https://www.runoob.com/java/java-tutorial.html
#### java object-oriented features of the three
Encapsulation, inheritance, polymorphism,
encapsulation: a description of the object's attributes and behavior encapsulated into a class, to achieve specific business logic function into a packaging method, followed by encapsulation significance is effectively protected by accessing the property attribute modifier privatization ( member variables), public methods.
Inheritance: implementation code reuse, all subclasses common extraction behavior and properties of a parent class, subclass inherits all the attributes and behavior of the parent class may be provided, having a succession of transfer and unity.
Polymorphism: referenced in the definition of the specific type and specific method call type variable points can not be determined at compile phase, but in order to determine the specific reference type variable to point to the statement in which the object is invoked at runtime which class of method.
Polymorphic forms are cast, up like shape, polymorphic polymorphic behavior can be divided into objects and polymorphism.
Polymorphic behavior: when there is different implementation methods, different object calls the same run () {}, when the cat call is run, when the fish swim call, when to call the birds are flying.
Object polymorphism: the same object, the shape may be a different type, such as the same person object that can be shaped to son, father, and other employees.
##### differences and relations between the JRE, JDK, JVM
##### Download and environmental structures.
https://www.jianshu.com/p/fe6b68295479
download link:
https://www.oracle.com/technetwork/java/javase/downloads/jdk12-downloads-5295953.html
