SELECTIONS - Introduction to Java Programming - page 82

3.1 Introduction
The program can decide which statements to execute based on a condition.

Key
Point

If you enter a negative value for radius in Listing 2.2, ComputeAreaWithConsoleInput.java,
the program displays an invalid result. If the radius is negative, you don't want the program to
compute the area. How can you deal with this situation?
Like all high-level programming languages, Java provides selection statements : statements
that let you choose actions with two or more alternative courses. You can use the following
selection statement to replace lines 12-17 in Listing 2.2:

problem

selection statements

if (radius < 0 ) {
System.out.println( "Incorrect input" );

}
else {
area = radius * radius * 3.14159 ;
System.out.println( "Area is " + area);

}

Selection statements use conditions that are Boolean expressions. A Boolean expression is
an expression that evaluates to a Boolean value : true or false . We now introduce Boolean
types and comparison operators.

Boolean expression
Boolean value

3.2 boolean Data Type
A boolean data type declares a variable with the value either true or false .

Key
Point

How do you compare two values, such as whether a radius is greater than 0 , equal to 0 , or less
than 0 ? Java provides six comparison operators (also known as relational operators ), shown
in Table 3.1, which can be used to compare two values (assume radius is 5 in the table).

boolean data type
comparison operators

T ABLE 3.1 Comparison Operators

Java Operator Mathematics Symbol Name Example (radius is 5) Result

less than

< radius < 0 false

<

- less than or equal to

<= radius <= 0 false

greater than

> > radius > 0 true

- greater than or equal to radius >= 0

>= true

equal to

== = radius == 0 false

/ not equal to

!= radius != 0 true

Note
You can also compare characters. Comparing characters is the same as comparing their
Unicodes. For example, a is larger than A because the Unicode of a is larger than the
Unicode of A . See Appendix B, The ASCII Character Set, to find the order of characters.

compare characters

Caution
The equality comparison operator is two equal signs ( == ), not a single equal sign ( = ).
The latter symbol is for assignment.

== vs. =

The result of the comparison is a Boolean value: true or false . For example, the follow-
ing statement displays true :

double radius = 1 ;
System.out.println(radius > 0 );