protected access in Object class

Clear me friends.

Object class is the base class for all classes. Thus the methods with protected access in the object class can be invoked by any object(Of any class).

I know the statement is wrong!.

But i cannot convince myself(Since i don't know it clearly).

Shed some light.

Thanks
Jerlad

JosAH

Chief Editor

7,787 Posts

May 15th, 2008
03:16 PM
#2

Re: protected access in Object class

Quote:

It's not wrong but it needs a bit more detail: a protected method is visible in any
of the (descending) classes where the method is defined. So not just any other
object can invoke that method on any other object, i.e. it has to be at least a
descendant of the other object's class. A short example:

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class B { protected void m() { ... } }

class D extends B { ... }

class E extends B { ... }

class Derived extends D { ... }

...

Now any D can call m() on any B and D. Any E can call m() on any E and B
and any Derived can call m() on any other Derived, D and B but not on an E object.

But none of the D objects can call m() on an E object, nor can any E object
call m() on any D object.

kind regards,

Jos

pronerd

Expert

334 Posts

May 15th, 2008
03:22 PM
#3

Re: protected access in Object class

Quote:

Object class is the base class for all classes. Thus the methods with protected access in the object class can be invoked by any object(Of any class).

I know the statement is wrong!.

I am not sure I follow the question. It is hard to make out what is being asked. Any class that extends java.lang.Object class can access its protected methods.

Protected status just means that the method or object in question can not be accessed outside of the package it is in. If you are extending that class you are actually making your object part of that object so you are inside the access boundaries.

This section of the Java tutorial describes what the access control arguments mean.
http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol.html

BigDaddyLH

Moderator

1,211 Posts

May 15th, 2008
03:54 PM
#4

Re: protected access in Object class

Quote:

Protected status just means that the method or object in question can not be accessed outside of the package it is in.

That's incorrect. You are thinking of the default access level:

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	package com.acme.utils;

public class Helper {

    public void pub() {}

    void packageLevel() {}

}

package elsewhere;
imports com.acme.utils.Helper;
...
Helper x = new Helper();
x.pub(); //okay, calling a public method
x.packageLevel(); //error. Can only be accessed with package com.acme.utils or from withing subclasses of Helper.

pjerald

Member

64 Posts

May 17th, 2008
09:03 AM
#5

Re: protected access in Object class

Quote:

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class B { protected void m() { ... } }

class D extends B { ... }

class E extends B { ... }

class Derived extends D { ... }

...

Sorry for the delayed replay. Jos I cannot understand any D can call m() on any B and D. Can you please explain that.

My understanding is this

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class F{

class B { protected void m() { ... } }

class D extends B { ... }

class E extends B { ... }

class Derived extends D { ... }

...
 
C c = new C();

c.m();

// c invokes m. and the m is not defined inside C. 

// this is valid since m has protected acccess.
 
D d = new D();

d.m(); //valid
 
E e = new e();

e.m(); //valid
 
Derived derived = new Derived();

derived.m(); //valid
 
}

I am introducing a class F here. Can you please explain "any D can call m() on any B and D."