Decision Making and Repetition with Reusable Objects - Java Programming: Comprehensive Concepts and Techniques - page 236

Table 4-5 shows the equality, relational, and logical operators used in Java,
including examples of each and the result of evaluating the condition.

Table 4-5 Operator Results in Selection Structures

OPERATOR MEANING EXAMPLE RESULT TYPE

= = equal to 2 = = 2 true equality

1 = = 6 false

!= not equal to 7 ! = 4 true equality

4 ! = 4 false

< less than 3 < 5 true relational

5 < 3 false

<= less than or equal to 4 < = 6 true relational

7 < = 6 false

> greater than 9 > 7 true relational

7 > 9 false

>= greater than or 8 > = 8 true relational

equal to 8 > = 10 false

&& logical AND (both (7 > 3) && (0 < 1) true logical

conditions must be (7 > 3) && (1 < 0) false

true in order to make
the condition true)

| | logical OR (one of the (7 > 3) | | (1 < 0) true logical

conditions must be (3 > 7) | | (1 < 0) false

true in order to make
the condition true)

! logical NOT (condition ! (5 = = 4) true logical

must evaluate to false ! (a = = a) false

in order to make the
condition true)

Coding an if Statement to Test the Cancel Button

As shown in the previous steps, when executed, the getSales() method
displays a dialog box prompting the user to enter a dollar amount. The dialog
box contains a text box for user input and two buttons: an OK button and a
Cancel button. If the user clicks the Cancel button, the program should termi-
nate. If the user clicks the OK button, the method validates the user input.
An if statement, as shown in line 33 of Figure 4-13, is used to test if the user
clicked the Cancel button. When clicked, the Cancel button in a JOptionPane
dialog box returns a blank value to the variable, answer. Line 33 uses the equal to
operator (= =) to compare the variable, answer, against a null. In Java, null is a
constant that represents the presence of no data. If the return value matches, or
is equal to, null, the condition is evaluated to be true and the finish() method is
called.