Protected methods in Object class

[ankur]

So I am a Java newbie and have a simple question here:

protected methods of a superclass are inheritable by the subclass. So
how come the protected methods of Object class namely clone and
finalize not inherited by every class by default ie why is overriding
for these methods necessary in the class which wants to use these
methods and not accessed directly.

Thanks,
Ankur

 

[Daniel Pitts]

So I am a Java newbie and have a simple question here:

protected methods of a superclass are inheritable by the subclass. So
how come the protected methods of Object class namely clone and
finalize not inherited by every class by default ie why is overriding
for these methods necessary in the class which wants to use these
methods and not accessed directly.

Thanks,
Ankur

All non-private member methods are inherited. Protected simply means
that you have to be at or below the class that declares it to access it.

The only thing that overriding does for it is to create a public method
with the same signature that delegates to the protected method.

 

[ankur]

All non-private member methods are inherited. Protected simply means
that you have to be at or below the class that declares it to access it.

The only thing that overriding does for it is to create a public method
with the same signature that delegates to the protected method.

I am still not able to understand the issue with these class
declarations:

I have following classes declared in the same package:


public class Sample implements Cloneable {
private int j;

public Sample(int h)
{
j = h;
}
public void display()
{
System.out.printf("The value of private variable is %d\n", j);
}
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~


public class TestSample{

/**
* @param args
*/
public static void main(String[] args) {
Sample obj = new Sample(78);
obj.clone();

}



}

I am not able to understand why can't obj refer to clone() method.
Afterall obj is of type Sample which extends Object class by default.

 

[Mike Schilling]

So I am a Java newbie and have a simple question here:

protected methods of a superclass are inheritable by the subclass. So
how come the protected methods of Object class namely clone and
finalize not inherited by every class by default ie why is overriding
for these methods necessary in the class which wants to use these
methods and not accessed directly.

Partly, because you're misunderstanding what "protected" means. It's
available in a subclass only to object whose type is that subclass.

class Class1
{
void method1(Object o)
{
Class1 dup = (Class1)this.clone(); // good
Object dup2 = o.clone() // will not compile
}
}

 

[ankur]

Partly, because you're misunderstanding what "protected" means. It's
available in a subclass only to object whose type is that subclass.

class Class1
{
void method1(Object o)
{
Class1 dup = (Class1)this.clone(); // good
Object dup2 = o.clone() // will not compile
}
}

Mark,

However in the examples below protected method of Superclass is
available in Testclass to object of type Subclass.


public class SuperClass {
private int j;
protected void messageinsuper()
{
System.out.printf("This is superclass\n");
}
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


public class SubClass extends SuperClass{

private int g;
public void messageinsub()

{
System.out.printf("This is a subclass\n");
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


public class TestClass {


public static void main(String[] args) {

SubClass obj = new SubClass();
obj.messageinsub();
obj.messageinsuper();
}

}

Earlier also I was doing the same thing. Sample extends Object and
TestSample declares an object of type Sample and tries to access
protected method Clone.

 

[alexander.maystrenko]

Partly, because you're misunderstanding what "protected" means. It's
available in a subclass only to object whose type is that subclass.

class Class1
{
void method1(Object o)
{
Class1 dup = (Class1)this.clone(); // good
Object dup2 = o.clone() // will not compile
}
}

Mark,

However in the examples below protected method of Superclass is
available in Testclass to object of type Subclass.

public class SuperClass {
private int j;
protected void messageinsuper()
{
System.out.printf("This is superclass\n");
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class SubClass extends SuperClass{

private int g;
public void messageinsub()

{
System.out.printf("This is a subclass\n");
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class TestClass {

public static void main(String[] args) {

SubClass obj = new SubClass();
obj.messageinsub();
obj.messageinsuper();
}

}

Earlier also I was doing the same thing. Sample extends Object and
TestSample declares an object of type Sample and tries to access
protected method Clone.- Hide quoted text -

- Show quoted text -

Hey, just take a look on the java api
http://java.sun.com/j2se/1.5.0/docs/api/
protected Object clone() -- so it's not public!

mad0master

 

[Lew]

public class SubClass extends SuperClass{

public class TestClass {
public static void main(String[] args) {

Please do not use TAB characters in Usenet posts.

SubClass obj = new SubClass();
obj.messageinsub(); // should capitalize each word part: messageInSub()
obj.messageinsuper();

You're in the same package.

}
}

A class can access protected members of another class in its same package.

However, when I try to use clone() I get an error:

===
package testit;
public class Super
{
protected void message()
{
System.out.println( "Super.message()" );
}
}

===
package testit;
public class Sub extends Super
{
public void tell()
{
message();
System.out.println( "Sub.tell()" );
}
}

===
package testit;
public class Checkit
{
public static void main(String[] args)
{
Sub sub = new Sub();
sub.tell();
sub.message();
Sub s2 = (Sub) sub.clone(); // line 9
}
}

===
Compilation of Checkit:
projects/testit/src/testit/Checkit.java:9: clone() has protected access in
java.lang.Object
Sub s2 = (Sub) sub.clone();

 

[Ravi]

Try this,

In order to call the protected method, u should be in the same package
as the base class. Not subclass.

Ravi

public class SubClass extends SuperClass{

public class TestClass {
public static void main(String[] args) {

Please do not use TAB characters in Usenet posts.

SubClass obj = new SubClass();
obj.messageinsub(); // should capitalize each word part: messageInSub()
obj.messageinsuper();

You're in the same package.

}
}

A class can access protected members of another class in its same package.

However, when I try to use clone() I get an error:

===
package testit;
public class Super
{
protected void message()
{
System.out.println( "Super.message()" );
}
}

===
package testit;
public class Sub extends Super
{
public void tell()
{
message();
System.out.println( "Sub.tell()" );
}
}

===
package testit;
public class Checkit
{
public static void main(String[] args)
{
Sub sub = new Sub();
sub.tell();
sub.message();
Sub s2 = (Sub) sub.clone(); // line 9
}
}

===
Compilation of Checkit:
projects/testit/src/testit/Checkit.java:9: clone() has protected access in
java.lang.Object
Sub s2 = (Sub) sub.clone();

 

[ankur]

Try this,

In order to call the protected method, u should be in the same package
as the base class. Not subclass.

Ravi

public class SubClass extends SuperClass{
public class TestClass {
public static void main(String[] args) {

Please do not use TAB characters in Usenet posts.

You're in the same package.

A class can access protected members of another class in its same package.

However, when I try to use clone() I get an error:

===
package testit;
public class Super
{
protected void message()
{
System.out.println( "Super.message()" );
}
}

===
package testit;
public class Sub extends Super
{
public void tell()
{
message();
System.out.println( "Sub.tell()" );
}
}

===
package testit;
public class Checkit
{
public static void main(String[] args)
{
Sub sub = new Sub();
sub.tell();
sub.message();
Sub s2 = (Sub) sub.clone(); // line 9
}
}

===
Compilation of Checkit:
projects/testit/src/testit/Checkit.java:9: clone() has protected access in
java.lang.Object
Sub s2 = (Sub) sub.clone();

Ravi,

I am using eclipse with these examples:
All three classes are in their own java files and in default package.

public class SuperClass {
private int j;
protected void messageinsuper()
{
System.out.printf("This is superclass\n");
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class SubClass extends SuperClass{

private int g;
public void messageinsub()

{
System.out.printf("This is a subclass\n");
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class TestClass {

public static void main(String[] args) {

SubClass obj = new SubClass();
obj.messageinsub();
obj.messageinsuper();
}

}

This works perfectly ie obj can call both messageinsub() and
messageinsuper()

Now I don't understand why this does not work:

public class Sample implements Cloneable {
private int j;

public Sample(int h)
{
j = h;
}
public void display()
{
System.out.printf("The value of private variable is %d
\n", j);
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class TestSample{

/**
* @param args
*/
public static void main(String[] args) {
Sample obj = new Sample(78);
obj.clone();

}

}

After all Sample extends object by default and just like the earlier
example I am trying to access protected method of superclass Object
which is clone() in a TestSample class. All are in the default package
in eclipse.

 

[ankur]

public class SubClass extends SuperClass{
public class TestClass {
public static void main(String[] args) {

Please do not use TAB characters in Usenet posts.

SubClass obj = new SubClass();
obj.messageinsub(); // should capitalize each word part: messageInSub()
obj.messageinsuper();

You're in the same package.

}
}

A class can access protected members of another class in its same package.

However, when I try to use clone() I get an error:

===
package testit;
public class Super
{
protected void message()
{
System.out.println( "Super.message()" );
}

}

===
package testit;
public class Sub extends Super
{
public void tell()
{
message();
System.out.println( "Sub.tell()" );
}

}

===
package testit;
public class Checkit
{
public static void main(String[] args)
{
Sub sub = new Sub();
sub.tell();
sub.message();
Sub s2 = (Sub) sub.clone(); // line 9
}

}

===
Compilation of Checkit:
projects/testit/src/testit/Checkit.java:9: clone() has protected access in
java.lang.Object
Sub s2 = (Sub) sub.clone();

Lew,
Agreed clone() has protected access but so does method message() which
belongs to superclass. Doesn't it ?

Ankur

 

[Lew]

Try this,

In order to call the protected method, u [sic] should be in the same package
as the base class.

Uhh, yeah, that was my point.

Not subclass.

Except that you *can* call a protected method from a subclass; that's the
purpose of 'protected'.

Please do not top-post.

 

[Lew]

Agreed clone() has protected access but so does method message() which
belongs to superclass. Doesn't it ?

Yes, and as I pointed out, Sub, Super and Checkit are also in the same
package. Object is not.

 

[Roedy Green]

I am not able to understand why can't obj refer to clone() method.
Afterall obj is of type Sample which extends Object class by default.

see http://mindprod.com/jgloss/scope.html
http://mindprod.com/jgloss/protected.html

 

[Ravi]

Ok, may be you are missing the trivial point here.
Object is in java.lang package.
default package isn't java.lang.
Does this make things clear?

 

[Thomas Fritsch]

I am still not able to understand the issue with these class
declarations:

I have following classes declared in the same package:


public class Sample implements Cloneable {
private int j;

public Sample(int h)
{
j = h;
}
public void display()
{
System.out.printf("The value of private variable is %d\n", j);
}

public Object clone()
{
try
{
return super.clone(); // call Object's clone() method
}
catch(CloneNotSupportedException e)
{
// cannot happen because Sample implements Cloneable
throw new InternalError();
}
}

}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

public class TestSample{
/**
* @param args
*/
public static void main(String[] args) {
Sample obj = new Sample(78);
obj.clone();
}
}

I am not able to understand why can't obj refer to clone() method.
Afterall obj is of type Sample which extends Object class by default.

Because Object's clone() method is protected, it is callable only from
subclasses (from class Sample in your case), but not from outside
Object's package java.lang (i.e. not from your class TestSample).

To solve your issue you have to insert a public clone method into your
class Sample. See above.

 

[Lew]

@Override

public Object clone()
{
try
{
return super.clone(); // call Object's clone() method
}
catch(CloneNotSupportedException e)
{
// cannot happen because Sample implements Cloneable
throw new InternalError();
}
}

 

[ankur]

Thomas Fritsch wrote:

@Override

Thanks everybody for help ! I understood now. Object class is not the
same package as other classes that extend it. Also protected members
have package access and are also inherited by subclasses where they
can be referenced using super. Cool !!!

Thanks for your help everybody !