Chapter 8 Inheritance and Polymorphism
继承和多态

头疼死了，不翻译成中文了，直接摘书上提纲性的文字。

1. A class C1 derived from another class C2 is called a subclass(extended class or derived class), and C2 is called a superclass(parent class or base class).
Private data fields and methods in a superclass are not accessible outside of the class. Therefore, they are not inherited in a subclass.

2. Keyword super
(1) to call a superclass constructor
(2) to call a superclass method
The constructors of a superclass are not inherited in the subclass. They can only be invoked from the constructors of the subclasses, using the keyword super.
In any case, constructing an instance of a class invokes the constructors of all the superclasses along the inheritance chain. This is called constructor chaining.
If a class is designed to be extended, it is better to provide a no-arg constructor to avoid programming errors, since constructors of its subclass will defaultly invoke super()

3. Overriding method (not overloading)
To override a method, the method must be defined in the subclasss using the same signature and return type as in its superclass.
A private method cannot be overridden, because it's not accessible outside its own class. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.
A static method can be inherited, but can't be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.

4. The Object class
Three frequently used methods:
(1) public boolean equals(Object object)
You should use equals(Object obj) to override the equals method in a subclass
(2) public int hashCode()
(3) public String toString()