The Java File class provides a very clear API to process files and directories on different platforms. Java offers different ways to write text to a file, and in this lesson, we will consider two of the simplest ways: using the java.io.FileWriter and the java.io.PrintWriter classes.

The FileWriter class

The Java FileWriter class has a set of constructors to write characters and strings to a specified file:

FileWriter(String fileName);
FileWriter(String fileName, boolean append);
FileWriter(File file);
FileWriter(File file, boolean append);

Two constructors take an additional boolean parameter append, which indicates whether to append (true) or overwrite (false) an existing file.

All these constructors can throw an IOException for several reasons:

if the named file exists but it is a directory;
if a file does not exist and cannot be created;
if a file exists but cannot be opened.

In this lesson, sometimes we will skip the exception handling mechanism to simplify our examples.

Let's consider the following code snippet:

File file = new File("/home/username/path/to/your/file.txt");
FileWriter writer = new FileWriter(file); // overwrites the file

writer.write("Hello");
writer.write("Java");

writer.close();

If the specified file does not exist, it will be created after executing this code. If the file already exists, this code overwrites the data.

The file will contain the text "HelloJava".

If you want to append some new data, you should specify the second argument as true.

File file = new File("/home/username/path/to/your/file.txt");
FileWriter writer = new FileWriter(file, true); // appends text to the file

writer.write("Hello, World\n");
writer.close();

This code appends a new line to the file. Run it multiple times to see what happens. Note that here we are using Unix-like OS line breaks. There is a difference between line break characters on different platforms:

\n Unix-like OS
\r\n Windows OS

Closing a FileWriter

It is important to close a FileWriter after using it to avoid a resource leak. This is done by invoking the close method:

writer.close();

Since Java 7, a convenient way to close an object of FileWriter is to use the try-with-resources construct.

File file = new File("/home/username/path/to/your/file.txt");

try (FileWriter writer = new FileWriter(file)) {
    writer.write("Hello, World");
} catch (IOException e) {
    System.out.printf("An exception occurred %s", e.getMessage());
}

It will close the writer automatically.

The PrintWriter class

The PrintWriter class in Java allows you to write formatted data to a file. It can output strings, primitive types and even an array of characters. The class provides several overloaded methods: print, println and printf.

File file = new File("/home/art/Documents/file.txt");
try (PrintWriter printWriter = new PrintWriter(file)) {
    printWriter.print("Hello"); // prints a string
    printWriter.println("Java"); // prints a string and then terminates the line
    printWriter.println(123); // prints a number
    printWriter.printf("You have %d %s", 400, "gold coins"); // prints a formatted string
} catch (IOException e) {
    System.out.printf("An exception occurred %s", e.getMessage());
}

This example first creates an instance of File and second, a PrintWriter in the try-with-resources statement to close it correctly. It writes "Hello" and "Java" on the same line, and then 123 on a new line. This example also calls the advanced printf method which can format a text using %d, %s and so on. Finally, the PrintWriter is closed.

The result contains:

HelloJava
123
You have 400 gold coins

The class has several constructors. Some of them are similar to constructors of FileWriter:

PrintWriter(String fileName);
PrintWriter(File file).

Others allow to pass FileWriter as a class that extends the Writer abstract class:

PrintWriter(Writer writer).

Conclusion

In Java, both FileWriter and PrintWriter extend the Writer abstract class and have many similarities. However, PrintWriter is a more versatile choice as it provides several useful methods. Among them are formatting methods and overloaded print methods for writing primitive types, and additional options for more flexible output handling.