CS51 - Spring 2010 - Lecture 11
Exercises 11.4.2, 11.4.3
- There is a JButton object that creates a usable button
- Instead of using a dropdown menu and clicking in the canvas, change it so that when we click a button, the object is created
- avoids accidentally creating a new object when we're trying to drag an object
- How do you think the GUI setup was constructed here? Why does it look funny?
- the FlowLayout doesn't wrap around
- splits the panel into a row by col grid
- each component takes up the same amount of space, i.e. the grid perfectly splits up the space
- elements are added in reading order, that is from left to right and then top to bottom
- specify rows then columns, for example 3, 2 is 3 rows and 2 columns
- first item added goes in upper left, then upper right, then middle left, then middle right, etc
- how do we create this?
- one large JPanel with a one row and two columns
- one JPanel (with the default FlowLayout) where we add the buttons
- add that JPanel to the large JPanel, so it will end up in the first position
- add the JComboBox to the large JPanel, so it will end up in the second, bottom position
- finally, add the large JPanel to the SOUTH of our content pane (and of course, validate :)
- how do you think we handle the button clicking?
- remember the things we need to do to handle GUI events
- implements ActionListener
- add the actionPerformed method
- add "this" object as a listener using addActionListener to the object(s) that we want to respond to
- need to addActionListener for all three of the buttons!
- what complication does this add?
- in actionPerformed, need to figure out which of the buttons caused actionPerformed to be called
- begin method looks like we predicted
- create new JButtons
- addActionListener(this) for all three
- notice that we're also listening to our colorMenu
- bottomPanel.setLayout(new GridLayout(2, 1));
- need to handle 4 different things (the three buttons, the one drop down menu)
- evt.getSource() gives us the object that generated the event
- why can we use '=='?
- we literally want to know if it was exactly that button that generated the event
- what do you think the big box at the top that changes color is?
- it's just a FilledRect
- how are we changing the color?
- we can adjust the sliders and in response to that, our program updates the color
- Another interface ChangeListener
- one method: public void stateChanged(ChangeEvent evt)
- similar to our other listeners, we have an: addChangeListener(this) method
- this pattern should start to look familiar:
- implements ChangeListener
- add the stateChange method
- tell the program that "this" will listen/respond to any changes made by the object (in this case, the sliders)
- why do we have two different interfaces ActionListener and ChangeListener
- action events happen much less frequently, i.e. based on a click
- change events are much more frequent and continuous, i.e. based on a dragging of the mouse
- we wouldn't want the overhead of all the action event code in the stateChanged method
- it's only the slider bar (not the text)
- JLabel for adding text (similar to our "Text" object, but it's a GUI component)
- let's look at the layout, how can we design this?
- two different ways:
- one large JPanel with a 3, 1 GridLayout (3 rows, 1 column)
- for each slider we have a JPanel
- 1, 3 GridLayout (1 row, 3 columns)
- one large JPanel with a 3, 3 GridLayout (3 rows, 3 colummns)
- don't need individual JPanels for each set of slider components
- just add them all to the large JPanel individually
- what information do you think we need to construct a new JSlider (i.e. what's going to go in the constructor)?
- need to know whether it's a vertical or horizonal slider
- range, specified by a minimum value and a maximum value
- initial, starting value
- the stateChanged method is going to be called everytime any of the sliders changes values
- what do we want to do when a slider changes value?
- get the value from each of the sliders
- getValue() method returns the value
- set the color of our FilledRect to be the new color indicated by the sliders
- update the text for all of the sliders
- setText(...) (similar to setText for Text)
- why do we do "" + redValue? what does this accomplish?
- we could try and figure out which of the sliders changed using evt.getSource(), but it would just make the code more complicated
- rather than using a JSlider, we can use a JTextField to allow the user to enter data
- getText() method allows us to get the text entered in the JTextField
- how do we know when to check the data?
- when the user hits "return" it triggers an event
- what type of listener do you think we would use?
- ActionListener (discrete, user generate event)
- we can addActionListener to respond to user inputs
- what does the layout look like?
- 3, 2 GridLayout
- each row has a JLabel and a JTextField
- when we're allowing a user to enter data, what do we need to be careful of?
- need to make sure the the user enters valid data
- in this case, it has to be a number and that number has to be between 0 and 255
- when we're creating a GridLayout layout manager, there are two additional parameters we can specify (if we want) that modify the amount of space in between the rows and columns
- new GridLayout(3, 2, 10, 5)
- 3 rows
- 2 columns
- 10 pixel horizontal gap (i.e. between each column)
- 5 pixel vertical gap (i.e. between each row)
- addActionListener(this) for all of the JTextFields
- actionPerformed method
- get the text values
- create a new color and set the color of the FilledRect
- what does the getColorMix method do?
- it's a private helper method
- why is it private?
- only used internally to the class
- gets the value from the JTextField and checks to see if it's the appropriate range
- what if it's not in the range?
- if it's less than the minimum value, it's set to the minimum value
- if it's larger than the maximum value, it's set to the maximum value
- why use a helper method?
- whenever you find yourself doing repeated work, make a method!
what if we want multiple lines of text, like a pararaph, etc?
- same basic premise as JTextField, but multiple lines
- look at the TextApplet example in the book