core technology

Inherit keywords ：extends, Literally , Subclasses can obtain some methods or variables of the parent class through inheritance . Inherited relationships , It is usually the relationship between classes , Radiation to nature generally refers to , Animals to birds are inherited , Birds inherit some characteristics of animals . From abstract to concrete , Birds to ostriches , It's also an inheritance relationship .
Method rewrite , Subclasses can use some division of the parent class private、final The method of decoration , You can also rewrite what it can use . Rewritten rules ： Keep the name of the parent method , Override the contents of the parent method , The range of modifiers is usually larger or equal .
create Test class , Define a method dolt(), Create subclasses Test2, Override parent method dolt()

Object Class is java The parent class of all objects in . Method instanceof Query whether the object belongs to a class , Belong to return true, Instead, return to false. It can also be used. getClass(),getName() Returns the class to which the current object belongs

Upward transformation , Instantiate the subclass object as a variable of the parent object ; Move down , Turn the parent object into a subclass object variable , Because the parent class rotor class is abstract to concrete , therefore , Cast required .

Interface ：interface, Interface is a pure abstract class , There is no code body in the defined method . Interfaces don't make much sense , Because his usage lies in inheritance , Subclasses rewrite methods according to their own characteristics , And this method , You can directly define . One interface per class ,implements The usage is and interface Interoperable ,interface Definition implements Then use

polymorphic ： stay java In inheritance, multiple subclasses can inherit the same parent , By turning up , It can be implemented in the parent class to replace the child class , Subclasses only need to call objects .