How to understand java polymorphism

1. Overview of polymorphism

Polymorphism is the third major feature of object-oriented after encapsulation and inheritance.

Understanding of the meaning of polymorphic reality:

Real things often take on multiple forms, such as students. Students are a type of human being, so a specific classmate Zhang San is both a student and a human being. There are two forms.

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As an object-oriented language, Java can also describe multiple forms of a thing. If the Student class inherits the Person class, a Student object is both a Student and a Person.

Polymorphism is reflected in the fact that parent class reference variables can point to subclass objects.

Prerequisite: There must be a child-parent class relationship.

Note: When calling a method using a polymorphic parent class reference variable, the overridden method of the subclass will be called.

Definition and usage format of polymorphism

父类类型 变量名=new 子类类型();

2. Characteristics of members in polymorphism

1. Polymorphic member variables: Compile and run to see Left

Fu f=new Zi();
System.out.println(f.num);//f是Fu中的值，只能取到父中的值

2. Polymorphic member methods: Compile and run on the left, run on the right

Fu f1=new Zi();
System.out.println(f1.show());//f1的门面类型是Fu,但实际类型是Zi,所以调用的是重写后的方法。

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3. Polymorphic transformation

Polymorphic transformation is divided into two types: upward transformation and downward transformation.

1. Upward transformation: Polymorphism itself is a process of upward transformation

Usage format:

父类类型 变量名=new 子类类型();

Applicable scenarios: When there is no need to face subclass types, pass Improve scalability, or use the functions of the parent class to complete the corresponding operations.

2. Downward transformation: A subclass object that has been upwardly transformed can use the format of forced type conversion to convert the parent class reference type to the subclass reference type.

Use format:

子类类型 变量名=（子类类型） 父类类型的变量;

Applicable scenarios: When you want to use the unique functions of subclasses.

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