More questions

1） Why String is immutable?
2) Can children's method be more public than the parent?
3) What's the relation ship between hashcode and equals? If two classes have same hashCode, do they have to be equals? No.
If two classes are equal, do they have to have same hashCode? Yes
4) What the difference between Primary Key and Unique Key? Primary key does not allow null, unique key allow null.
5) How Hashtable is implemented?
6) boolean match(String pattern, String text), pattern: ab*a, ab?a
7) How the Object.equals() are implemented internally? Comparing which parameter?
-- The memory space of the object.