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Message: [QUOTE="Joshua Cranmer, post: 2980447"] What do you mean, "Not a symbol because it is abstract"? Any type can be used for reference, including abstract classes and interfaces. This would be partial code for what I think you want to do: LinkedList<Symbol> stack = new LinkedList<Symbol>(); Queue<Symbol> queue = new LinkedList<Symbol>(); // ... Fill queue ... while (!queue.isEmpty()) { Symbol head = queue.remove(); if (head instanceof Operand) { stack.addLast(head); } else if (head instanceof Function) { // ... manipulate stack, etc. } } [/QUOTE]