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copied!<p>As far as I can see the problem here is that you are defining how the tree will be traversed in the tree itself and not in the visitor. While this can be a valid approach (design patterns do have variations) I think that in this case is better to decouple the tree structure from the traversing order (pre-order, in-order, post-order). As a matter of fact a typical exercise when teaching this pattern is to write the three visitors, each one performing a different traversal.</p> <p>In your case I would:</p> <ol> <li><p>Represent the expressions as a tree just like you did, but removing form the accept the traversing parts. In your code the accept would look like:</p> <pre><code>public void accept(Visitor visitor) { visitor.visit(this); } </code></pre></li> <li><p>Define public getters for the childs of your nodes, so that the visitor can access them (<code>getChildA()</code>, <code>getChildB()</code> and <code>getValue()</code>).</p></li> <li><p>Write a visitor for the type of traversal that you need. For evaluating the expression you will generally use a post-order while for printing the expression you can use in-order (as in your example). So, for evaluating the expression you will end with something that looks like this:</p> <pre><code>class EvalVisitor implements Visitor{ public Integer visit(IntegerNode node) { return node.getValue(); } public Integer visit(PlusTreeExpression tree) { return this.visit(tree.getChildA()) + this.visit(tree.getChildB()); } public Integer visit(MultiplyTreeExpression tree) { return this.visit(tree.getChildA()) * this.visit(tree.getChildB()); } } </code></pre></li> </ol> <p>HTH</p>

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