public StringBuilder(int capacity) {
    super(capacity);
}
AbstractStringBuilder(int capacity) {
        value = new char[capacity];
}
public StringBuilder(String str) {
    super(str.length() + 16);
    append(str);
}
public StringBuilder append(String str) {
    super.append(str);
        return this;
}
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);
    count = newCount;
    return this;
}
虽然走的路不一样，最终的结果是一样的。

------解决方案--------------------
Java code

public StringBuilder() {
    super(16);
}

------解决方案--------------------
你可以看StringBuilder的源码，默认空的构造方法 是将StringBuilder的容量capacity设置为16 然后返回对象实例，以String为参数的构造方法是构造一个capacity为字符串长度+16的对象，然后将 String字符串append进去，

对于你说的这两者因为String为""长度为0，所以效果一样，不过这个多执行了一步append方法而已
Java code

public StringBuilder() {
        super(16);
    }

  /**
     * 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) {
        super(capacity);
    }