A.4. Interacting with the User

Accepting arguments from the command line is helpful, but our programs would be very limited if we had no other way to interact with the user. We also need to be able to print text to the screen and read input from the keyboard.

Printing text is accomplished with the System.out.println() method, as used previously. There is a related method System.out.print(), which does not move on to the next line.

Before Java 1.5, reading from the keyboard was fantastically awkward. The introduction of the new Scanner class makes it reasonable. Before reading from the keyboard, a program must create a Scanner with the following incantation:

java.util.Scanner input = new java.util.Scanner(System.in);

This may seem somewhat cryptic at this point, but we have seen this kind of statement already. This is a declaration and initialization of a variable input. The type of the variable is java.util.Scanner, which will make more sense after we discuss packages (Section 3.3). The part to the right of the equals sign gives input its initial value. Specifically, it is the invocation of a constructor (Section 1.2). It doesn't matter that we don't understand these things just yet.

Now that we have input, we can read from the keyboard. If we store the next line typed by the user in a variable answer, we do this:

String answer = input.nextLine();

We can also ask for input.nextInt() to get an int from the user, input.nextDouble() to get the next double, and so on. A technical detail is that these methods only "use up" the number in question, not the entire line of input. If we want to go on to ask the user for non-numeric input, we should invoke nextLine() to clear out the rest of the line. In a program that only asks for numbers, this can be ignored. We will do more elaborate things with the Scanner class over the course of the book.

Exercises