java--polymorphism

One, polymorphic overview

　　　Polymorphism is the third most characteristic of object-oriented after encapsulation and inheritance.

Real things often reflect a variety of forms, such as students, students are a kind of people, then a specific classmate Zhang San is a student is also a person, that appears two forms.

Java, as an object-oriented language, can also describe the various forms of a thing. If the student class inherits the person class, a student object is both student and person.

Format: Refers to the parent class reference to the Child class object

Parent class type variable name =new subclass type ();

Ii. Characteristics of polymorphic members

　　　　A: polymorphic member variables

When a member variable of the same name appears in the child parent class, when the variable is called by polymorphic:

Compile time: Refer to whether the referenced variable belongs to a class that has a called member variable. No, compilation failed.

Run time: is also a member variable in the class to which the reference variable belongs.

Simple: Compile and run all refer to the left side of the equals sign. Compile run look to the left.

B: Polymorphic Member method

Compile time: The reference variable belongs to the class, and if there are no methods called in the class, the compilation fails.

Run Time: Refer to the class to which the object refers to the reference variable, and run the member method in the class to which the object belongs.

In short: Compile to see the left, run to the right

Third, transition up and down in polymorphic

The transformation of polymorphism is divided into two types: upward transformation and downward transformation.

A: Upward transformation: When there are sub-class objects assigned to a parent class reference, is the upward transformation, polymorphism itself is the process of upward transformation.

Use format: When the student class inherits the person class

Person P=new Student ();

B: A downward transition: a subclass object that has been transformed upward can use a format that enforces type conversions and converts a parent class reference to a subclass reference, which is a downward transformation. If the parent class object is created directly, it cannot be transformed downward.

Use format: Student stu= (Student) p;

Iv. Advantages and Disadvantages

　　Advantages and disadvantages of polymorphism

Advantages: Improved maintainability (guaranteed by polymorphic prerequisites) and improved Code extensibility

Disadvantage: You cannot directly access members that are specific to subclasses

java--polymorphism