What are the necessary conditions for the existence of polymorphism and how to implement it?

Three necessary conditions for the existence of polymorphism:

(Recommended tutorial:java introductory tutorial)

1. Inheritance

2. Rewriting

3. The parent class reference points to the subclass object (the parent class reference points to the subclass object)

For example:

Parent p = new Child(); List myList=new LinkedList<>();

(Learning video recommendation:java course)

Polymorphic implementation method:

Method 1: Rewriting

Method 2: Interface

Method 3: Abstract classes and abstract methods

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