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Day_ twelve

2022-06-25 16:25:00 【grp_ grp_ grp】

Day12 object-oriented

1.Final

final For the embellishment , Indicates final , Unalterable

final The decorated class cannot be inherited , Decorated member methods cannot be overridden , Decorated variables cannot be assigned twice , And there is no default value

final Declaration of static variables ：

public static final data type Variable name = value ;

2. polymorphic

Definition ：

Parent class reference Point to Subclass object

Parent class reference ： refer to A type variable declared with a parent type

Point to ： It refers to which object can be found through the memory address

Subclass object ： new Heap memory object created by subclass

Statement ：

Parent type Variable name = new Subclass （）;

Advantages of polymorphism ： Embrace variety , Reduce the coupling between classes and details

The disadvantages of polymorphism ： Missing subclass specific properties

call ：

1 If the parent class doesn't have , Direct error

2 If the parent class has , Subclasses don't have , Execute parent

3 If both parent and child classes have , Member method execution subclass , Other execution parent classes

form ：

1 Direct polymorphism

2 Formal parameters and actual parameters , The method parameter list is the parent type , Call methods to pass in subclass objects

3 The return value is polymorphic , The return value type is the parent type , But return subclass objects

instanceof

Determine whether an object is instantiated from a class

3. abstract

What is it? ：

It's a modifier , The decorated class is an abstract class , The decorated method is abstract method

abstract class ： Object cannot be instantiated

Abstract method ： There is no method body , Define only the functions , No functional implementation , And the abstract method must be in the abstract class , But there can be no abstract methods in an abstract class

4. Interface interface

Interface ： It can be understood as a completely abstract class , Class contains only abstract methods and constants
1.8 Start allowing Static methods and default methods

grammar ： Modifier interface The interface name {}

Abstract methods in interfaces , No need to add abstract modification , Method The default is public abstract
Interface , No variables , Constant only , also public static final It can be omitted
Between classes and interfaces , No longer an inheritance relationship , It becomes an implementation relationship , from extends Instead of implements

The interface name Variable = new Subclass implementation () Polymorphism also occurs

One class Only one class can be inherited , however Can achieve N Interface , Separated by commas , It can solve the problem of weak single inheritance function
class Class name implements Interface 1 , Interface 2, Interface 3....{}

Interface and interface , It's multi inheritance , Separated by commas
   interface The interface name extends Parent interface name 1, Parent interface name 2,...{}

One class If you implement an interface , Then you must implement all the abstract methods in the interface ,. Otherwise, it is necessary to add abstract modification
An abstract class , Implement an interface , Can achieve 0~N Abstract methods

1.8 There can be static methods , There can be default Method ( It can be understood as a member method )
   Static methods , Call with the interface name
   default Method needs to be called by the sub implementation class , You can also override

1.9 Start Support private Method

If both interfaces and abstract classes can do one thing , Interface is preferred , Because this will preserve the inheritance of the class