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User interfaces for object-oriented systems, Part 6: The RPN calculator

The RPN-calculator application demonstrates object-oriented UI principles

By Allen Holub, JavaWorld.com, 03/01/00

This month we continue the process begun in the January Java Toolbox by completing the RPN (Reverse Polish Notation) calculator application. The RPN calculator is a small but nontrivial application that demonstrates some of the important object-oriented UI principles we've looked at in previous Java Toolbox articles, specifically the visual-proxy design pattern. This application isn't just a toy; I use it virtually every day -- every time I need a calculator, in fact.

A word of warning: the following discussion will be incomprehensible if you haven't read the earlier installments of this series.

TEXTBOX: TEXTBOX_HEAD: Build user interfaces for object-oriented systems: Read the whole series!

• Part 1. What is an object? The theory behind building object-oriented user interfaces
• Part 2. The visual-proxy architecture
• Part 3. The incredible transmogrifying widget
• Part 4. Menu negotiation
• Part 5. Build an application that puts user-interface principles into practice
• Part 6. An RPN-calculator application demonstrates object-oriented UI principles :END_TEXTBOX
  
  

Reverse Polish Notation was championed for years by Hewlett-Packard, though I've been told that their most recent calculators don't support it anymore (a pity). It is one of those things that are difficult to learn but wonderful to use once you understand them. The RPN's basic notion is built around an arithmetic stack. Numbers, when entered, are pushed on the stack, and all operations use stack items as operands.

For example, when you press the add (+) key, the two items closest to the top of the stack are popped and added together, and the resulting sum is pushed, effectively replacing the original operands. Although that might seem like a strange way to do things, you never need to use parentheses, and once you get used to it, you'll probably rather like it. I've been using my PalmPilot as a handheld calculator, using Russ Webb's great RPN calculator (see Resources), but I wanted one for my computer too. Being a programmer, I thought building one was the easiest way to get exactly what I wanted.

The analysis model

Since the structure of the calculator's UI is closely related to the object model, let's start by looking at the analysis-level static model, shown in Figure 1.

As is the case with all object-oriented designs, the analysis-level classes are found in the implementation as well. Figure 1, therefore, is really an implementation-level diagram of the original analysis diagram. In object-oriented systems, the design-level model is nothing but the analysis-level model with implementation-level detail added.

Figure 1. The analysis-level static model

The first analysis-level class of interest -- Rpn -- contains the main() method and little else. Though they aren't shown in the diagram, the class also contains a constructor and implementations of all the methods required by the interfaces. In an attempt to reduce the clutter a bit, I usually don't show these in a Unified Model Language (UML) diagram. As is the case with many object-oriented programs, main() (and the Rpn class, for that matter) aren't much to look at. Object-oriented systems tend to be networks of cooperating objects, with no central God class that controls everything from above. There's no spider sitting in the middle of the web pulling strands. An object-oriented system's main() method, as a consequence, typically creates a few objects, hooks them up to each other, and then terminates. That's exactly what happens here: main() creates an instance of the Parser and Math_stack, hooks them up to each other, and terminates.

 

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