当前位置：网站首页>2022/07/26 learning notes (day16) abstraction and interface

Concept ： Further abstraction of classes , A method without a method body , At the same time, abstract methods must also use keywords abstract Decorate .

2. Grammar format ：

Add... Before the return value type abstract

3. Instance object ：
step ：

1. You can't create abstract class objects directly
2. You must inherit the parent class with a subclass
3. Non Abstract subclasses must override all abstract methods in the overriding Abstract parent class
4. Remove abstract keyword , Fill in the method body braces
5. Create subclass objects for use

4. data type

Reference data type

5. Construction method

Used to call subclasses

6. Abstract method ：

Judge ：
           When it is impossible to determine whether the method body of a method is executed, it is an abstract method
       Format ：
           public abstract void Method name ( );
       Conditions ：
           The class of the abstract method must be an abstract class

Interface ：

1. brief introduction ：

In the life
                 Interface is a kind of public standard
             stay java in
                 An interface is a common specification for multiple classes
                     The most important content is abstract methods

2. Defining interfaces ：

Grammar format ：
List of modifiers interface The interface name { }

3. Realization ：

1. The interface cannot be used directly , There must be an implementation class to implement this interface
       Format ：
           public class Implementation class name implements The name of the interface
   2. The implementation class of the interface must override （ Realization ） All abstract methods in the interface
       Override override abstract methods
           Get rid of abstract keyword , Add method body braces
   3. Create objects that implement classes , And use

   Be careful ：
       If the implementation class does not cover all the methods in the interface , Then the implementation class must be an abstract class

4. When inheritance and interface appear at the same time ：
extends Keywords before ,implement The key word comes after

5. Downward transformation between classes and interfaces ：

In an inheritance relationship ：
                 If there is no inheritance
                     Compilation failed
                 There must be an inheritance relationship to transition down
                     Compile and pass
                         There may be ClassCastException
             Between class and interface ：
                 There is no inheritance relationship can also be transformed downward
                     Compilation can pass
                         There may be ClassCastException

6. The transformation of ：

          Classes and interfaces can be forcibly converted without inheritance ：
                 Premise : This class is an interface
                     Otherwise ClassCastException
             Interfaces can be forcibly transferred without inheritance ：
                 The implementation class pointed to by the interface reference All interfaces for forced conversion should be implemented

                         Compilation and operation can pass
               The implementation class pointed to by the interface reference Not all interfaces for forced conversion are implemented

                         Compilation can pass , Run appears ClassCastException abnormal

7. Content ：

Abstract method ：

Be careful ：
           Abstract methods in interfaces , Modifier must be public abstract Two fixed keywords
           These two modifiers , You can optionally omit ( Not recommended for beginners )
           The three elements of the method can be defined at will

Constant ：
       Format ：
           public static final constant type Constant names = Face value ;
       Naming specification ：
           All letters are capitalized , Each word is separated by an underscore
       Be careful ：
           public static final It can be omitted
           Constants in the interface , Assignment must be made. , You can't do without assignment
   There is no way to construct

Be careful ：
   Interfaces cannot have static method blocks and constructor
   Multiple inheritance
       A class can only have one parent , But a class can implement multiple interfaces
   If there are multiple interfaces implemented in the implementation class , There are repetitive abstract methods , Then you only need to cover it once
   If the implementation class does not cover all abstract methods , Then the implementation class must be an abstract class

In inheritance, there must be an inheritance relationship to compile in order to pass （ Static binding ）