Java Calculator Programming Exercise

COMP 585 Java Calculator Programming Exercise Calculator GUI This is an exercise that demonstrates how to create a simple GUI that uses a non-default layout manager, and how to use a simple state machine to keep track of buttons pushed The GUI looks like this: The components consist of the following: • A JTextField to represent the calculator display • 10 JButtons to represent the digits 0 through 9 • 4 JButtons to represent the arithmetic operations add, subtract, multiply, divide This GUI can be created by using a 5-by-4 GridLayout with 5-pixel gaps between cells. Cells can be left empty by placing a “dummy” component such as an empty JPanel into the layout at that cell. Note that you cannot add a null pointer as a component to a container. Another option is to use absolute positioning. Trace for Input Processing We will keep the logic simple by not implementing other buttons such as equals, clear, clear memory. To understand the necessary input processing, it’s helpful to perform a trace of a simple series of inputs. We will keep track of several items of calculator state. For example, suppose the user presses the keys “35 + 7 – 5 + 16” Current Key 3 5 + 7 5 + Value One 35 35 35 42 42 42 37 Last Fn + + + + Value Two Disp 7 3 35 35 7 42 5 5 37 State Machine Model for Input Processing The behavior in the above table can be described by a simple finite state machine (FSM). Here’s an example, but feel free to create your own. There are many possible solutions. This models the simple calculator with no equals or clear operation. Adding these buttons will require a more complex FSM. Event Listeners Use your trace table and FSM to design two action listeners: one for number buttons and one for function buttons. Number Button Listener State == 0 Set Display to text of current button State = 1 State == 1 Append text of current button to Display State = 1 … Function Button Listener State == 0 Illegal operation State == 1 Copy Display to Value1 (Value1 = Display) Set Last Function to text of current button State = 2 … Fonts In this example you will need to experiment with the font used to display text associated with a component such as a button. Fonts are represented by class java.awt.Font. All Swing components have a default Font object, which can be examined by the appropriate accessor. JButton b = new JButton(“click”); Font f = b.getFont(); One of the more useful constructors is Font(String name, int style, int pointsize) Where name is the font family name, style is a static constant indicating plain, bold, italic, etc., and pointsize is point size. There are also several useful font derivation methods that create a new font based on properties of an existing font. For example Font deriveFont(float size); is applied to an existing font and returns a new font which is identical to the current font except with a different point size. A small set of logical fonts are built in to the virtual machine and are therefore available on all installations. In addition, Java Swing may use physical fonts which have been installed on the local machine. These will of course vary from one platform to the next. To see which fonts are available on a given platform at runtime, use one of the following static methods: Font[] GraphicsEnvironment.getAllFonts() String[] GraphicsEnvironment.getAvailableFontFamilyNames() In simple cases, it will not be necessary to exhaustively list all font families just to find a useable font. For example, the following will usually work on most systems: JButton b = new JButton(“Click”); b.setFont(new Font(“Times”, Font.BOLD, 18)); where font family is chosen to be something very common, i.e, the “Times” family. When in doubt, get the exact list of available font families on the current system and pick one. Event Listeners To add event handling capability to your calculator app, implement an action listener for each button. A good strategy for this app is to write two event listeners, one to respond to numerical buttons, and another to respond to function buttons. You only need to instantiate one object of each listener, then attach that listener to multiple buttons. In the actionPerformed() callback method, you will need to examine either the source or the action event of the event to determine specifically which button was clicked. The source returns a reference to the actual object that triggered the event. The action command equates to the text displayed on the button. Either property can be used to tell clicks on different buttons apart. The steps in brief are: 1 Define the listeners (as nested classes or anonymous classes, for example) 2 During the GUI building phase instantiate objects of the listener class 3 Also during the GUI building phase, “register” the objects as listeners for the appropriate button. The ActionListener interface is defined in package java.awt. A useful debugging strategy at this stage is to send lots of diagnostic output to the console window via System.out.print()/println().