File I/O

    FileInputStream in = new FileInputStream( "/etc/passwd" );

try ( FileInputStream fin = new FileInputStream( "/etc/passwd" ) ) {
    ....
    // Fin will be closed automatically if needed upon exiting the try clause.
}

When the stream is first created, its available() method and the File object’s length() method should return the same value.

To read characters from a file as a Reader, you can wrap an InputStreamReader around a FileInputStream. If you want to use the default character-encoding scheme for the platform, you can use the FileReader class instead, which is provided as a convenience. FileReader is just a FileInputStream wrapped in an InputStreamReader with some defaults. For some crazy reason, you can’t specify a character encoding for the FileReader to use, so it’s probably best to ignore it and use InputStreamReader with FileInputStream.

The following class, ListIt , is a small utility that sends the contents of a file or directory to standard output:

    //file: ListIt.java
    import java.io.*;

    class ListIt {
        public static void main ( String args[] ) throws Exception {
            File file =  new File( args[0] );

            if ( !file.exists() || !file.canRead() ) {
                System.out.println( "Can't read " + file );
                return;
            }

            if ( file.isDirectory() ) {
                String [] files = file.list();
                for ( String file : files )
                    System.out.println( file );
            } else
                try {
                    Reader ir = new InputStreamReader( 
                        new FileInputStream( file ) );

                    BufferedReader in = new BufferedReader( ir );
                    String line;
                    while ((line = in.readLine()) != null)
                        System.out.println(line);
                }
                catch ( FileNotFoundException e ) {
                    System.out.println( "File Disappeared" );
                }
        }
    }

ListIt constructs a File object from its first command-line argument and tests the File to see whether it exists and is readable. If the File is a directory, ListIt outputs the names of the files in the directory. Otherwise, ListIt reads and outputs the file, line by line.

For writing files, you can create a FileOutputStream from a String pathname or a File object. Unlike FileInputStream, however, the FileOutputStream constructors don’t throw a FileNotFoundException. If the specified file doesn’t exist, the FileOutputStream creates the file. The FileOutputStream constructors can throw an IOException if some other I/O error occurs, so you still need to handle this exception.

If the specified file does exist, the FileOutputStream opens it for writing. When you subsequently call the write() method, the new data overwrites the current contents of the file. If you need to append data to an existing file, you can use a form of the constructor that accepts a Boolean append flag:

    FileInputStream fooOut =
        new FileOutputStream( fooFile ); // overwrite fooFile
    FileInputStream pwdOut =
        new FileOutputStream( "/etc/passwd", true ); // append

Another way to append data to files is with RandomAccessFile, which we’ll discuss shortly.

    String s = new BufferedReader(
        new InputStreamReader( System.in ) ).readLine();
    File out = new File( "/tmp/foo.txt" );
    FileWriter fw = new FileWriter ( out );
    PrintWriter pw = new PrintWriter( fw )
    pw.println( s );pw.close();

    try {
        RandomAccessFile users = new RandomAccessFile( "Users", "rw" )
     } catch (IOException e) { ... }

    users.seek( userNum * RECORDSIZE );
    users.writeUTF( userName );
    users.writeInt( userID );
    ...

In this naive example, we assume that the String length for userName, along with any data that comes after it, fits within the specified record size.

    URL resource = MyApplication.class.getResource("/config/config.xml");

Instead of constructing a File reference to an absolute file path, or relying on composing information about an install directory, the getResource() method provides a standard way to get resources relative to the classpath of the application. A resource can be located either relative to a given class file or to the overall system classpath. getResource() uses the classloader that loads the application’s class files to load the data. This means that no matter where the application classes reside—a web server, the local filesystem, or even inside a JAR file or other archive—we can load resources packaged with those classes consistently.

    URL data = AnyClass.getResource("/config/config.xml");

On the other hand, a relative URL does not begin with a slash (for example, mydata.txt). In this case, the search begins at the location of the class file on which getResource() is called. In other words, the path is relative to the package of the target class file. For example, if the class file foo.bar.MyClass is located at the path foo/bar/MyClass.class in some directory or JAR of the classpath and the file mydata.txt is in the same directory (foo/bar/mydata.txt), we can request the file via MyClass with:

    URL data = MyClass.getResource("mydata.txt");

In this case, the class and file come from the same logical directory. We say logical because the search is not limited to the classpath element from which the class was loaded. Instead, the same relative path is searched in each element of the classpath—just as with an absolute path—until it is found. Although we’d expect the file mydata.txt to be packaged physically with MyClass.class, it might be found in another JAR file or directory at the same relative and corresponding location.

For example, here’s an application that looks up some resources:

    package mypackage;
    import java.net.URL;
    import java.io.IOException;
      
    public class FindResources {
      public static void main( String [] args ) throws IOException {
        // absolute from the classpath
        URL url = FindResources.class.getResource("/mypackage/foo.txt");
        // relative to the class location
        url = FindResources.class.getResource("foo.txt");
        // another relative document
        url = FindResources.class.getResource("docs/bar.txt");
      }
    }