原因：JAVA核心编程1卷7版P116
Employee E1 = new Employee("Alice",...);
Employee E2 = new Employee("Bob",...);
swap(E1, E2);
void swap(Employee a, Employee b)
{
Employee temp;
temp = a;
a = b;
b = temp;
}
无法实现，E1， E2 的引用互换！！！故是按值传递的。