020-024 polymorphism review

1. polymorphic ： A reference to a parent type can point to an object of a child type .

2. Parent child = new Child(); When a method is invoked in a polymorphic manner , First, check whether the parent class has sing() Method , If there are no compilation errors ; If there is , Then call the subclass sing() Method .

3. There are two types of cast ：

   1. Upward type conversion （upcast）： For example, I will dog Type conversion to Animal type , Convert subtype to parent type . For upward type conversions , There is no need to display the specified .
   2. Downward type conversion （downcast）： For example, will Animal Type conversion to dog type . Convert parent type to child type . For downward type conversion , You must show the specified （ You must use cast ）.

4. abstract class （abstract class ）: Use abstract The tired modified by keywords is called abstract class . The abstract class cannot be instantiated , in other words , You can't new Come up with an abstract class object （ example ）.

5. Abstract method （abstract method）: Use abstract The method modified by keyword is called abstract method . Abstract methods need to be defined in abstract classes . As opposed to abstract methods , The previously defined method is called concrete method （ There is a statement , There is realization ）.

6. If a class contains abstract methods , Then this class must be an abstract class .

7. If a class is abstract , Then this class can contain concrete methods （ There is a statement , There is realization ）.

8. If a class contains abstract methods , Then this class must be declared as abstract class , in other words , This class must be an abstract class ; conversely , If a class is abstract , Then this class can contain both abstract methods , It can also include specific methods .

9. In any case , As long as a class is abstract , Then this class cannot be instantiated .

10. Inherit the parent class in the subclass （ The parent class is an abstract class ） Under the circumstances , Then the subclass must implement all the abstract methods defined in the parent class ; otherwise , This subclass needs to declare a abstract class .

11. Interface （interface）： The status of the interface is equivalent to class, All methods in an interface are abstract methods . When declaring a method in an interface , have access to abstract keyword , You can also not use . Usually , Will be omitted abstract keyword .（ There is a statement , No implementation ）.

12. You can think of interfaces as special abstract classes （ Abstract classes can have concrete methods , There can also be abstract methods , There can only be abstract methods in the interface , There can be no specific method ）.

13. Class can implement interfaces , Realize the use of keywords implements Express , Represents that a class implements an interface .

14. A class implements an interface , Then this class must implement all the methods declared by the interface . If the class is an abstract class , Then there is no need to implement the methods in the interface .

15. Java It's a single inheritance , In other words, a class can only have one parent ; A class can implement multiple interfaces , Multiple interfaces are separated by commas .

16. polymorphic ： Polymorphism , That is, the reference of the parent type can point to the object of the child type , Or a reference to an interface type can point to an instance of the class that implements the interface . The cast method between the interface and the class implementing the interface is exactly the same as that between the parent class and the child class .

17. Static keyword ： Can be used to modify properties , It can also be used to modify methods , It can also be used to modify classes .

18. static Modify properties ： No matter how many objects a class generates , All these objects share a unique static member variable ; An object modifies the static member variable , The value of the static member variable of other objects also changes . If a member variable is static Of , Then we can pass Class name . Member variable name The way to use it （Java It is recommended that we use this method ）.

19. static Modification methods ：static The decorated method is called static method . For static methods , You can use the class name . Method name to access .

20. Static methods can only inherit , Can not rewrite （Override）.

21. final keyword ：final You can modify properties 、 Method 、 class .

22. final decorator ： When a class is final When it is modified , Indicates that the class is a terminal class , That is, it cannot be inherited .

23. final Modification methods ： When a method is final When it is modified , Indicates that the method is a final state method , That is, it cannot be rewritten （Override） .

24. final Modify properties ： When an attribute is final When it is modified , Indicates that the property cannot be overridden .

25. When final When decorating a native data type , The value representing the native data type cannot be changed （ For example, you can't 10 become 20）; If final When modifying a reference type , Indicates that the reference type cannot point to other objects ; But the content of the object that the reference refers to can change .

26. The static code block of the class will be executed only once , It is executed when the class is loaded , Because each class will only be loaded once , All static code blocks will be executed only once ; The construction method is not , Every time an object is generated, the constructor of the class will be called , therefore new The constructor will be called once .

27. If there is an existing construction method in the inheritance system , There are also static code blocks , Then first execute the static code block of the topmost class , Execute all the way to the static code block of the lowest class . Be careful ： Static code blocks are executed only once .

28. Non static member variables cannot be accessed in static methods ; You can access static member variables in static methods . Static member variables can be accessed in non static methods .

29. summary ： Static can only access static ; Non static access to everything

30. Cannot be used in static methods this keyword .