CS2007: Undo

1
CS2007 Undo

• Assessment 1
• What is Undo?
• GUI State
• Stacks
• Reading Malik, first part of chap 17 (Stacks)

2
Programming Aside

• String comparison
• assignment, not equality test
• useless (do not use this)
• equals() tests for equal strings
• equalsIgnoreCase() ignores case when testing
• text will be regarded as equal to TEXT
• java.text.Collator most flexible, can be set to
  ignore accents.
• text will be regarded as equal to tèxt
• Similar comparison methods (eg, compareTo)
• is String A alphabetically before String B?

3
Undo

• Interfaces should allow users to reverse actions
• In other words, undo!
• Error recovery
• Gives user confidence to experiment
• Essential for high usability

4
Undo Functionality

• Granularity (character? word?)
• Depth (1 undo? 10 undos?)
• Redo (undo undo)?
• User interface
• Compare Notepad, Word, NetBeans

5
Whats best?

• Opinions
• Granularity, depth, redo, interface

6
Whats best?

• Should determine by studying user needs and
  expectations
• CS2506

7
Programming Undo

• Must remember previous GUI state
• Must be able to restore GUI to a previous state
• (if depthgt1) must keep track of many previous
  states, in right order

8
Example Currency converter

• Input poundsField
• Output eurosField

9
GUI State

• Need to save state of GUI
• Inputs (eg, poundsField), outputs (eg,
  eurosField)
• Not just inputs (poundsField)
• Probably as Strings
• Could also save as doubles (JTextField?)
• But Strings easier, more general

10
Option 1 Class Variables

• Declare class variables to hold old values
• String oldPoundsText, oldEurosText
• .
• at beginning of Convert or saveState method
• oldPoundsText poundsField.getText()
• oldEurosText eurosField.getText()

11
Example

• Add Save button as above
• Add Restore button with code
• poundsField.setText(oldPoundsText)
• eurosField.setText(oldEurosText)

12
Option 2 State Class

• Define a new class that holds GUI state
• Class variables hold information
• Constructor creates this class from the GUI
• Method to set GUI to this state
• Can have other methods as well
• Read/write from file
• Compare to other state

13
Example

• public class GUIState
• String poundsText, eurosText
• GUIState (GUI g) // construct from GUI
• poundsText g.poundsField.getText()
• eurosText g.eurosField.getText()
• void setGUI (GUI g) // set GUI to this state
• g.poundsField.setText(poundsText)
• g.eurosField.setText(eurosText)

14
NetBeans Note

• Only works if poundsField, etc accessible in
  guiState
• Not accessible by default in NB (private)
• For each widget
• Select widget in GUI designer
• select code properties
• Change Variable modifiers from private to
  ltdefaultgt
• In theory, change tools, options, misc, GUI
  Builder, variable modifier to ltdefaultgt

15
Algorithm Design Choice 1

• How should state be stored?
• Class variables?
• State class?
• Opinions? Rationale?

16
State Class is Better

• Much better programming practice!
• Especially for more complex GUIs (more state to
  remember)
• Especially if other things done with state (eg,
  saving to a file)

17
Just record changes

• What if GUI state is very large?
• Eg, entire Document for MS Word
• Just record changes between old state and current
  state
• Eg, if only poundsBox has changed (because
  Convert not pressed), just record its old value,
  dont bother with eurosBox
• Alternative undo algorithm

18
Algorithm Design Choice 2

• Record
• Full state
• Change in state
• Advantages of recording full state
• Easier to program
• Quicker (less computation needed)
• Advantages of recording changes
• Needs much less memory

19
Changes vs Full State

• Which is better?
• Depends on circumstances
• Size of GUI state
• Computer speed, memory
• Cost of programmer time
• MS Word vs CS2007 Currency Convert

20
Algorithm Design

• Store state in variables vs class
• Class is almost always better!
• Store full state vs changes
• Depends on circumstances
• Art of algorithmic problem solving Designing
  (choosing) appropriate algorithms for particular
  circumstances

21
Keeping Track of States

• If undo depth is 1, just save GUI status in a
  class variable
• GUIState oldState
• .
• oldState new GUIState(this)
• Exercise single Undo for currency convertor

22
Multiple states

• If depthgt1, we must keep track of many previous
  states
• GUI before most recent change
• GUI before second most recent change
• GUI before third most recent change
• etc

23
Stacks

• Use a Stack data structure
• Last-in, first-out collection
• Elements are added and removed only from one end
  (the top of the stack).
• Very common data structure in computer
  programming

24
Stack concepts (2)

• Illustration (stack of books)

25
Stacks in Java

• Stack class in java.util
• Usually we add a type in ltgt
• StackltStringgt stringStack
• StackltGUIStategt guiStack

26
Stack methods

• push (element) -- add element to the top of the
  stack
• peek() -- return object at top of stack (dont
  change stack)
• pop() return object at top of stack (and take
  it off the stack)
• size() number of things in stack
• empty() is stack empty?

27
Stack example

• StackltStringgt stringStack new StackltStringgt()
• stringStack.push(Joe)
• stringStack.push(Susan)
• System.out.println(stringStack.peek()) //
  prints Susan
• System.out.println(stringStack.pop()) // prints
  Susan
• System.out.println(stringStack.pop()) // prints
  Joe

28
Stack in Undo

• Main idea
• Whenever GUI changes, push new state onto a stack
• When undo pressed, pop state off the stack, and
  set GUI to it
• Many details and caveats, to-be-discussed