Reading Files in Java

The standard Java class library provides several ways to read data from files. Some of them are quite old, others have appeared recently. In this topic, we will consider only two basic methods, which is quite enough for now. You can choose the one that is most suitable for you.

Reading data using Scanner

It is possible to use java.util.Scanner to read data from files. This class is a high-level approach to reading input data. It allows reading primitive types or strings by using regular expressions.

First of all, we need to create an instance of java.io.File and then an instance of Scanner passing the file object. After that, we can get data from the file by using the scanner in the same way as we read from the standard input.

Suppose you have a string called pathToFile. It stores the path to a file that contains a sequence of numbers separated by spaces.

Let's create a file object and then a scanner to read data from the file.

File file = new File(pathToFile);
Scanner scanner = new Scanner(file); // it throws FileNotFoundException (checked)

When you create an instance of Scanner passing a file, you must handle the checked exception FileNotFoundException. You can also declare the method as throwing this exception.

Now, we can use methods of Scanner to read data as strings, integers and so on.

Let's read a line from the file:

while (scanner.hasNext()) {
    System.out.print(scanner.nextLine());
}

This code reads each line from the file and outputs it to the standard output.

After using a scanner, we should close the object to avoid leaks. A convenient way to close scanners and handle exceptions is to use the try-with-resources statement as below.

The full example of Scanner usage is presented below:

File file = new File(pathToFile);
 
try (Scanner scanner = new Scanner(file)) {
    while (scanner.hasNext()) {
        System.out.print(scanner.nextLine() + " ");
    }
} catch (FileNotFoundException e) {
    System.out.println("No file found: " + pathToFile);
}

Then, for a file containing:

first line
second line
third line

The program outputs to the console the following result:

first line second line third line

The scanner also allows you to read integers, boolean, doubles, and other types. The corresponding methods have names like nextInt, nextBoolean, and etc. In case no new data is available, the next methods throw java.util.NoSuchElementException.

Instead of displaying the read data in the standard output, you can store it in an array or a string.

Reading all text from a file as a single string

Since Java 1.7 there has been a set of new classes and methods for handling files. Within this topic, we will confine ourselves to learning how to read an entire text file. Note that this method should be used only for small text files. By small we mean that their size is smaller than the RAM available to the JVM. That's more than enough for learning and performing small tasks.

First of all, make the following imports:

import java.nio.file.Files;
import java.nio.file.Paths;

The Files class consists of methods that operate on files, the Paths class contains a set of methods that return a special object to represent the path to a file.

The following method returns all text from a specified file:

public static String readFileAsString(String fileName) throws IOException {
    return new String(Files.readAllBytes(Paths.get(fileName)));
}

Let's try to use the readFileAsString method to read the source code from the HelloWorld.java file and print it to the standard output. HelloWorld.java contains a traditional basic program mentioned in one of the earliest topics "The first program".

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ReadingFileDemo {
    public static String readFileAsString(String fileName) throws IOException {
        return new String(Files.readAllBytes(Paths.get(fileName)));
    }

    public static void main(String[] args) {
        String pathToHelloWorldJava = "/home/username/Projects/hello-world/HelloWorld.java";
        try {
            System.out.println(readFileAsString(pathToHelloWorldJava));
        } catch (IOException e) {
            System.out.println("Cannot read file: " + e.getMessage());
        }
    }
}

It prints the source code:

package org.hyperskill;

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }
}

Note that it is not difficult to modify the code above so that it would read the path to the target file from the standard input instead of hardcoding it.

Conclusion

The File class provides a very clear API to process files and directories on different platforms. We have covered two methods of reading files in Java. The method using java.util.Scanner is convenient for reading various data types from a file. When reading using Scanner, you need to handle the FileNotFoundException exception or declare it. It is recommended to close the Scanner after use with try-with-resources. The method using the Files class and Paths class provides the ability to read the entire text from a file as a single string. Reading the entire text from a file as a single string is suitable for small files. These methods provide convenient tools for working with files in Java.