Create subclass from superclass static main

Question

I have a generic, abstract class ( SuperClass ). I want to have there a main method, that would be a default main for each subclass and would do the same, but with proper subclass object that derived and called it.

Like this:

 public abstract class SuperClass {

    // some code here...

    public static void main(String args[]) {
       // here instantiate the subclass
       // extending this SuperClass, and call
       // some methods
    }
 }

 public SubClass extends SuperClass {
      // here just implement some 
      // abstract methods from SupeClass
      // and NOT implement main()
 }

And now I would like to be able to run the SubClass as standalone program, that executes the default main derived from the SuperClass . How to instantiate the proper SubClass object in the main method?

• I cannot do just new because in SuperClass I don't know the actual name of a SubClass
• I cannot do it using reflection, because I cannot get the name of the SubClass from a static method implemented in the SuperClass ( Getting the name of a sub-class from within a super-class )

In C++, AFAIR, there is something like virtual modifier for a method, that I guess would be useful here. How to do in in Java?

Answer 1

如果希望子类成为应用程序入口点，则不继承静态方法，在子类中编写main方法。

Answer 2

You could use Spring IOC for example.

Create an xml file like the following and put in your classpath:

appconfig.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/context
           http://www.springframework.org/schema/context/spring-context-2.5.xsd">

    <bean id="someBean" class="com.company.SubClass"/>

</beans>

Then in your main code you could do something like this:

   public static void main(String args[]) {
       ApplicationContext context = ClassPathXmlApplicationContext("appconfig.xml");
       SuperClass startClass = (SuperClass) context.getBean("someBean");
       startClass.someMethod();
   }

Then your SuperClass will not know about its subclasses (but will know about Spring instead...).

You will have to add some Spring jar files to your classpath as well.

Answer 3

您不能在子类中继承静态方法，但如果您想在c ++中创建类似虚拟的方法，请将您的方法设为抽象或受保护

Answer 4

If by I would like to be able to run the SubClass as standalone program you mean that you want to be able to run something like java my.app.SubClass , that doesn't work, because as everyone already pointed out, static methods are not inherited.

Depending on why you want this strange subclass nesting, you would find a workaround by implementing a non-static main like this:

public class SuperClass{ 
  public static void main(String[] args) {
     SuperClass c = //figure out which class to load via a factor or something
     c.nonStaticMain(args);
  }
  protected void nonStaticMain(String[] args) {
    //do everything from your old main() here
  }
}