Section 11.6 Writing Files
Plain text files can be written as well as read using Java. The use of only one Java class is required to write a plain text file, the FileWriter class. The FileWriter must be closed after it is open, and it implements the AutoClosable interface. Consequently, it is best to open it using a try-with-resources so that we allow Java to close the file for us.
There is an important option to be aware of when opening a file to write using the FileWriter class. That is, where in the file should writing begin? While it is not uncommon to read an entire file all at once, files are often written incrementally, a small amount at a time. For example, you might open a file and write a few lines and then close it. Open it again later in your program and close it again. The implication here is that writing will begin at the end of the file. If we begin writing from the beginning each time, then the file will contain only the last data written because the file content is being overwritten each time.
When intending to append to the end of a file instead of writing from the beginning, we must be careful to open a file using a FileWriter in append mode. By default, the FileWriter opens in write mode, which starts writing from the beginning of a file. This may or may not be what you want. As a demonstration of FileWriter, let’s write a utility function that appends a message to a log file.
The FileWriter class is in java.io.FileWriter and it has a constructor that take a file name String to be opened. If the file does not exist on your file system, FileWriter will create it for you. If it does exist, the existing file will be opened for writing.
The following code snippet demonstrates how to create two FileWriter objects using a file name String. The first
fw1 variable references a FileWriter object that will write from the beginning of the file, overwriting anything that is in the file currently, and the second fw2 variable references a FileWriter object that will append to the file by starting to write new data at the end of the file. Note that the only difference between these two constructors is the addition of a true argument after the file name when intending to append to the file.// Open a file named log.txt and start writing from the beginning of the file
FileWriter fw1 = new FileWriter("log.txt");
// Open a file named log.txt and write by appending to the end of the file
FileWriter fw2 = new FileWriter("log.txt", true);
Because FileWriter throws the checked IOException, the object must be instantiated in a try-catch. Also, because the FileWriter must be closed to avoid a resource leak, and it implements the
AutoCLosable interface, we use a try-with-resources so that Java takes care of closing the FileWriter for us. Beyond these two items, the only additional thing to know is that the FileWriter object uses its write(…) method to write to a file.In Listing 11.6.1 we demonstrate the use of the FileWriter class by defining a utility method named
log(…) which takes a message String parameter and appends it to a file named "log.txt". This static method can very useful when you need to monitor a long running program that you cannot monitor continuously.The implementation of
log(…) in Listing 11.6.1 includes all the required components: the IOException thrown by a FileWriter is caught, and the FileWriter object is instantiated in a try-with-resources to ensure it is closed.// WriteFile1.java
import java.io.FileWriter; // Import classes
import java.io.IOException;
public class WriteFile1 {
public static void main(String[] args) {
log("First message"); // Log a message
log("Second message"); // Log another message
log("Third message"); // And another message
}
// Utility method to log messages to a file
public static void log(String msg) { // Open a file to append
try (FileWriter fw = new FileWriter("log.txt", true)) {
fw.write(msg + "\n"); // Write added to end of file
}
catch (IOException e) { // Log // Handle exceptions
System.out.println(e);
}
}
}
WriteFile1.javajavac WriteFile1.java java WriteFile1 type log.txt First message Second message Third message
