Simple Input from the Keyboard

The class Scanner

A Java program can read data from either the keyboard or another source, such as a disk, and place it into memory. The programs in this course will use the keyboard as their input device.

The Java Class Library provides the class Scanner that we read data typed at the keyboard and place it into variables that we specify.

The Java Class Library is organized into units called packages. The class Scanner, for example, is in the package java.util. To use Scanner in our program, we must tell the compiler where to find the class. One way to do so is to write the class’s name as java.util.Scanner each time we need to use it. This fully qualified name is tedious to write over and over again, so Java provides another way. We can write the following import statement before the rest of our program:

import java.util.Scanner;

This statement tells the compiler to look in the package java.util for the class Scanner. Then, anytime we write Scanner in our program, the compiler will understand it as java.util.Scanner. As we introduce other classes in the Java Class Library, we will show you the appropriate import statement to use.

📝 Syntax: The import statement

To use a class from the Java Class Library, you can write an import statement before the rest of the program, including its initial comments. This statement has the following form:

Assuming that a statement such as:

import package-name.class-name

It allows you to use just the class name within the program instead of its fully qualified name, package-name.class-name. Although you can avoid import statements by always using fully qualified class names, most programmers do use import statements.

Scanner objects

Before we can use any of the methods in Scanner, we must create a Scanner object by writing a statement such as

Scanner keyboard = new Scanner(System.in);

This statement assigns the Scanner object associated with the input device that System.in represents to the variable keyboard—which could have any name of our choosing. This input device, by convention, is the keyboard.

Scanner methods

Scanner provides several methods that read input data. We can use any of these methods by writing a statement that has the following form, where keyboard is the Scanner object that we defined previously:

variable = keyboard.method_name();

The named method reads a value from the keyboard and returns it. That is the expression keyboard.method_name() represents the value that was read. The previous statement then assigns this value to the indicated variable.

Data, like Java statements, can contain white space. Any cluster of symbols other than white space is called a token. A Scanner object identifies these tokens and converts them into values of various data types according to the Scanner method we use.