(c) Copyright 2000, 2001, 2002, 2003 by Daniel Berleant 

Reading from a Local File

There is a class FileInputStream

It has methods associated with file input streams (surprise!)

. . . for example, read( )

We must declare an object of class FileInputStream

 (Objects are like, unlike classes)

An object is an instance of a class

…like a variable is an instance of a type:

//myInt is a variable of type int:
int myInt;

//f is an object of class FileInputStream:    
FileInputStream f;

      To use a method in a class – must be declared static

      To use a method in an object – must declare the object

 Example:

FileInputStream myInput;

 myInput is now an object for reading a file

 . . . but which file?

 . . . we must provide myInput with memory to use, and say what file it refers to:

myInput=

new FileInputStream("fileRead.dat");

 I/O can fail, so Java requires we account for that:

try{myInput=

    new FileInputStream("fileRead.dat");}

...

 If it does fail, we must "catch" the failure:

try{myInput=

    new FileInputStream("fileRead.dat");

   }

catch (IOException e)
   {System.out.println(e);}

 Let's analyze that code...

  Sample codes

We might want to know when we are done reading from the file:

Let's see an example:

(If an example ever does not work, please send email)

import java.lang.*;

import java.io.*;  

public class FileRead

  {

  static FileInputStream myInput;

  public static void main (String argv[ ])

     {

     int Cbuffer=-1;

     char C;

     System.out.println("About to read a file...");

     try {myInput = 

        new FileInputStream( 

          "FileRead.java");

     }

     catch(IOException e){System.out.println(e);}

     do {

        try {Cbuffer=myInput.read();}

        catch (IOException e){ };

        C=(char)Cbuffer;

        if (Cbuffer!=-1) 

           System.out.print(C);

     }

     while (Cbuffer!=-1);

     System.out.println("...Finishing.");

     try {myInput.close();} 

     catch(IOException e) {System.out.println(e);}

    }

 }

 Let's analyze it . . .

Writing to a Local File

Here is an example

import java.lang.*;

import java.io.*;  

public class FileWrite

  {

  static FileOutputStream output;

  static DataOutputStream data;

  public static void main 

         (String argv[ ])

     {System.out.println

        ("About to write a file...");

        try {output = 

           new FileOutputStream("data.dat");}

        catch (IOException e)

          {System.out.println("Sorry...");}

        data=new DataOutputStream(output);

        try {data.writeBytes

           ("Hello there everyone!!"+"\n");} 

        catch (IOException e){System.out.println(e);}

        System.out.println("...Finishing.");

        try {data.close();} 

        catch(IOException e) { }

        try {output.close();} 

        catch(IOException e) { }

     }

  }

 Let's analyze it

Files and URLs

    Both must be converted to streams

    Files: use e.g. the FileInputStream class

    URLs: see code example below (also appears in a later session)

import java.net.*;

import java.io.*;

public class URLtest

{

private static URL location;

public static void main(String argv[])

  {

  try {location=new URL("http://www.iastate.edu");}

  catch(MalformedURLException e)

     {System.err.println ("Invalid URL: "+location);}

  InputStream myInput;

  BufferedReader dataInput;

  String lineOfText;

  try {

      myInput=location.openStream();

      dataInput=new BufferedReader(new

            InputStreamReader(myInput));

      System.out.println

        (dataInput.readLine());

      while

        ((lineOfText=dataInput.readLine())

            !=null)

        System.out.println(lineOfText);  

        dataInput.close();

      myInput.close();

      }

  catch(IOException e)

      {System.out.println("oops!!!!!");

      System.out.println

        ("Exception: "+e.toString() );

      }

  }

}