2022-07-21 group 4 polymorphism

Polymorphic basic syntax ：

        1、 Before learning basic grammar , Two concepts need to be popularized ：

                Upward transformation

                Son ----> Father

                Move down

                Father ----> Son  

                java Allow upward transformation in , It also allows downward transformation .

                        Whether it's up or down , There must be an inheritance relationship between the two types , No inheritance , The compiler will report an error

Polymorphism refers to ：

        A parent type reference points to a child type object

        Including compile phase and run phase

        Compile phase ： Method of binding parent class

        Operation phase ： Methods for dynamically binding subtype objects         ·

        In many forms

  Parent type Variable name =new Subclass object ;

Variable name . Method name （）;

Parent type ： Refers to the type of parent class inherited by the child class object , Or the interface type of the parent class of the implementation

When a method is invoked in a polymorphic manner , First check that the method is present in the parent class , without , Compile error ; If there is , What is executed is the method after subclass rewriting

Reference type conversion

          Upward transformation ： Polymorphism itself is the process of up conversion from subclass type to parent type , This process is the default . When a parent reference points to a subclass object , It's the upward transformation .

 Parent type   Variable name =new  Subclass type ();
 Such as ：Animal a=new Cat();

          Move down ： The process of converting a parent type down to a child type , This process is mandatory , A subclass object that has been transformed upward , Convert a parent class reference to a child class reference , You can use the cast format , It's a downward shift

When you need to access subclass objects " specific " Methods , At this time, it must be a downward transformation .

 Subclass type   Variable name =new ( Subclass type ) Parent variable 
 Such as ：Cat c=new (Cat)a;

Abnormal transformation

Animal a=new Cat();
a.eat();// It's called Cat Of eat

Dog d=new (Dog)a;

d.run();// It's called Dog Of run, Operation error reporting 

This is because Cat and Dog There is no inheritance , Does not meet the definition of type conversion

instranceof

If the variable belongs to this data type, return true

If the variable does not belong to the data type, return false

Animal a=new Cat();
a.eat;

if(a instanceof Cat){
    Cat c=new (Cat)a;
    c.run();
}else if(a instanceof Dog){
    Dog d=new (Dog)a;
    d.run();
}

Anonymous object

Most of them are used to transmit parameters

Anonymous objects are used with constructors

grammar ：

new   Class name （）;

function ： It has the same function as normal named objects . It still has the call attribute , Function of method .

Use scenarios ： Most of them are used in parameter transmission , Actual parameters , In most cases, it is used with constructors

benefits ： Saving resource

Arrays class , Array tool class

Is an array a class ：

1. Standing on the JVM From the perspective of , It's a class , stay JVM When parsing an array , Will generate an array of parsing arrays
2. From the perspective of compilation , Not class , Because there is no class name , There is no structure .

Linked list ： It's a data structure

There is no subscript in the linked list

In memory , Arrays and linked lists are the most basic data structures , surface , Or linear table .

The linear table , Linear structure , It is one containing n>=0 A finite sequence of nodes , There is only one previous node , There is also a next node . A line with ends

One way linked list ： While maintaining the value of a node itself , Also maintain the direction of its next value

Double linked list ： While maintaining the value of a node itself , Also maintain the direction of its previous and next values

ArrayList and LinkedList What's the difference? ？

The disadvantages and advantages of arrays ：

shortcoming ： Too complex space

advantage ： Easy to query and modify

What are the disadvantages and advantages of linked lists .