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POIs it possible to design a type-safe linked list preventing getNext() at the tail node?
 

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  1. POIs it possible to design a type-safe linked list preventing getNext() at the tail node?
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    <p>I'm wondering if it is possible to design, for example, a type-safe singly linked list structure such that it is impossible to ask for the next node from the tail node. </p> <p>At the same time, the client would need to be able to traverse (recursively or otherwise) through the list via <code>node.getChild()</code> but be prevented at compile time (at least with human-written explicit type checking) from going past the tail. </p> <p>I'm wondering:</p> <ol> <li>Is there a name for this type of problem?</li> <li>Is there an object oriented or other approach that would help to avoid explicit run-time type checking? </li> </ol> <p>The implementation language isn't important, but here's a Java example of what I'm thinking of. </p> <p><strong>Edit after Joop's answer:</strong></p> <pre><code>public class TestHiddenInterfaces { interface Node { HasNoChildNode getTail(); } interface HasNoChildNode extends Node {}; interface HasChildNode extends Node { Node getChild(); } class HasNoChild implements HasNoChildNode { @Override public HasNoChildNode getTail() { return this; } } class HasChild implements HasChildNode { final Node child; @Override public Node getChild() { return child; } HasChild(Node child) { this.child = child; } @Override public HasNoChildNode getTail() { if(child instanceof HasChild) return ((HasChild) child).getTail(); else if(child instanceof HasNoChild) return (HasNoChildNode) child; else throw new RuntimeException("Unknown type"); } } @Test public void test() { HasNoChild tail = new HasNoChild(); assertEquals(tail, tail.getTail()); HasChild level1 = new HasChild(tail); assertEquals(tail, level1.getTail()); HasChild level2 = new HasChild(level1); assertEquals(tail, level2.getTail()); } } </code></pre>
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    1. USJeff Axelrod
    1. USJeff Axelrod
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    1. COHave you thought of having the node which is being asked for the parent check for a null parent (if it wasn't set, it would default as null) and either throwing an exception at that point (for which the client of the node would check for) or having some other smart default behavior that would be documented in your javadoc and checked for by the client?
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      1. USJack
    2. COI don't know what you mean by "//Not type safe". In your last assertion, you say: level2.getParent().getParent(). That returns the Root node. Why would that assertion fail? You wouldn't be able to call getParent() on the Root unless you cast it to a (NonRoot) type, which would cause a ClassCastException. So yes, the only way to identify the root is to use some kind of runtime check.
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      1. USSam Goldberg
    3. CO@Jack I intend to use language support to make the `getParent()` method nonexistent as I showed in the `interface RootNode extends Node {};` line.
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      1. USJeff Axelrod
 

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