Lecture 3

Chat: Assignments 2 and 3 Undo Capability Fine-tuning appearance Adding Customizability Save/Restore Assignment 4

Undo/Redo

Expected by user Apply consistently Second undo = redo? More depth is nice What counts as an event?

Easily and non-easily reversible events Warn user when taking irreversible actions Persistency: can undo vector be saved?

1-step Undo

After each reversible action, save state (backend) If Undo command issued:

Restore state. (backend, possibly frontend).

Save undone state in case of redo. Disable undo, enable redo.

Multi-step undo/redo

Keep stack of recent states. Undo pops back to previous state. New actions pushed onto the top. Immediately after a series of undos, can redo actions. Enhanced stack. More complex behavior possible Emacs’ tree undo

Resource issues with undo

Saving/restoring state may cost too much. Can store difference between

before/after states. Can store command itself. Need

special undo code for each command.

May only want to support limited undo depth.

Supporting undo in Java

Vectors to store states/commands. Dynamically sized and untyped. Need one vector for undoable events and one for redoable events. Can just store ActionEvents generated by your UI, rather than state, if you are able to reverse them. Allows highly reusable code.

Images

“A picture is worth a thousand words, and uses less screen real estate.” Attach to a JComponent:

ImageIcon ii = new ImageIcon(“images/icon1.gif”);JButton b = new JButton(ii);JLabel l = new JLabel(“Hello”,ii);JMenuItem mi = new JMenuItem(“test”);mi.setIcon(b.getIcon());

Manipulate using Graphics or Graphics2D object

What can I do in 1 line?

Can add images to any AbstractButton (button, checkbox, menu, menuitem), JLabel, JList, JComboBox Only Icons (fixed-size images) supported so far. Use ImageIcon. Dubious support for animated gifs, but these distract even when they do work.

Other Easy Graphic Effects

Buttons/Menus can have several icons to indicate status. Change background color of any JComponent using setBackground. setForeground too. Use JLayeredPane to superimpose text, components, on top of image. Can also setFont().

Example: JButton icons

ImageIcon ic1 = new ImageIcon(filenameOrURL);

// … Get icons ic2 through ic6.

JButton but = new Jbutton(“text”);

but.setIcon(ic1);

but.setRolloverIcon(ic2);

but.setPressedIcon(ic3);

but.setSelectedIcon(ic4);

but.setDisabledIcon(ic5);

but.setDisabledSelectedIcon(ic6);

// etc.

Customization

“A happy user is a productive user.” What should we allow the user to change? Almost everything Don’t let the user trap himself.

e.g. no “hide Edit Preferences”, no “disable Quit”.

Session vs. Permanent prefs

Easy part first:

Cosmetic customization, such as colors, background screen, menu shortcuts, all easy. Font also easy, except font size may cause problems. Same goes for Icons. Examples

Hard parts (sort of)

Permanent prefs. Need to save everything relevant in a file. Prioritized prefs. “I like blue text, except in buttons with icons, except for the Back button.” Multiple preference settings. Resizing/rearranging GUI components.

Save/Restore

A quick overview of stream I/O in Java. Note: easy to write OS specific code

here, because of path name conventions

Serialization

Streams

InputStream

OutputStream

ObjectInputStream

FilterInputStream

Integer

Wrapping

int

Objectint

FileInputStream

A wrapper class Stream wrapping

A technique for “borrowing methods”

Vector.add()

Example (SaveRestoreDemo.java)

// In actionPerformed(ActionEvent e) method.

// code handles “Save” menuitem by storing a

// serialized object to file.

if (e.getActionCommand().equals("Save")) {

try {

FileOutputStream out = new FileOutputStream(fname);

ObjectOutputStream s = new ObjectOutputStream(out);

s.writeObject(mySerializableObject);

s.flush();

} catch (Exception ex) {

System.out.println("Save exception." + ex);

}

Object Serialization

Uniform way of converting objects to strings, for RMI and saving/restoring from files. Thread-safe. Object cannot be modified during serialization. Objects can only be serialized once by a given serializer. Further references are by handle. This is efficient, but somewhat limiting. Compatibility issues.

Using Serialization

Use classes ObjectOutputStream, ObjectInputStream. Methods writeObject(ob), readObject() convert any Serializable object to a data stream. Serializable interface is empty.

Only purpose is security. If you make a class Serializable, its private fields can be accessed.

Can override this by providing custom writeObject and readObject methods in Serializable class, or by declaring variables transient

Custom Serialization

Default serialization is inefficient May want to serialize part of a field, but protect another part. May wish to switch between allowing and disallowing serialization, or change methods.

Save/Restore

1-step save: serialize everything, write it to file. Wasteful. Better: save only as much as necessary, but serialize where handy. Restore: Read from file in reverse order data was written.

Assignment 4

Lots of work, start early. Get totally familiar with SaveRestoreDemo before starting. The new functionality is mostly here.