1. Java Programming: Guided Learning with Early Objects
Chapter 5
Control Structures II: Repetition

2. Objectives Learn about repetition (looping) control structures
Explore how to construct and use:
Counter-controlled repetition structures
Sentinel-controlled repetition structures
Flag-controlled repetition structures
EOF-controlled repetition structures
Examine break statements in loops
Java Programming: Guided Learning with Early Objects

3. Objectives (continued)
Discover how to form and use nested control structures
Learn how to avoid bugs by avoiding patches
Learn how to draw shapes using the class Graphics
Learn how to use JtextAreas in a GUI
Java Programming: Guided Learning with Early Objects

4. Why Is Repetition Needed?
Many situations in which it is necessary to repeat a set of statements
Three looping (repetition) structures:
while
for
do…while
Allow repeating statements over and over until certain conditions are met
Java Programming: Guided Learning with Early Objects

5. while Looping (Repetition) Structure
Syntax:
while (logical expression)
statement
logical expression is called a loop condition
statement may be simple or compound
If logical expression evaluates to true the statement executes
Java Programming: Guided Learning with Early Objects

6. while Looping (Repetition) Structure (continued)
Infinite loop: executes indefinitely
Loop control variable must be initialized before executing the loop
If semicolon is at the end of the loop, the action of the loop is empty or null
Java Programming: Guided Learning with Early Objects

7. Figure 5-1 while loop
Java Programming: Guided Learning with Early Objects

8. Designing while loops
Body of while loop executes only when logical expression evaluates to true
Typically, logical expression determines if loop control variable (LCV) satisfies condition
LCV must be initialized before while loop
Value of LCV must eventually make condition false
Value of LCV updated in loop body
counter = 0;
while (counter < n)
Java Programming: Guided Learning with Early Objects

9. Counter-Controlled while Loops
Number of loop iterations known in advance
Assume n iterations
Counter initialized to 0 before while statement
Before executing loop body, counter compared to n
If counter less than n, loop body executes
Inside loop body, counter incremented
Java Programming: Guided Learning with Early Objects

10. Sentinel-Controlled while Loops
Sentinel: special value that signals end of processing
Read first item before entering while statement
If item is not the sentinel, loop body executes
Loop continues as long as sentinel value not encountered
while (variable != sentinel)
Java Programming: Guided Learning with Early Objects

11. Flag-Controlled while Loops
Uses a boolean variable to control loop
The boolean variable is known as a flag
Must eventually be set to true in loop body
boolean found = false;
while (!found)
Java Programming: Guided Learning with Early Objects

12. EOF-Controlled while Loops
If data file is frequently altered, avoid using a sentinel value
Sentinel might accidentally be erased
Programmer may not know sentinel value
EOF-controlled while loop depends on stream object used to input data
Java Programming: Guided Learning with Early Objects

13. EOF-Controlled while Loops (continued)
Scanner object:
static Scanner console;
console = new Scanner(System.in);
Method hasNext returns true if there is an input in the input stream
Java Programming: Guided Learning with Early Objects

14. EOF-Controlled while Loops (continued)
Expression console.hasNext() acts as loop condition
console acts as loop control variable
Expression like console.nextInt() updates value of loop control variable
while (console.hasNext())
Java Programming: Guided Learning with Early Objects

15. More on Expressions in while Statements
while loop may be controlled by single variable
Logical expression in while statement may be complex
while ((numGuess < 7) && (!guessRight))
Java Programming: Guided Learning with Early Objects

16. for Looping (Repetition) Structure
while loop general enough to implement all forms of repetitions
for loop syntax:
for (initial expr;logical expr;update expr)
statement
logical expr is the loop condition
Java Programming: Guided Learning with Early Objects

17. for Looping (Repetition) Structure (continued)
All three expressions are for loop control expressions
for loop body may contain simple or compound statements
Java Programming: Guided Learning with Early Objects

18. Figure 5-3 for loop
Java Programming: Guided Learning with Early Objects

19. for Looping (Repetition) Structure (continued)
Execution of for loop:
initial expr executes
logical expr evaluated
If loop condition evaluates to true, execute statement
Execute update statement
Repeat until loop condition evaluates to false
Java Programming: Guided Learning with Early Objects

20. for Looping (Repetition) Structure (continued)
Primarily used to implement counter-controlled loops
Called counted or indexed
Loop control variable declared within for statement
After for loop executes, using loop control variable generates a syntax error
Loop body does not execute if logical expr initially false
Java Programming: Guided Learning with Early Objects

21. for Looping (Repetition) Structure (continued)
update expr should change value of loop control variable
Semicolon at end of for statement makes the for loop null
If logical expr omitted, it is assumed true
Can omit all three statements:
for (;;)
Java Programming: Guided Learning with Early Objects

22. do…while Looping (Repetition) Structure
Syntax: do
statement
while (logical expr);
statement may be simple or compound
Compound statement enclosed in braces
logical expr is the loop condition
Java Programming: Guided Learning with Early Objects

23. Figure 5-4 do…while loop
Java Programming: Guided Learning with Early Objects

24. do…while Looping (Repetition) Structure (continued)
statement executes first
logical expr evaluated
If logical expr true, statement executes again
Loop body must contain statement that ultimately makes logical expr false
Java Programming: Guided Learning with Early Objects

25. do…while Looping (Repetition) Structure (continued)
while and for loops have entry conditions
May never execute
do…while loop has exit condition
Always executes at least once
Pretest loop: loop condition evaluated before executing loop body
Posttest loop: loop condition evaluated after executing loop body
Useful for input validation
Java Programming: Guided Learning with Early Objects

26. Choosing the Right Looping Structure
for loop: number of repetitions known in advance
while loop: number of repetitions cannot be determined in advance
do…while loop: number of repetitions not known, but must be at least one
Java Programming: Guided Learning with Early Objects

27. Loops and the break Statement
break statement alters flow of program control
In switch structure, provides immediate exit from structure
Used in while, for, do…while loops to exit from structures before loop ends
Java Programming: Guided Learning with Early Objects

28. Loops and the break Statement (continued)
Two purposes:
Exit early from a loop
Skip remainder of switch structure
After break statement executes, loop terminates
Remaining loop statements skipped
Program continues at first statement after loop
Java Programming: Guided Learning with Early Objects

29. Loops and the break Statement (continued)
Use of break statement eliminates certain boolean variables
Should be used sparingly
Java Programming: Guided Learning with Early Objects

30. Nested Control Structures
Nested for loops:
for (int i = 1; i <= 5; i++) {
for (int j = 1; j <= i; j++) {
while loop nested in for loop:
for (int i = 0; i <= 4; i++) {
while (num != 999) {
Nested while loops:
while (inFile.hasNext()) {
Java Programming: Guided Learning with Early Objects

31. Avoiding Bugs by Avoiding Patches
Software patch is code written on top of existing code to remedy deficiency in original code
Better to correct original deficiency
Patches represent poor programming practice
Java Programming: Guided Learning with Early Objects

32. GUI Graphics and JTextAreas (Optional)
class Graphics contained in package java.awt
Provides methods for drawing items on the screen
Methods draw shapes, bitmap images
Class contains methods to set properties such as font and color
Java Programming: Guided Learning with Early Objects

33. GUI Graphics and JTextAreas (Optional) (continued)
Java coordinate system
Identifies every point on the screen
Origin: upper-left corner of GUI component
Coordinates (0,0)
Every coordinate pair has x-coordinate and y-coordinate
x-coordinate specifies horizontal position
y-coordinate specifies vertical position
Java Programming: Guided Learning with Early Objects

34. Figure 5-5 Java’s coordinate system
Java Programming: Guided Learning with Early Objects

35. Figure 5-6 Chair and a table
Java Programming: Guided Learning with Early Objects

36. Figure 5-7 Coordinates of the lines that make up the chair and table
Java Programming: Guided Learning with Early Objects

37. GUI Graphics and JTextAreas (Optional) (continued)
Method drawLine takes coordinates as parameters
Methods of class Graphics are abstract
Cannot create an instance of the class
class JFrame has method paint
Draws Graphics object on a JFrame object
Coordinate system starts from upper-left corner of the window and contains window title
Java Programming: Guided Learning with Early Objects

38. GUI Graphics and JTextAreas (Optional) (continued)
JPanel: a container placed inside a JFrame object below title of window
class JPanel has method paintComponent
Draws Graphics objects on JPanel object
public void paintComponent
(Graphics g)
Java Programming: Guided Learning with Early Objects

39. GUI Graphics and JTextAreas (Optional) (continued)
Use class Graphics without creating Graphics object
Override paintComponent to draw chair and table
super.paintComponent(g);
Java Programming: Guided Learning with Early Objects

40. GUI Graphics and JTextAreas (Optional) (continued)
Create class ChairAndTable by extending class JPanel
Definition of paintComponent inside ChairAndTable
Method paintComponent executed when ChairAndTable instantiated
Java Programming: Guided Learning with Early Objects

41. Figure 5-8 Output of the ChairAndTableTestProg program
Java Programming: Guided Learning with Early Objects

42. GUI Graphics and JTextAreas (Optional) (continued)
random method of class Math generates random number
Used to generate random:
Number of figures
Color
Width
Height
Shape
Java Programming: Guided Learning with Early Objects

43. Figure 5-9 Sample run of OvalsAndRectsTestProg
Java Programming: Guided Learning with Early Objects

44. JTextArea class JTextField used to input or display a line of text
JTextField cannot handle multiple lines
class JTextArea collects multiple lines of input, displays multiple lines of output
JTextField and JTextArea derived from class JTextComponent
JTextComponent is an abstract class
Java Programming: Guided Learning with Early Objects

45. Table 5-3 Methods Inherited by the class JTextArea from the Parent class JTextComponent
Java Programming: Guided Learning with Early Objects

46. Summary Java has three repetition structures: while loops: while for
do…while
while loops:
Counter-controlled
Sentinel-controlled
Flag-controlled
EOF-controlled
Java Programming: Guided Learning with Early Objects

47. Summary (continued) Infinite loop never terminates
Loop conditions may be simple or complex
while loop general enough to implement all forms of repetition
for loop implements counter-controlled loops
Control variable declared in loop statement
Number of iterations known
do…while loop executes statement before evaluating condition
Java Programming: Guided Learning with Early Objects

48. Summary (continued)
Pretest loops evaluate condition before executing loop body
Posttest loops execute body then evaluate condition
break statement used for two purposes:
Exit early from loop
Skip remainder of switch structure
After executing break statement in loop, remaining loop statements skipped
Java Programming: Guided Learning with Early Objects

49. Summary (continued) Control structures may be nested
Software patches are poor programming practice
Better to fix original deficiency
class Graphics provides methods for drawing items on the screen
Java coordinate system origin in upper-left corner
Java Programming: Guided Learning with Early Objects

50. Summary (continued) Abstract classes cannot be instantiated
class Graphics is abstract
Pass Graphics object to paintComponent method in JPanel
Method random in class Math generates random numbers
JTextField inputs or displays single line of text
JTextArea inputs or displays multiple lines of text
Java Programming: Guided Learning with Early Objects