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POExtendable double-linked tree implementation in Java
 

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    <p>I want to extend an existing double-linked tree implementation by some extra data. </p> <p>Therefor I could refactor the basic <code>TreeNode</code> implementation, but I want to have a separate extended TreeNode implementation, because in my real-world szenario building-up an <code>ExtandedTreeNode</code> will be much more expensive than building-up the basic <code>TreeNode</code>, and the extra data is only necessary for some use-cases.</p> <h1>Rudimental Code</h1> <p>Here my first rudimental approach for an extendable double-linked tree implementation in Java:</p> <h2>Basic tree node interface:</h2> <pre class="lang-java prettyprint-override"><code>interface TreeNode { // tree node getters: TreeNode getParent(); List&lt;? extends TreeNode&gt; getChildren(); // tree node setters: void setParent(TreeNode parentNode); void addChild(TreeNode childNode); // some basic operations: boolean isSelectable(); // [...] } </code></pre> <h2>Extended tree node interface:</h2> <pre class="lang-java prettyprint-override"><code>interface ExtandedTreeNode extends TreeNode { // narrow types of tree node getters of super-interface: @Override ExtandedTreeNode getParent(); @Override List&lt;? extends ExtandedTreeNode&gt; getChildren(); // narrowing types of tree node setters of super-interface is not possible! // some additional operations: boolean isRemoveable(); // [...] } </code></pre> <h2>Basic tree node implementation:</h2> <pre class="lang-java prettyprint-override"><code>class TreeNodeImpl implements TreeNode { private TreeNode parent; private List&lt;TreeNode&gt; children = new ArrayList&lt;TreeNode&gt;(); private boolean isSelectable; // // implement tree node getters: // @Override public TreeNode getParent() { return parent; } @Override public List&lt;? extends TreeNode&gt; getChildren() { return children; } // // implement tree node setters: // @Override public void setParent(TreeNode parent) { this.parent = parent; } @Override public void addChild(TreeNode childNode) { children.add(childNode); } // // implement basic operations: // @Override public boolean isSelectable() { return isSelectable; } // [...] } </code></pre> <h2>Extended tree node implementation:</h2> <pre class="lang-java prettyprint-override"><code>class ExtandedTreeNodeImpl implements ExtandedTreeNode { private ExtandedTreeNode parent; private List&lt;ExtandedTreeNode&gt; children = new ArrayList&lt;ExtandedTreeNode&gt;(); private TreeNode treeNode; private boolean isRemoveable; public ExtandedTreeNodeImpl() { treeNode = new TreeNodeImpl(); } // // implement tree node getters: // @Override public ExtandedTreeNode getParent() { return parent; } @Override public List&lt;? extends ExtandedTreeNode&gt; getChildren() { return children; } // // implement tree node setters: // @Override public void setParent(TreeNode parent) { this.parent = (ExtandedTreeNode) parent; // &lt;--- How to avoid this type cast!! } @Override public void addChild(TreeNode childNode) { children.add((ExtandedTreeNode) childNode); // &lt;--- How to avoid this type cast!! } // // implement basic operations by delegating to composite TreeNode: // public boolean isSelectable() { return treeNode.isSelectable(); } // [...] // // implement additional operations: // @Override public boolean isRemoveable() { return isRemoveable; } // [...] } </code></pre> <h2>Question</h2> <p>For me the type hierarchy looks great for the read-only and non-tree-node-specific methods, but it leaks for the setters <code>setParent(..)</code> and <code>addChild(..)</code>. Particularly the risky typecast to <code>ExtandedTreeNode</code> is quite evil and I want to get rid of it. </p> <p>I thought about extracting the <code>setParent(..)</code> and <code>addChild(..)</code> methods to another two separate Interfaces like <code>TreeNodeWriteable</code> and <code>ExtandedTreeNodeWriteable</code>, but maybe there are some better design solutions. </p> <p>Does anyone know a design pattern or blue print which could solve my problem with the writeable tree-node-specific setters?</p>
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