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POType-safe way to store two types of object in a collection
 

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    <p>I've been implementing an enhanced Shunting-Yard algorithm for parsing an arithmetic expression. One aspect of the algorithm, is that it maintains a <code>Queue</code>, and a <code>Stack</code>.</p> <p>In my implementation, the <code>Queue</code> contains <code>Expressions</code> and <code>Operators</code>. The <code>Stack</code> contains <code>Operators</code> and <code>Parenthesis</code>.</p> <p><code>Expressions</code>, <code>Parenthesis</code>, and <code>Operators</code> have nothing in common that warrants any two of them having a shared interface.</p> <p><strong>Approaches:</strong></p> <ul> <li><p>My current implementation consists of <code>Expression</code> and <code>Operator</code> implementing a <code>INotParanthesis</code>. <code>Operator</code> and <code>Paranthesis</code> implement a <code>INotExpression</code>. I then declare <code>Queue &lt;INotParanthesis&gt;</code>, and <code>Stack &lt;INotExpression&gt;</code>.</p> <p>I don't like this implementation - these interfaces seem very much a hack for the purpose of cleaner algorithm code. I also believe that interfaces should describe what an object is, as opposed to what it isn't.</p></li> <li><p>On the other hand, I also don't want to use collections of <code>&lt;Object&gt;</code>, as it can be difficult to be certain of the correctness of such code.</p></li> <li><p>The only one I've come up with, so far, is implementing my own <code>NonParanthesisQueue</code> and <code>NonExpressionStack</code> containers. This has the advantage of more consistent type checking on objects getting pulled out of those containers - and the disadvantage of a lot more code.</p></li> </ul> <p>Are there any reasonable alternatives to my approaches? </p>
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