polymorphic cases in Java

There are actually two kinds of polymorphism:

1. Polymorphism of the method:

1.1 Method Overloading: The same method name, which performs different methods depending on the type and number of parameters passed in

1.2 Method Overwrite: The same method name, depending on the sub-class to implement different functions

2. Object polymorphism: Refers to the inheritance of the class, and the child and parent classes are converted to each other

1.1 Upward Transformation (auto-complete): Parent class Parent Class object = Subclass Instance

2.2 Downward transformation: Subclass Subclass Object = (parent Class) parent class instance;

Because the parent class can have more than one child class, and a subclass does not have too many parent classes, there is no compilation error in the upward transition, and a compile-time error may occur

Class a{public void print () {System.out.println (method in "a");}} Class B extends a{public void print () {System.out.println (method in "B");}} public class Test{public static void Main (String args[]) {A A =new a (); b b = (b) a;b.print ();}} /*exception in thread "main" java.lang.classcastexception:a cannot is cast to B at        test.main (test.java:14) */

The above code has a class conversion exception, which refers to the exception caused by a forced downward transformation of two two non-relational class objects.

The use of the instanceof keyword, which is primarily to determine whether an object is an instance of a class:

Object instanceof class//Returns a Boolean

classa{ Public voidprint () {System.out.println ("Method in a"); }}classBextendsa{ Public voidprint () {System.out.println ("Method in B"); }} Public classtest{ Public Static voidMain (String args[]) {A A=NewB (); if(AinstanceofB) {b b=(b) B;    } b.print (); }}

Note: Before a downward transformation occurs, it is important that the object's upward transformation occur first

　　

Example: an upward transition instance

classa{ Public voidprint () {System.out.println ("Method in a"); }}classBextendsa{ Public voidprint () {System.out.println ("Method in B"); }} Public classtest{ Public Static voidMain (String args[]) {A A=NewB ();//Upward TransformationA.print ();//the method in B    }}

polymorphic cases in Java