【java】StringBuffer 插入字符串源码

public AbstractStringBuilder insert(int offset, String str) {
	if ((offset < 0) || (offset > length()))
		//如果offset不在索引范围，则抛出异常：
		throw new StringIndexOutOfBoundsException(offset);
		
	if (str == null)
		str = "null";
		
	int len = str.length();
	
	//原来字符串数组内部容量确保没有问题，传入当前数组存放字符串长度+插入字符串长度
	ensureCapacityInternal(count + len);
	
	//把原来数组从offset位置及以后的字符串复制到offset+len去，复制的长度是原来数组长度减去offset前的长度
	System.arraycopy(value, offset, value, offset + len, count - offset);
	
	//把待插入的字符串复制到offset的位置去
	str.getChars(value, offset);
	count += len;
	return this;
}

/**
 * This method has the same contract as ensureCapacity, but is
 * never synchronized.
 */
private void ensureCapacityInternal(int minimumCapacity) {
	// 如果插入字符串后总长度比原来申请的总长度大，则需要进行容量扩展
	if (minimumCapacity - value.length > 0)
		expandCapacity(minimumCapacity);
}


/**
 * This implements the expansion semantics of ensureCapacity with no
 * size check or synchronization.
 */
void expandCapacity(int minimumCapacity) {
	//先试着扩展到原来容量的2倍+2
	int newCapacity = value.length * 2 + 2;
	
	//如果插入的字符串太长，直接用传入的参数
	if (newCapacity - minimumCapacity < 0)
		newCapacity = minimumCapacity;
		
    //如果计算的容量小于0，溢出了，表示出了内存问题
	if (newCapacity < 0) {
		if (minimumCapacity < 0) // overflow
			throw new OutOfMemoryError();
			
		//直接给一个很大的值作为扩展后的容量
		newCapacity = Integer.MAX_VALUE;
	}
	
	//基于新的数组长度，把原来的数组内容复制到新申请长度的数组
	value = Arrays.copyOf(value, newCapacity);
}


public static char[] copyOf(char[] original, int newLength) {
	//申请扩展后字符串长度的字符数组
	char[] copy = new char[newLength];
	
	//把原来数组内容复制到新的数组
	System.arraycopy(original, 0, copy, 0,
					 Math.min(original.length, newLength));
	
	return copy;
}