Is Polymorphic Bean reference in conformity with Java polymorphism

[Rajat Sharmanaive]

Hi

I am new to J2EE. Learning through HFSJ by K&B. Following is my query from the same.

The jsp standard action:

<jsp:useBean id=personBean type="PersonBean" class="EmployeeBean" scope="request">
</jsp:useBean>

In the generated servlet, translates to:

PersonBean personBean = new EmployeeBean();

Following are my 2 beans:

abstract public class PersonBean {
	public String getName(){
		return name;
	}
	
	public void setName(String name){
			this.name=name;
	}
	private String name;
}

public int getEmpId(){
		return empId;
	}
	
	public void setEmpId(int empId){
			this.empId=empId;
	}
        private int empId;
}

But according to the book (which is indeed true, as this is what happens). The following action sets all the properties.

<jsp:setProperty name="personBean" property="*"/>

From what i have learnt w.r.t to Java Polymorphism is that a reference variable can invoke (at runtime, through dynamic dispatch) only the methods that it knows about. This means (assume B is subclass of A)

A a = new B();

a can invoke methods that are overridden.
a cannot invoke:
methods that are only in B // This is what seems to happen here
methods that are only in A //off course, because the actual object instantiated is of type B.

How then, is personBean able to set empId which is not overridden and which solely belongs to the subclass. Please help. It is not a very encouraging feeling to have when you are thinking about giving SCJP and find your Inheritance fundamentals challenged. Thanks.

[bhanu chowdary]

a cannot invoke:

methods that are only in A //off course, because the actual object instantiated is of type B.

When you are extending a class the subclass inherits all the methods of the super class. The below code prints abcd.

abstract class poly {
	void abcd() {
		System.out.println("abcd");
	}
	void efgh() {
		System.out.println("efgh");
	}
}

public class PolyTest extends poly{	
	void efgh() {
		System.out.println("efgh Test");
	}	
	void ijkl() {
		System.out.println("ijkl");
	}
	public static void main(String[] args) {
		poly p = new PolyTest();
		p.abcd();
	}
}

[Rajat Sharmanaive]

Hi Bhanu,

I stand corrected. Thanks for pointing that out. That was very basic. Somehow, i lost it. The main doubt however, still remains unresolved.

Hoping that also does not get solved in a way, that exposes my "expertise" . Looking forward to help.

[Rajat Sharmanaive]

Hi Bear

Apologies to you and to those who were inconvenienced in any way by this careless gesture. Din't pay attention.

Rajat