src/org/nutz/el/arithmetic/RPN.java
private Object calculate(LinkedList<Object> el2){
        if(el2.peek() instanceof Operator){
            Operator obj = (Operator) el2.peek();
            return obj.calculate();
        }
        if(el2.peek() instanceof Elobj){
            return ((Elobj) el2.peek()).fetchVal();
        }
        return el2.peek();
    }
private Object calculate(LinkedList<Object> el2){
        if(el2.peek() instanceof Operator){
            Operator obj = (Operator) el2.peek();
            return obj.calculate();
        }
        if(el2.peek() instanceof Elobj){
            return ((Elobj) el2.peek()).fetchVal();
        }
        return el2.peek();
    }
计算