Programming and Problem Solving with Java

(^70) | Java Syntax and Semantics, Classes, and Objects
In contrast,readLineis called from within an expression. Java supports two kinds of
methods: value-returning methodsand void methods. They are distinguished by how they
are called.readLine, a value-returning method, is called within an expression. When
it returns, a value that it has computed takes its place in the expression and can be
used for assignment or further computation.println, a void method, doesn’t re-
turn a value. We call it as a separate statement and, when it has finished, execution
picks up with the statement that follows it.

Interactive Input and Output

When an application inputs and outputs values that a human user supplies and

reads, it performs interactive input/output. To make the application more user-

friendly, the programmer has to consider additional information that the user

needs beyond the raw input and output. For example, the user must be prompted to enter

values; otherwise, he or she won’t know when or where to type the input. When the appli-

cation outputs its results, it must label them so that they are meaningful. Labeling output

also helps the user to distinguish among different values displayed by the application.

The following code segment inputs the three parts of a name, using System.out.printto

display a prompting message for each input value:

System.out.print(“Enter first name: “); // Prompt for first name

first = in.readLine(); // Get first name

System.out.print(“Enter last name: “); // Prompt for last name

last = in.readLine(); // Get last name

System.out.print(“Enter middle initial: “); // Prompt for middle initial

middle = in.readLine(); // Get middle initial

The next code segment outputs the name with System.out.println, using a label to in-

dicate what is displayed:

System.out.println(“You entered the name: “+

first + “ “+ middle + “. “+ last);

When an application outputs data that has just been entered, it confirms to the user that the

data was typed correctly or indicates that an error occurred. This kind of input–output se-

quence is known as echo printing.

2.2 Application Construction

So far in this chapter, we have looked at the basic elements of Java code: identifiers, decla-

rations, variables, constants, expressions, assignment statements, method calls, input, out-

put, and comments. Now let’s see how to collect these elements into an application. A Java

application is a class containing class declarations: fields and methods. For the class to be a

Java application, one of the methods must be namedmain. Execution of the Java application

begins in themainmethod.

Value-returning method A

method that is called from

within an expression and

returns a value that can be used

in the expression

Void method A method that is

called as a separate statement;

when it returns, processing con-

tinues with the next statement

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