Reading and Writing Files

2017-11-03 09:05:01

You've already learned how to chain an OutputStreamWriter to a FileOutputStream and an InputStreamReader to a FileInputStream. Although this isn't hard, Java provides two simple utility classes that take care of the details, java.io.FileWriter and java.io.FileReader .

20.8.1. FileWriter

The FileWriter class is a subclass of OutputStreamWriter that writes text files using the platform's default character encoding and buffer size. If you need to change these values, construct an OutputStreamWriter on a FileOutputStream instead.

public class FileWriter extends OutputStreamWriter

This class has four constructors:

public FileWriter(String fileName) throws IOException public FileWriter(String fileName, boolean append) throws IOException public FileWriter(File file) throws IOException public FileWriter(FileDescriptor fd)

The first constructor opens a file and positions the file pointer at the beginning of the file. Any text in the file is overwritten. For example:

FileWriter fw = new FileWriter("36.html");

The second constructor allows you to specify that new text is appended to the existing contents of the file rather than overwriting it by setting the second argument to TRue. For example:

FileWriter fw = new FileWriter("36.html", true);

The third and fourth constructors use a File object and a FileDescriptor, respectively, instead of a filename to identify the file to be written to. Any preexisting contents in a file so opened are overwritten.

You use the standard Writer methods like write( ), flush( ), and close( ) to write the text in the file.

20.8.2. FileReader

The FileReader class is a subclass of InputStreamReader that reads text files using the platform's default character encoding. If you need to change the encoding, construct an InputStreamReader chained to a FileInputStream instead.

public class FileReader extends InputStreamReader

This class has three constructors that differ only in how the file to be read is specified:

public FileReader(String fileName) throws FileNotFoundException public FileReader(File file) throws FileNotFoundException public FileReader(FileDescriptor fd)

Only the constructors are declared in this class. You use the standard Reader methods like read( ), ready( ), and close( ) to read the text in the file.

FileReader and FileWriter always use the local default encoding for converting characters to and from bytes. This is rarely what you want. You should almost always specify the encoding explicitly, or perhaps autodetect it. FileReader and FileWriter are a minor convenience at most. Instead of using them, you can easily chain an InputStreamReader to a FileInputStream or an OutputStreamWriter to a FileOutputStream. Even if you know you want the default encoding, you're better off requesting it explicitly to make your intention clear.

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