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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Confusing interface
Date: Fri, 7 Jul 2017 09:45:44 -0400
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On 7/7/2017 9:10 AM, Eric Douglas wrote:
> Oracle defines this
> public interface TreeNode
> 
> which contains this
>      TreeNode getParent();
> 
> Then they define this
> public interface MutableTreeNode extends TreeNode
> 
> which contains this
>      void setParent(MutableTreeNode newParent);
> 
> Then they define this
> public class DefaultMutableTreeNode implements Cloneable,
>         MutableTreeNode, Serializable
> 
> which contains this
>      @Transient
>      public void setParent(MutableTreeNode newParent) {
> 
>      public TreeNode getParent() {
> 
> Now I define
> public class MyCustomTreeNode extends DefaultMutableTreeNode
> 
> and Eclipse allows
> @Override
> public MyCustomTreeNode getParent()
> 
> but I must keep the signature
> public void setParent(MutableTreeNode newParent)
> 
> If I put @Override on the set and declare the parameter as MyCustomTreeNode, it says must override or implement a supertype method.
> If the get and set are both using interface parameters, and my custom class is simply extending a class which implements one interface which extends the other, why is one allowed and not the other?

     Because `setParent(MyCustomTreeNode newParent)' does not override
the setParent() method of the superclass.  You can write the method
with the signature you like, but it's not an override.  Consider:

	MyCustomTreeNode parent = ...;
	MyCustomTreeNode child = ...;
	child.setParent(parent);

So far, all is well and behaves as you intend.  Next, however:

	DefaultMutableTreeNode kidnapper = ...;
	child.setParent(kidnapper);

Question: Which setParent() method runs, the one in your class or the
one in the superclass?  Right you are, it's the superclass' method --
which means your class has not overridden it.

     I imagine many people encounter this same confusion -- I confess
that I did, too, many years ago.  I implemented

	class MyMap extends AbstractMap<Thing,Wotsit>

... and provided the method

	Wotsit get(Thing key)

To my surprise, some calls to get() went directly to the get() of
AbstractMap and not to my code!  I had made the same mistake then
that you are making now. (My error was before @Override came onto
the scene, so the compiler didn't alert me.  Be thankful that the
tools have improved so you're faced with a compile error instead of
with a baffling mystery.)

-- 
esosman@comcast-dot-net.invalid
Twelve hundred ninety-three days to go.