StringBuilder底层代码研究

public AbstractStringBuilder append(String str) {
    //校验
    if (str == null) 
        str = "null";
    int len = str.length();
    if (len == 0)
        return this;
    
    //新长度
    int newCount = count + len;
    
    //确定是否需要给当前的value扩容
    if (newCount > value.length)
        expandCapacity(newCount);
    
    //把字符串转化为char[]。然后增加到value后方
    str.getChars(0, len, value, count);
    count = newCount;
    return this;
    }
    
    
void expandCapacity(int minimumCapacity) {
        //自动构造长度为两倍当前长度
        int newCapacity = (value.length + 1) * 2;
        //如果自动构造长度超长则只能取最大Integer长度
        if (newCapacity < 0) {
            newCapacity = Integer.MAX_VALUE;
        //如果新长度大于自动构造长度那么构造长度为新长度
        } else if (minimumCapacity > newCapacity) {
        newCapacity = minimumCapacity;
        }
        //字符串相关数组扩容
        value = Arrays.copyOf(value, newCapacity);
    }