeeded;return this;}//添加long型数据public AbstractStringBuilder append(long l) {if (l == Long.MIN_VALUE) {append("-9223372036854775808");return this;}int appendedLength = (l < 0) ? Long.stringSize(-l) + 1: Long.stringSize(l);int spaceNeeded = count + appendedLength;ensureCapacityInternal(spaceNeeded);Long.getChars(l, spaceNeeded, value);count = spaceNeeded;return this;}//删除指定间隔中的字符public AbstractStringBuilder delete(int start, int end) {if (start < 0)throw new StringIndexOutOfBoundsException(start);if (end > count)end = count;if (start > end)throw new StringIndexOutOfBoundsException();int len = end - start;if (len > 0) {System.arraycopy(value, start+len, value, start, count-end);count -= len;}return this;}/*** 删除指定位置的字符* @param index* @return*/public AbstractStringBuilder deleteCharAt(int index) {if ((index < 0) || (index >= count))throw new StringIndexOutOfBoundsException(index);//数组拷贝System.arraycopy(value, index+1, value, index, count-index-1);count--;return this;}//字符替换public AbstractStringBuilder replace(int start, int end, String str) {if (start < 0)throw new StringIndexOutOfBoundsException(start);if (start > count)throw new StringIndexOutOfBoundsException("start > length()");if (start > end)throw new StringIndexOutOfBoundsException("start > end");if (end > count)end = count;int len = str.length();int newCount = count + len - (end - start);//检查容量ensureCapacityInternal(newCount);//数组拷贝System.arraycopy(value, end, value, start + len, count - end);str.getChars(value, start);count = newCount;return this;}//字符串截取 开始位置到最后位置public String substring(int start) {return substring(start, count);}//字符串截取 start开始位置 end结束位置public String substring(int start, int end) {if (start < 0)throw new StringIndexOutOfBoundsException(start);if (end > count)throw new StringIndexOutOfBoundsException(end);if (start > end)throw new StringIndexOutOfBoundsException(end - start);return new String(value, start, end - start);}@Overridepublic abstract String toString();//返回char[]final char[] getValue() {return value;}}