1. Java Programming: Guided Learning with Early Objects
Chapter 4
Control Structures I: Selection

2. Objectives Learn control structures
Examine relational and logical operators
Explore how to form and evaluate logical (Boolean) expressions
Learn how to use the selection control structures if, if…else, and switch in a program
Java Programming: Guided Learning with Early Objects

3. Objectives (continued)
Learn how to avoid bugs by avoiding partially understood concepts and techniques
Learn about JTextFields and JPanels
Java Programming: Guided Learning with Early Objects

4. Control Structures Four ways a computer processes statements:
Sequentially
Selection
Repetition
Method calls
Branching: alter execution flow by selection
Looping: alter execution flow by repetition
Java Programming: Guided Learning with Early Objects

5. Figure 4-1 Flow of execution examples: (a) sequential, (b) selection, (c) repetition
Java Programming: Guided Learning with Early Objects

6. Relational Operators Express conditions
Make comparisons to make decisions
Logical (Boolean) expression: has a value of either true or false
Relational operator: allows comparisons in a program
Relational operators are binary
Binary operators require two operands
Result of comparison is true or false
Java Programming: Guided Learning with Early Objects

7. Table 4-1 Relational Operators in Java
Java Programming: Guided Learning with Early Objects

8. Relational Operators and Primitive Data Types
Relational operators used with integral and floating-point data types
Expression
Meaning
Value
8 < 15
8 less than 15
True
6 != 6
6 not equal to 6
False
2.5 > 5.8
2.5 greater than 5.8
5.9 <= 7.5
5.9 less than or equal to 7.5
true
Java Programming: Guided Learning with Early Objects

9. Comparing Floating-Point Numbers for Equality
Check absolute value of difference of two floating-point numbers
Difference less than given tolerance
Use method abs from class Math
Example: Math.abs(x – y) <
Java Programming: Guided Learning with Early Objects

10. Comparing Characters
Expression using relational operators evaluates to true or false based on collating sequence
Example: ‘R’ > ‘T’
Java Programming: Guided Learning with Early Objects

11. Table 4-2 Evaluating Expressions Using Relational Operators and the Unicode (ASCII) Collating Sequence
Java Programming: Guided Learning with Early Objects

12. Comparing Strings Strings compared character by character
Comparison continues until:
Mismatch found
Last characters compared and are equal
One string exhausted
Shorter string less than larger string if comparison equal through shorter string
Use method compareTo of class String
Java Programming: Guided Learning with Early Objects

13. Strings, the Assignment Operator, and the Operator new
Example 1:
String str1 = “Hello”;
String str2 = “Hello”;
(str1 == str2) evaluates to true
str1.equals(str2) evaluates to true
Example 2:
String str3 = new String(“Hello”);
String str4 = new String(“Hello”);
(str3 == str4) evaluates to false
str3.equals(str4) evaluates to true
Java Programming: Guided Learning with Early Objects

14. Figure 4-3 Variables str1, str2, and the objects they point to
Java Programming: Guided Learning with Early Objects

15. Figure 4-4 Variables str3, str4, and the objects they point to
Java Programming: Guided Learning with Early Objects

16. Wrapper Classes (Revisited)
Use method compareTo to compare values of two Integer objects
Use method equals to compare values of two Integer objects for equality
Relational operators compare values of Integer and Double objects
Using autoboxing and auto-unboxing
Assignment operator always uses operator new to create Double object
Java Programming: Guided Learning with Early Objects

17. Logical (Boolean) Operators and Logical Expressions
Logical (Boolean) operators enable you to combine logical expressions
Logical operators take logical values as operands
Binary operators: && and ||
Unary operator: !
Java Programming: Guided Learning with Early Objects

18. Table 4-6 && (and) Operator
Table 4-7 || (or) Operator
Java Programming: Guided Learning with Early Objects

19. Order of Precedence
Expression might contain arithmetic, relational, and logical operators
Relational and logical operators evaluated left to right
Left-to-right associativity
Parentheses clarify meaning, override precedence of operators
Java Programming: Guided Learning with Early Objects

20. Table 4-8 Precedence of Operators
Java Programming: Guided Learning with Early Objects

21. Short-Circuit Evaluation
Logical expressions evaluated using efficient algorithm
Short-circuit evaluation:
Logical expression evaluated left to right
Stops when value of entire expression known
Java Programming: Guided Learning with Early Objects

22. boolean Data Type and Logical (Boolean) Expressions
boolean data type has values true and false
Logical expressions manipulated using boolean data type
boolean, true, false are reserved words
Java Programming: Guided Learning with Early Objects

23. Selection: if and if…else
Logical values permit programs to incorporate decision-making
Alters processing flow
Three selection or branch control structures: if
if…else
switch
Java Programming: Guided Learning with Early Objects

24. One-Way Selection One-way selection syntax: if (logical expression)
statement
logical expression also called condition
Determines whether to execute statement
If expression true, statement executes
statement also called action statement
Java Programming: Guided Learning with Early Objects

25. Figure 4-5 One-way selection
Java Programming: Guided Learning with Early Objects

26. Two-Way Selection May need to choose between two alternatives
Java provides if…else statement
Two-way selection syntax:
if (logical expression)
statement1
else
statement2
If logical expression is true, statement1 executes
Otherwise statement2 executes
Java Programming: Guided Learning with Early Objects

27. Figure 4-7 Two-way selection
Java Programming: Guided Learning with Early Objects

28. Compound (Block of) Statements
if and if…else structures select one statement at a time
Compound statement (block) permits more complex statements
Syntax:
{ statement1
statement2 …
statementn }
Java Programming: Guided Learning with Early Objects

29. Multiple Selections: Nested if
Some problems require implementation of more than two alternatives
Include multiple selection paths using nested control statements
Java associates else with the most recent incomplete if
Java Programming: Guided Learning with Early Objects

30. Comparing if…else Statements with a Series of if Statements
a. if (month == 1)
System.out.println(“January”);
else if (month == 2)
System.out.println(“February”);
else if (month == 3)
System.out.println(“March”);
b. if (month == 1)
if (month == 2)
if (month == 3)
Example a executes faster than b
Java Programming: Guided Learning with Early Objects

31. Conditional Operator (? :) (Optional)
Conditional operator written as ?:
Ternary operator
Syntax:
expr1 ? expr2 : expr3
Performs an evaluation and returns a value
Example:
max = (a >= b) ? a : b;
Java Programming: Guided Learning with Early Objects

32. switch Structures
switch structure does not require evaluation of logical expression:
switch (expr) {
case val1:
statements1
break;
case val2:
statements2
default:
statements }
Java Programming: Guided Learning with Early Objects

33. Figure 4-8 switch statement
Java Programming: Guided Learning with Early Objects

34. switch Structures (continued)
switch statement executes according to value of expression matched against a case label
Statements execute until break statement or end of structure
If value of expression does not match case labels, default label executes
If no default statements, structure skipped
break statement causes exit from switch structure
If no break statement, execution continues
Java Programming: Guided Learning with Early Objects

35. Choosing Between an if…else and a switch Structure
No fixed rules to decide whether to use if…else or switch
If multiple selections involve a range of values:
Use either if…else or switch structure
Convert each range to a distinct value
If range of values is large and cannot be reduced:
Use if..else structure
Java Programming: Guided Learning with Early Objects

36. Avoiding Bugs by Avoiding Partially Understood Concepts and Techniques
Not sufficient to be mostly correct in use of concepts and techniques
Many ways to solve a problem
Must make correct use of concepts and techniques
Do not use a concept or technique until understanding is complete
Learn concepts one at a time in a logical order
Take time to understand each one
Java Programming: Guided Learning with Early Objects

37. Thermometer Class (Revisited)
class Thermometer provides current, maximum, minimum temperature for the day
Before method definition, include comments specifying preconditions and postconditions
Precondition: statement specifying conditions that must be true before method called
Postcondition: statement specifying what is true after method call completed
Java Programming: Guided Learning with Early Objects

38. GUI JTextFields and JPanels (Optional)
Display all input and output
Text fields: white areas to the right of labels
After entering the radius
User clicks Calculate button
Program calculates and displays the area and circumference
User clicks Exit button, program terminates
Java Programming: Guided Learning with Early Objects

39. Figure 4-9 GUI to find and display the circumference and area of a circle
Java Programming: Guided Learning with Early Objects

40. JTextField Text field objects belong to class JTextField
Instantiate an object of type JTextField
JTextField used for both input and output
Complete the design:
Create three labels
Create two buttons Calculate and Exit
Place labels, text fields, buttons in content pane
Set title of window, height, width
Make window visible
Java Programming: Guided Learning with Early Objects

41. Handling the Events Clicking buttons will generate an event
Must write code to handle each event and register listeners
class CalculateButtonHandler and class ExitButtonHandler inner classes
Inside class CircleProgram
Method main instantiates single object of class CircleProgram
Java Programming: Guided Learning with Early Objects

42. Figure 4-10 Sample run for the final CircleProgram
Java Programming: Guided Learning with Early Objects

43. JPanel Sometimes want to vary the number of components in each row
Set layout of container to null
Specify position of components
Use class JPanel
JPanel objects are containers
Create panels and subpanels
Specify layout of each panel
Subpanels added to panel below
Panels added to content pane
Java Programming: Guided Learning with Early Objects

44. Handling the Events Clicking buttons generates action event
One inner class handles all three events
public void actionPerformed(ActionEvent e)
Parameter e identifies source of event
Use method getActionCommand of class ActionEvent
Returns command associated with action
String str = e.getActionCommand();
Java Programming: Guided Learning with Early Objects

45. Figure 4-12 Graphical user interface after entering the text
Figure 4-13 Graphical user interface after clicking the three buttons
Java Programming: Guided Learning with Early Objects

46. Summary Program statements processed in four ways:
Sequentially
Selection (branching)
Repetition (looping)
Method calls
Logical expression has value of true or false
Also called Boolean expression
Relational operator makes comparisons
Floating-point numbers compared, given tolerance
Java Programming: Guided Learning with Early Objects

47. Summary (continued) char values compared using collating sequence
Strings compared character by character
class String provides compareTo method
Strings instantiated with assignment statement
Strings instantiated with new operator
Integers compared for equality using:
equals method
compareTo
Java Programming: Guided Learning with Early Objects

48. Summary (continued)
Assignment operator always uses operator new to create Double object
Logical (Boolean) operators allow combining logical expressions
Relational and logical operators left-to-right associative
Java logical expressions use short-circuit evaluation
One-way selection: if
Java Programming: Guided Learning with Early Objects

49. Summary (continued) Two-way selection: if…else
Compound statements allow more than one statement to execute in selection structure
Selection structures may be nested to allow multiple selection
Conditional operator more concise form of if…else statement
switch structure does not require evaluation of logical expression
Java Programming: Guided Learning with Early Objects

50. Summary (continued) Learn concepts one at a time in logical order
JTextField used for input and output
JPanel objects are containers
Create a hierarchy of JPanels
One handler can handle multiple events
Use parameter e of type ActionEvent
Use method getActionCommand of class ActionEvent
Java Programming: Guided Learning with Early Objects