Re: How is tag interface functionality implemented in Java ?

Lew <>
Mon, 08 Sep 2008 03:00:09 -0400
Lasse Reichstein Nielsen wrote:

The clone() method also checks for Clon[e]able-implementation on each
class in the inheritance chain, and only copies the fields of those
that do implement Clon[e]able. If somewhere up the chain there is a class
that doesn't implement Clon[e]able, then it stops its copying there.

EJP wrote:

Really? Where does it say that? What does it do with the members which
don't implement Cloneable? How come the description in the Javadoc for
java.lang.Object.clone() says something completely different?

This is fantasy folks.

I read over the Javadocs and worked to think it through. Here's what I've got
so far:

If one does not implement 'clone()' with 'super.clone()', then of course the
proposition is false unless the override does that logic itself.

If one does use 'super.clone()', and all classes in the inheritance also do
so, then the question is what happens when some intermediate class does not
implement 'Cloneable'.

Let's say 'C' extends 'B' extends 'Object' ('C' -> 'B' -> 'Object'). Assume
'B' does not implement 'Cloneable' and 'C' does. 'C#clone()' should not throw
any exceptions, and all fields should be copied, because the Javadocs promise
that fields will be (shallowly) copied if the class in question implements
'Cloneable'. Since 'C' does, there should be no problem. The check happens
with something equivalent to 'getClass()', which is polymorphic and returns
the leaf type, so the intermediate type would not figure into the decision.

You can check this with:

// testit/
// Java 5+

package testit;
class B
   private int belem;
   public final void setBelem( int v ) { belem = v; }
   public final int getBelem() { return belem; }

public class C extends B implements Cloneable
   public C clone()
     return (C) super.clone();

   public static void main( String [] args )
     C cloneMe = new C();
     cloneMe.setBelem( -17 );
       C copy = cloneMe.clone();
       System.out.println( "Cloned: belem = "
           + copy.getBelem() );
     catch ( CloneNotSupportedException ex )
       System.out.println( "clone() not supported: "
           + ex.getMessage() );


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