Getting input from the user

Getting input from the user in Java requires us to get familiar with a built-in Java class called Scanner, which essentially allows us, programmers, to process user input from the keyboard. Similar to the System class, in which we used the println operation (in programming terms, println is a method) to display a statement to the screen, there are several methods in Scanner to input data of different types. Before we can refer to Scanner in our program, we have to have an import statement to tell the compiler where the Scanner class is located among the many other Java built-in classes. Import statements go in the very beginning of the program and in this case, it will be as follows:

import java.util.Scanner;

Now we need to create a variable of type Scanner in order to start using it at getting input. Since the variable’s type is a class (not a simple data type like int or double), this variable refers to what Java calls an object. Compared to a variable of type int, an object is a more complex entity and will require special handling. Here, beyond the variable declaration, an additional statement is needed to create the object (i.e. setting aside the memory for it). Let’s begin with the variable declaration:

Scanner input;

Just as before, the type of the variable goes first (in this case it’s the class Scanner), followed by the variable name, which is input in this case. To create a Scanner object, we will need the following statement:

input = new Scanner(System.in);

Ok, so it looks like a new Scanner object is being created using the new operation and it’s going to be referred to by the variable input (the variable input is the object reference). But what about System.in? Let’s think back at output statements first. The System.out prefix (it’s really an object), refers to the computer screen, so the println operation will send whatever is between the parentheses to the screen. In the case for input, System.in must refer to the input device, which is just the keyboard. So essentially, the above Java statement creates a Scanner object that will intercept input from the keyboard using a variable called input. Before we look at operations (or methods) to get input from, it’s helpful to know that we can combine the last 2 statements into one as follows:

Scanner input = new Scanner(System.in);

This will accomplish the declaration and object creation at the same time. Remember it’s possible to do the same with variables of simple types. So instead of having the two statements

int plants;

plants = 16;

we can have the single statement:

int plants = 16;

Ok, now we’re ready to actually do something with our Scanner object. Let’s start with a simple program of asking the user to enter an integer value, the number of books he or she read this year. Whatever value the user enters, it would make sense to create a variable for it since we can expect to do something with its value later. Also, if we’re prompting the user to enter some input, we will need an output statement. So here’s what we have so far:

int books;

System.out.println(“How many books have you read this year? “);

At this point, we would expect the user to enter some integer value on the keyboard. To intercept that value or read it into our program, we will use our Scanner object input. As we already know, a Scanner has different methods to deal with different types. In this case, since we’re expecting an integer value, we will use the method nextInt (interpretation: get me the next value entered on the keyboard as an integer). Here’s the Java statement:

books = input.nextInt();

The equal sign (the assignment operator), will take the value that the user inputs, which is intercepted by the method nextInt, and store it in the variable books. What’s to the right of the assignment operator causes the method nextInt to execute – in programming terms, the method is invoked. The syntax for invoking a method is: objectReference.methodName. If we’d like to see that this worked properly, we can now output the value of the variable:

System.out.println(“You have read “ + books + “ books.”);

A couple minor things about the program. The cursor where the user types in a value is on the line following the prompt. This is because println inserts a ‘new line’ character. To change it, we can use print instead of println. Also, instead of declaring the variable shoes then using it, we could have accomplished everything in one statement:

int books = input.nextInt();

Let’s put everything together in a full program. Everything that comes after the ‘//’ are comments, which are ignored by the compiler.

// The program prompts the user to enter number of books and

// then displays it back to the screen

// import to tell the compiler where Scanner is

import java.util.Scanner;

public class InputTest

{

public static void main(String args[])

{

// Create a Scanner object ‘input’

Scanner input = new Scanner(System.in);

// prompt the user to enter number of books

System.out.print("How many books have you read this year? ");

// store the result in ‘books’

int books = input.nextInt();

// display the value of ‘books’

System.out.println("You have read " + books + " books");

}

}

Let’s try something a little more sophisticated. Instead of displaying the value that the user entered, we will square it first then display the result. For this program, we’re going to need to variable to store the user input, and another variable to store the result. Here’s how the program goes:

// this program will square a number that the user enters

import java.util.Scanner;

public class SquareANumber

{

public static void main(String args[])

{

// Create a Scanner object ‘input’

Scanner input = new Scanner(System.in);

// prompt the user to enter number of shoes

System.out.print("Enter a whole number: ");

// store the result in ‘number’

int number = input.nextInt();

// compute the square and store in 'square'

int square = number * number;

// display the result

System.out.println(number + " squared is " + square + ".");

}

}