Java file overview

Java File class is represented of  file and directory pathnames. A pathname has two components:

• An optional system-dependent prefix string,
  such as a disk-drive specifier, "/" for the UNIX root directory, or "\\" for a Win32 UNC pathname, and
• A sequence of zero or more string names.

Creating Java File Objects

There have three constructors for creating File objects. The simplest accepts a String object as an argument that specifies a path for a file or a directory. For example,

File myfile = new File("C:/temp/tempfile");

You can also create a File object that represents a pathname for a file by using a constructor that allows you to specify the directory that contains the file and the file name separately.

For example,

File tempDir = new File("C:/temp");  // Parent directory
File tempFile = new File(tempDir, "Hwllo.java");         // The path to the file

The fourth constructor allows you to define a File object from an object of type URI that encapsulates a Uniform Resource Identifier

For example,

 File remoteFile = new File(new URL(http://www.devx.biz/java));

Absolute and Relative Paths

In general, the pathname that you use to create a File object has two parts: an optional prefix, followed by a series of names separated by the system default separator character for pathnames. Under MS Windows the prefix for a path on a local drive will be a string defining the drive, such as "C:\\" or "C:/". Under Unix the prefix will be a forward slash, "/". A path that includes a prefix is an absolute path and since it includes a prefix it is not system-independent. A path without a prefix is a relative path and as long as it consists of one or more names separated by characters specified as File.separator or File.separatorChar it should be portable across different systems.

On a computer with the name myLT, with a shared directory shared, you could create a File object as follows:

File myFile = new File("\\\\myLT\\shared\\temp\\src\\java\\io",
                         "Hello.java");

Relative Paths

File myFile = new File(File.separator + File.separator + "myLT" +
                       File.separator + "shared" + File.separator +
                       "temp", "Hello.java");

Deleting a File

    boolean success = (new File("filename")).delete();
    if (!success) {
        // Deletion failed
    }

Copying Java Files

 Using file streams to copy the contents of one file to another file. 

    void copy(File src, File dst) throws IOException {
        InputStream in = new FileInputStream(src);
        OutputStream out = new FileOutputStream(dst);
        byte[] buf = new byte[1024];
        int len;
        while ((len = in.read(buf)) > 0) {
            out.write(buf, 0, len);
        }
        in.close();
        out.close();
    }

Using file channel

A file channel defines two methods

Example

import java.io.*;
import java.nio.*;
import java.nio.channels.FileChannel;

public class FileCopy {
  public static void main(String[] args) {
    if(args.length==0) {
      System.out.println("No file to copy. Application usage is:\n"+
                         "java -classpath . FileCopy \"filepath\"" );
      System.exit(1);
    }
    File fromFile = new File(args[0]);

    if(!fromFile.exists()) {
      System.out.println("File to copy, "+fromFile.getAbsolutePath()
                       + ", does not exist.");
      System.exit(1);
    }
   
    File toFile = createBackupFile(fromFile);
    FileInputStream inFile = null;
    FileOutputStream outFile = null;
    try {
      inFile = new FileInputStream(fromFile);
      outFile = new FileOutputStream(toFile);

    } catch(FileNotFoundException e) {
      e.printStackTrace(System.err);
      assert false;
    }

    FileChannel inChannel = inFile.getChannel();   
    FileChannel outChannel = outFile.getChannel();

    try {
      int bytesWritten = 0;
      long byteCount = inChannel.size();
      while(bytesWritten<byteCount)
        bytesWritten += inChannel.transferTo(bytesWritten,
                                             byteCount-bytesWritten,
                                             outChannel);
     
      System.out.println("File copy complete. " + byteCount
                       + " bytes copied to "
                       + toFile.getAbsolutePath());
      inFile.close();
      outFile.close();

    } catch(IOException e) {
      e.printStackTrace(System.err);
      System.exit(1);
    }
    System.exit(0);
  }
 }

Questions about java file

Which gets the name of the parent directory file “file.txt”?

A. String name= File.getParentName(“file.txt”);
B. String name= (new File(“file.txt”)).getParent();
C. String name = (new File(“file.txt”)).getParentName();
D. String name= (new File(“file.txt”)).getParentFile();
E. Directory dir=(new File (“file.txt”)).getParentDir(); String name= dir.getName();

What writes the text “<end>” to the end of the file “file.txt”?

A. OutputStream out= new FileOutputStream (“file.txt”);
                     Out.writeBytes (“<end>/n”);
B. OutputStream os= new FileOutputStream (“file.txt”, true); DataOutputStream out = new DataOutputStream(os); out.writeBytes (“<end>/n”);
C. OutputStream os= new FileOutputStream (“file.txt”); DataOutputStream out = new DataOutputStream(os); out.writeBytes (“<end>/n”);
D. OutputStream os= new OutputStream (“file.txt”, true); DataOutputStream out = new DataOutputStream(os); out.writeBytes (“<end>/n”);