java.lang.StringBuilder（二）构造器
StringBuilder的父类为：java.lang.AbstractStringBuilder这一抽象类，在JDK的源码中可以查到，但在JDK6.0的文档却没有此类不知为何，这里就不讨论它。
?? StringBuilder共有4个构造器，如下：
1、StringBuilder() 构造一个其中不带字符的字符串生成器，初始容量为 16 个字符（理解这里返回一个长度为0，初始容量为16的StringBuilder。
??? 源码：
/** * Constructs a string builder with no characters in it and an * initial capacity of 16 characters. */ public StringBuilder() {super(16);//这里直接调用了父类构造器：AbstractStringBuilder(int ..) }?AbstractStringBuilder（int）源码：
/** * Creates an AbstractStringBuilder of the specified capacity. */ AbstractStringBuilder(int capacity) { value = new char[capacity];//得到一个char数组 }?所以从上面两源码片段，可见StringBuilder（）所得到的容量是16的，但由于count没有值，所以长度为0，这一点以后讨论，看第二个构造器
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2、StringBuilder(CharSequence?seq)? 构造一个字符串生成器，包含与指定的 CharSequence 相同的字符。
/** * Constructs a string builder that contains the same characters * as the specified <code>CharSequence</code>. The initial capacity of * the string builder is <code>16</code> plus the length of the * <code>CharSequence</code> argument. * * @param seq the sequence to copy. * @throws NullPointerException if <code>seq</code> is <code>null</code> */ public StringBuilder(CharSequence seq) {//对CharSequence 要理解哟 this(seq.length() + 16);//这里调用了this(int)构造器，设置当前StringBuilder的容量为(seq+16) append(seq);//调用本类的的append(seq)方法，见下面： } /** * @throws IndexOutOfBoundsException {@inheritDoc} 父类中抛出 */ public StringBuilder append(CharSequence s) { if (s == null) s = "null"; if (s instanceof String) return this.append((String)s); if (s instanceof StringBuffer) return this.append((StringBuffer)s); if (s instanceof StringBuilder) return this.append((StringBuilder)s); return this.append(s, 0, s.length());//见以下函数 } /** * @throws IndexOutOfBoundsException {@inheritDoc} */ public StringBuilder append(CharSequence s, int start, int end) { super.append(s, start, end);//父类的append见个代码片段 return this; }父类的 super.append(s, start, end)源码： public AbstractStringBuilder append(CharSequence s, int start, int end) { if (s == null) s = "null";if ((start < 0) || (end < 0) || (start > end) || (end > s.length()))//长度溢出判断 throw new IndexOutOfBoundsException(//抛出异常 "start " + start + ", end " + end + ", s.length() " + s.length());int len = end - start;//len为从中截取的长度if (len == 0) return this;//如果起始位置相同，则对内容不作处理，返回原对象int newCount = count + len;//count为当前对象的长度 + 截取的长度lenif (newCount > value.length)//如果当前长度大于value的容量则增加对象容量 expandCapacity(newCount);//把当前对象的容量设置为newCount,这里不再讨论此方法 for (int i=start; i<end; i++)//把串中截取部分的值，追加到当前对象的value[]数组，也就是最终的值中 value[count++] = s.charAt(i);//每次加一个 count = newCount;//修改当前对象的长度（理解长度与容量的变化过程）return this;返回当前对象 }??3、StringBuilder(int?capacity)?构造一个其中不带字符的字符串生成器，初始容量由 capacity 参数指定。
???? 只初始化容量，这个构造器在上面提到过，现在来看看它的实现过程：
???? 构造器源码：
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/** * Constructs a string builder with no characters in it and an * initial capacity specified by the <code>capacity</code> argument. * * @param capacity the initial capacity. * @throws NegativeArraySizeException if the <code>capacity</code> * argument is less than <code>0</code>. */ public StringBuilder(int capacity) {//只初化一个没有字符内容的（长度为0），容量为capacity的StringBuildersuper(capacity);//调用父类构造器：AbstractStringBuilder(int ..)，具体实现见第一构造中有 }?
?4、StringBuilder(String?str)???构造一个字符串生成器，并初始化为指定的字符串内容。
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/** * Constructs a string builder initialized to the contents of the * specified string. The initial capacity of the string builder is * <code>16</code> plus the length of the string argument. * * @param str the initial contents of the buffer. * @throws NullPointerException if <code>str</code> is <code>null</code> */ public StringBuilder(String str) {super(str.length() + 16);//调用父构造器：AbstractStringBuilder(int ..)，注意容量加了16append(str); }? public StringBuilder append(String str) {?super.append(str);??????? return this;??? }父类的append(String)
/** * Appends the specified string to this character sequence. * <p> * The characters of the <code>String</code> argument are appended, in * order, increasing the length of this sequence by the length of the * argument. If <code>str</code> is <code>null</code>, then the four * characters <code>"null"</code> are appended. * <p> * Let <i>n</i> be the length of this character sequence just prior to * execution of the <code>append</code> method. Then the character at * index <i>k</i> in the new character sequence is equal to the character * at index <i>k</i> in the old character sequence, if <i>k</i> is less * than <i>n</i>; otherwise, it is equal to the character at index * <i>k-n</i> in the argument <code>str</code>. * * @param str a string. * @return a reference to this object. */ public AbstractStringBuilder append(String str) {if (str == null) str = "null"; int len = str.length();if (len == 0) return this;int newCount = count + len;if (newCount > value.length) expandCapacity(newCount);str.getChars(0, len, value, count);//赋值过程，不具体讨论了count = newCount;return this; }?好了，写到这里，写得很乱，没有添加什么东西，呵呵，那就写给自己看哈。
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????????????????????????????????? 2011-01-19? 卓