Reading From a File Using Scanners

Note

Above, you’ll see there’s a video titled CSE 121: Reading From a File & Scanner Methods and Processing Tokens Video Walkthrough. The video and the reading both have the same information! You’re not required to go through both the video and the reading, as the video just walks through the reading to help contextualize it!

So far in this course, we have been limited as to how we can take input from the user into our program; we recently learned how to take user input from the console using Scanners. What if we want to take input from a more standard, consistent source, such as a file?

By using files, we can store and use data that outlives the run time of our program. It also allows us to manipulate data that isn’t given to us by the user through the console at runtime, giving our programs much more flexibility in the kinds of tasks that they can do. As it turns out we can also use Scanners and all their associated methods to read data from a file!

Files and File Scanners in Java

Let’s first take a look back at how we made Scanners to take input from the console:

Scanner console = new Scanner(System.in);

We pass System.in as a parameter to our new Scanner() call in order to let the Scanner know it’s reading from the console. So it would make sense if we could pass our file into the Scanner to let it know it’s reading from said file. It turns out this is exactly what you have to do! We can’t just pass the name of the file into our Scanner though, as this doesn’t explicitly tell the Scanner we are processing a file (it also turns out you can make Scanners over a String, which we will cover later in this lesson). In order to make the distinction we need to make a new variable of the datatype File. The syntax for making a File is similar to that of the Scanner, but with the word Scanner replaced with File, and the System.in argument replaced with the name of our file, in this case the file we will be reading from is called "newFile.txt".

File newFile = new File("newFile.txt");

It turns out we have to include a different import statement to access the code for the File object! When using Files we have to include the import java.io.*; statement as the File object is contained in Java’s IO package, which stands for Input Output.

Caution

We have to remember to import Java’s util package when using Scanners to access their code using the import java.util.*; statement.

Notice that we pass the filename as a String into the new File() call. Now that we have a File, we can make the Scanner to process input from the file by passing the File as the argument to the new Scanner() call:

File newFile = new File("newFile.txt");
Scanner fileScan = new Scanner(newFile);

Caution

It’s very likely that a program will contain more than one type of Scanner. Keeping track of which Scanner does what can be difficult if your naming scheme is not consistent and descriptive. Try and use the same names for each type of Scanner across all programs that you write to minimize the confusion, and make sure their names clearly show the type of input data that the Scanner is processing. console, and fileScan are both good names for Scanners.

Now that we know how to setup our File and Scanner objects, let’s take a look at putting it all together in code on the next page.