Lecture Notes for the NC009 Complete Course on Java

Lecture Notes Eight: Basic Java Networking

Part One: Basic I/O.

Before I forget here's the URL for the JMF2.1 main page. Now let's get started.

So far we know how to generate data for the user, graphically or textually. We are also able to get input from the user from the command line, as a sequence of Strings, or from the mouse in a graphical, or a graphical user interface, context.

Now we want to be able to read text from a file, and from the terminal, and also to write text to a file.

If you want to be able to make phone calls you need to buy a telephone and use it according to the manufacturer's instructions. Same thing with I/O in Java: we just need to know what type of objects we need to work with (and how) in order to read from or write to a file, and read from the terminal.

FileInputStream
InputStreamReader
BufferedReader
StreamTokenizer
System.out
System.in
FileOutputStream
PrintWriter

Once we have these things under control, we can look at

other formats in which data can be stored to, or retrieved from, files and
how networking is not much more different from reading from, or writing to, a text file.

Here are the programs, step by step.

1. FileInputStream

Find the documentation for it here.

oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%vi file
oldschool.cs.indiana.edu%ls -l
total 2
-rw-------   1 dgerman       289 Jul 19 12:49 Example.java
-rw-------   1 dgerman        59 Jul 19 12:45 file
oldschool.cs.indiana.edu%cat file
This is a sample
file that has three
lines of text in it. 
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileInputStream stream = new FileInputStream(args[0]); 
    
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%ls -l
total 3
-rw-------   1 dgerman       793 Jul 19 12:50 Example.class
-rw-------   1 dgerman       289 Jul 19 12:50 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%java Example file
oldschool.cs.indiana.edu%java Example A.java
Something went wrong: java.io.FileNotFoundException: A.java
oldschool.cs.indiana.edu%java Example
Something went wrong: java.lang.ArrayIndexOutOfBoundsException: 0
oldschool.cs.indiana.edu%

So now you know how you can get a connection, and what happens if you're asking for an inconsistent or incorrect kind of connection. Let's now read the character one by one and put parens around them when we print them back to the user to see if we can get inside the contents of the file.

2. InputStreamReader

Find documentation for this kind of objects here.

oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileInputStream stream = new FileInputStream(args[0]); 
      InputStreamReader reader = new InputStreamReader(stream);
      int data = reader.read(); 
      while (data != -1) {
        System.out.print("(" + data + ")"); 
        data = reader.read(); 
      }     
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example file
(84)(104)(105)(115)(32)(105)(115)(32)(97)(32)(115)(97)(109)(112)(108)(101)(10\
)(102)(105)(108)(101)(32)(116)(104)(97)(116)(32)(104)(97)(115)(32)(116)(104)(\
114)(101)(101)(10)(108)(105)(110)(101)(115)(32)(111)(102)(32)(116)(101)(120)(\
116)(32)(105)(110)(32)(105)(116)(46)(32)(10)
oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileInputStream stream = new FileInputStream(args[0]); 
      InputStreamReader reader = new InputStreamReader(stream);
      int data = reader.read(); 
      while (data != -1) {
        System.out.print("(" + (char)data + ")"); 
        data = reader.read(); 
      }     
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example file
(T)(h)(i)(s)( )(i)(s)( )(a)( )(s)(a)(m)(p)(l)(e)(
)(f)(i)(l)(e)( )(t)(h)(a)(t)( )(h)(a)(s)( )(t)(h)(r)(e)(e)(
)(l)(i)(n)(e)(s)( )(o)(f)( )(t)(e)(x)(t)( )(i)(n)( )(i)(t)(.)( )(
)oldschool.cs.indiana.edu%

So you see how you can get to the characters, by first getting their integer codes and then converting them into char data. Now let's see how we can get a whole line of characters at a time.

3. BufferedReader

Find documentation about this kind of objects here.

oldschool.cs.indiana.edu%ls -l
total 2
-rw-------   1 dgerman       408 Jul 19 13:12 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileInputStream stream = new FileInputStream(args[0]); 
      InputStreamReader reader = new InputStreamReader(stream);
      BufferedReader b = new BufferedReader(reader); 
      String line = b.readLine();
      while (line != null) {
        System.out.println("(" + line + ")"); 
        line = b.readLine(); 
      }      
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example file
(This is a sample)
(file that has three)
(lines of text in it. )
oldschool.cs.indiana.edu%

This was easy and you knew all about it already.

4. StreamTokenizer

Find documentation about these kind of objects here.

oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%ls -l
total 2
-rw-------   1 dgerman       763 Jul 19 13:33 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileInputStream stream = new FileInputStream(args[0]); 
      InputStreamReader reader = new InputStreamReader(stream);
      BufferedReader b = new BufferedReader(reader); 
      StreamTokenizer s = new StreamTokenizer(b); 
      s.nextToken(); 
      while (s.ttype != StreamTokenizer.TT_EOF) { 
        if (s.ttype == StreamTokenizer.TT_WORD)
          System.out.println("(" + s.sval + ")"); 
        else if (s.ttype == StreamTokenizer.TT_NUMBER) 
          System.out.println("(" + s.nval + ")");
        s.nextToken(); 
      }  
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example file
(This)
(is)
(a)
(sample)
(file)
(that)
(has)
(three)
(lines)
(of)
(text)
(in)
(it.)
oldschool.cs.indiana.edu%

EXERCISE

Use a StringTokenizer on lines returned from a BufferedReader to achieve same result.

5. System.out

You can read about this kind of objects here. You have used this kind of objects a lot.

6. System.in

You can read about this kind of objects here.

oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      InputStreamReader reader = new InputStreamReader(System.in);
      BufferedReader b = new BufferedReader(reader); 
      System.out.println("Welcome to the Echo program!"); 
      System.out.print("Type: "); 
      String line = b.readLine(); 
      while (! line.equals("quit")) { 
        System.out.println("You typed: " + line);      
        System.out.print("Type: "); line = b.readLine(); 
      }
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } finally { 
      System.out.println("Good-bye."); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example
Welcome to the Echo program!
Type: Test
You typed: Test
Type: Hello!
You typed: Hello!
Type: I am talking to you.
You typed: I am talking to you.
Type: Hi there.
You typed: Hi there.
Type: quit
Good-bye.
oldschool.cs.indiana.edu%

6. FileOutputStream

You can read the documentation about this kind of objects here.

oldschool.cs.indiana.edu%ls -l
total 2
-rw-------   1 dgerman       667 Jul 19 13:51 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%pico Example.java
oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%ls -l
total 2
-rw-------   1 dgerman       350 Jul 19 14:00 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileOutputStream stream = new FileOutputStream(args[0]); 
      
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } finally { 
      System.out.println("Good-bye."); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example
Something went wrong: java.lang.ArrayIndexOutOfBoundsException: 0
Good-bye.
oldschool.cs.indiana.edu%java Example file.out
Good-bye.
oldschool.cs.indiana.edu%ls -l
total 3
-rw-------   1 dgerman       850 Jul 19 14:00 Example.class
-rw-------   1 dgerman       350 Jul 19 14:00 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
-rw-------   1 dgerman         0 Jul 19 14:01 file.out
oldschool.cs.indiana.edu%

7. PrintWriter

Read the documentation about this kind of objects here.

oldschool.cs.indiana.eduls -l
total 2
-rw-------   1 dgerman       701 Jul 19 14:09 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
oldschool.cs.indiana.edu%vi Example.java
oldschool.cs.indiana.edu%cat Example.java
import java.io.*;
 
class Example {
  public static void main(String[] args) {
    try {
      FileOutputStream stream = new FileOutputStream(args[0]); 
      PrintWriter p = new PrintWriter(stream, true);
      BufferedReader b = new BufferedReader(new InputStreamReader(System.in));
      System.out.print("Hello, please type: "); String line = b.readLine(); 
      while (! line.equals("quit")) {
        p.println(line); 
        System.out.print("Thanks, please go on: "); line = b.readLine();  
      }  
      stream.close(); 
    } catch (Exception e) {
      System.out.println("Something went wrong: " + e.toString()); 
    } finally { 
      System.out.println("Good-bye."); 
    } 
  } 
} 
oldschool.cs.indiana.edu%javac Example.java
oldschool.cs.indiana.edu%java Example file.out
Hello, please type: I am typing lines that   
Thanks, please go on: my program should write
Thanks, please go on: to the file that I have
Thanks, please go on: indicated on the command
Thanks, please go on: line, that is: file.out
Thanks, please go on: quit
Good-bye.
oldschool.cs.indiana.edu%ls -l
total 5
-rw-------   1 dgerman      1362 Jul 19 14:09 Example.class
-rw-------   1 dgerman       701 Jul 19 14:09 Example.java
-rw-------   1 dgerman        59 Jul 19 12:50 file
-rw-------   1 dgerman       120 Jul 19 14:10 file.out
oldschool.cs.indiana.edu%cat file.out
I am typing lines that
my program should write
to the file that I have
indicated on the command
line, that is: file.out
oldschool.cs.indiana.edu

That's basically it, as far as text goes.

We don't know it yet, but we have almost everything worked out.

Our next stop: Socket and ServerSocket for client/server.

I am providing this as a separate document, that we may actually cover (finish covering) next time.

Last updated: Apr 8, 2002 by Adrian German for NC009