Programming and Problem Solving with Java

(^130) | Arithmetic Expressions

Converting Strings to Numeric Values

Many of the problems that we use computers to solve involve the entry of numerical

data—that is, values of the typesint,long,float, anddouble.ABufferedReaderobject enables

us to enter aString. How, then, do we input a number? The answer is that we can’t—at least

not directly. Java provides only for the input of strings. We must enter a number as a string

and then convert the string into one of the numeric types using methods from Java’s

library.

Java’s standard library includes a set of classes that correspond to the built-in numeric

types. These classes provide methods and constants that are useful in working with the

numeric types. Like the Mathclass, they are auto-

matically imported to every Java class. Table 3.2

lists these predefined classes and the built-in type

to which each one corresponds.

As you can see, the general rule is that the

class name is the same as the name of the built-

in type except that its first letter is capitalized.

The lone exception is that the class correspon-

ding to intis called Integer.

Among the methods associated with each

of these classes is one that takes a string as its

argument and returns a value of the correspon-

ding type. Table 3.3 shows the relevant methods.

For example, we can write

number = Double.parseDouble("–435.82E27");

to convert"–435.82E27"into a value of typedouble

and store it innumber. Of course, what we really

want to do is convert an input string into a num-

ber. We can replace the string"–435.82E27"in the

preceding statement with a call to thereadLinemethod associated with aBufferedReader:

number = Double.parseDouble(in.readLine());

Let’s look at an example of inputting an integer value:

int intNumber;

intNumber = Integer.parseInt(in.readLine());

We now have a single statement that reads a numerical value from the screen. What if

the user types something other than a number? At this point in our knowledge of Java, the

result is that the application halts and displays a message such as “Number Format Error.”

In Chapter 9, we will see how an application can catch such an error (another example of an

exception) and respond to it without stopping.

Built-in Type Object Type

int Integer

long Long

float Float

double Double

Table 3.2 Predefined Classes Corresponding to Built-in Numeric Types

Object Type Method Argument Returns

Integer parseInt String int

Long parseLong String long

Float parseFloat String float

Double parseDouble String double

Table 3.3 String-to-Numeric Type Conversion Methods

T

E

A

M

F

L

Y

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