I'm unable to perform virtual method invocation

This is my code:

class base1
{

}
class der1 extends base1
{
 public static void main(String []args)
 {
  base1 b=new der1();
  b.showTest();
 }
 public void showTest()
 {
 System.out.println("Hello i am a der开发者_如何学Pythonive class");
 }

}

it will not compile.

you need showTest() in base class also to make it compilable

base1 b=new der1(); b.showTest();

here b is the reference of base , you need showTest() in base at compile time it wil check for showTest() in base and at run time due to

base1 b=new der1(); it will invoke der1's version of showTest()

Also See

• Polymorphism

The method showTest() is defined in the subclass. To be able to use it you have to:

1.- Declare it in the base class

class base1 {
     public void showTest(){
     }
}

Or 2.- Declare b as der1

public static void main( String [] args ) { 
    der 1 b = new der1();
    ...