CSDN 21 Days Learning Challenge - Polymorphism (05)

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Event address: CSDN 21-day Learning Challenge

The biggest reason for learning is to get rid of mediocrity. One day earlier, one more wonderful life; one day later, one more day of mediocrity.Dear friends, if you:
Want to systematically/deeply learn a certain technical knowledge point...
It is difficult to persevere in groping and learning alone, and want to study efficiently in a group...
Want to write a blog but can't start, and urgently need the injection of dry goodsEnergy...
love to write, willing to make myself a better person...
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What is polymorphism?

means that objects can have multiple shapes.

Common forms of polymorphism

superclass type object name = new subclass constructor

Member access features in polymorphism

Method call: compile on the left, run on the right.
Variable call: look on the left when compiling, and look on the left when running.(note)

Prerequisites for polymorphism

There is an inheritance/implementation relationship; there is a parent class reference pointing to a subclass object; there is method overriding (polymorphism focuses on behavioral polymorphism).

Advantages

In the polymorphic form, the object on the right can be decoupled, which is convenient for expansion and maintenance.

Animal a = new Dog();a.run(); // Subsequent business behavior changes with objects, and subsequent code does not need to be modified

When defining a method, use the parent type as a parameter, and the method can receive all subclass objects of the parent class, reflecting the extensibility and convenience of polymorphism.

A problem with polymorphism:

Polymorphism cannot use the unique features of subclasses

Automatic type conversion (child to parent):

Animal c = new Cat();

Coercion (from parent to child)

From parent to child (must perform forced type conversion, otherwise an error will be reported): Subclass object variable = (subclass) variable of parent class type
Function: It can solve the disadvantage of polymorphism, and can call subclassesUnique function.
Note: Classes with inheritance/implementation relationships can be casted at the compile stage; however, if the converted type and the real object type of the object are not the same type, a ClassCastException will occur when running the code

Animal c = new Cat();Dog d = (Dog)c; // An exception ClassCastException occurred

Java recommends using instanceof to determine the real type of the current object before coercion conversion, and then perform coercion conversion

Variable name instanceof Real type
Determines the real type of the object pointed to by the variable on the left of the keyword, whether it is the type on the right or its subclass type, and returns true if it is, and vice versa.

Summary

1. How many ways are there for the type conversion of reference data types?
Automatic type conversion, forced type conversion.
2. What problems can be solved by forced type conversion?
Can be converted into a true subclass type, thus calling the subclass's unique functions.
3. What should be paid attention to in mandatory type conversion?
Two types with inheritance relationship/implementation can be casted, and there is no problem in compiling.
At runtime, if the casted type is found not to be the real type of the object, an error (ClassCastException) will be reported
4. What should be done before cast and how?
Use instanceof to determine the real type of the current object, and then perform forced conversion
Object variable name instanceof real type

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