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Java Streams Basics

Lesson 5

Most of the programs work with external data stored either in local files or coming from other computers on the network. Java has a concept of working with so-called streams of data. After a physical data storage is mapped to a logical stream, a Java program reads data from this stream serially - byte after byte, character after character, etc. Some of the types of streams are byte streams (InputStream, OutputStream) and character streams (Reader and Writer). The same physical file could be read using different types of streams, for example, FileInputStream, or FileReader.

Classes that work with streams are located in the package java.io. Java 1.4 has introduced the new package java.nio with improved performance, which is not covered in this lesson.

There are different types of data, and hence different types of streams.

Here's the sequence of steps needed to work with a stream:

  1. Open a stream that points at a specific data source: a file, a socket, URL, etc.
  2. Read or write data from/to this stream.
  3. Close the stream.

Let's have a closer look at some of the Java streams.

Byte Streams

If a program needs to read/write bytes (8-bit data), it could use one of the subclasses of the InputStream or OutputStream respectively. The example below shows how to use the class FileInputStream to read a file named abc.dat. This code snippet prints each byte's value separated with white spaces. Byte values are represented by integers from 0 to 255, and if the read() method returns -1, this indicates the end of the stream.

import java.io.FileInputStream;
import java.io.IOException;
public class ReadingBytes {
public static void main(String[] args) {  FileInputStream myFile = null;  try {
  myFile = new FileInputStream("c:\\abc.dat");  // open the  stream
  boolean eof = false;
   while (!eof) {
    int byteValue = myFile.read();  // read  the stream
    System.out.println(byteValue);
    if (byteValue  == -1){
    eof = true;
    }
    //myFile.close();  // do not do it here!!!
   }
  }catch (IOException e) {
      System.out.println("Could not read file: " + e.toString());
  } finally{
   try{
     if (myFile!=null){
       myFile.close(); // close the stream
     }
   } catch (Exception e1){
    e1.printStackTrace();
   }
  }
 }
}

Please note that the stream is closed in the clause finally. Do not call the method close() inside of the try/catch block right after the file reading is done. In case of exception during the file read, the program would jump over the close() statement and the stream would never be closed!

The next code fragment writes into the file xyz.dat using the class FileOutputStream:

int somedata[]={56,230,123,43,11,37};
FileOutputStream myFile = null;
try {
  myFile = new  FileOutputStream("c:\xyz.dat");
  for (int i = 0; i <somedata.length;I++){
      myFile.write(data[I]);
  }
} catch (IOException e)
      System.out.println("Could not write to a file: " + e.toString()); }
finally
    // Close the file the same way as in example above }

Buffered Streams

So far we were reading and writing one byte at a time. Disk access is much slower than the processing performed in memory. That's why it's not a good idea to access disk 1000 times for reading a file of 1000 bytes. To minimize the number of time the disk is accessed, Java provides buffers, which are sort of "reservoirs of data". For example, the class BufferedInputStream works as a middleman between the FileInputStream and the file itself. It reads a big chunk of bytes from a file in one shot into memory, and, then the FileInputStream will read single bytes from there. The BufferedOutputStream works in a similar manner with the class FileOutputStream. Buffered streams just make reading more efficient.

You can use stream chaining (or stream piping) to connect streams - think of connecting two pipes in plumbing. Let's modify the example that reads the file abc.dat to introduce the buffering:

FileInputStream myFile = null;
BufferedInputStream buff =null
try {
  myFile = new  FileInputStream("abc.dat");
  BufferedInputStream buff = new BufferedInputStream(myFile);
  boolean eof = false;
  while (!eof) {
    int byteValue = buff.read();
    System.out.print(byteValue + " ");
    if (byteValue  == -1)
      eof = true;
  }
} catch (IOException e) {
  e.printStackTrace();
} finally{
  buff.close();
  myFile.close();
}

It's a good practice to call the method flush() when the writing into a BufferedOutputStream is done - this forces any buffered data to be written out to the underlying output stream.

While the default buffer size varies depending on the OS, it could be controlled. For example, to set the buffer size to 5000 bytes do this:

BufferedInputStream buff = new BufferedInputStream(myFile, 5000);

Character Streams

Java uses two-byte characters to represent text data, and the classes FileReader and FileWriter work with text files. These classes allow you to read files either one character at a time with read(), or one line at a time with readLine().

The classes FileReader and FileWriter also support have buffering with the help of BufferedReader and BufferedWriter. The following example reads text one line at a time:

FileReader myFile = null;
BufferedReader buff = null;
try {
  myFile = new FileReader("abc.txt");
  buff = new BufferedReader(myFile);
  boolean eof = false;
  while (!eof) {
    String line = buff.readLine();
    if (line == null)
      eof = true;
    else
      System.out.println(line);
    }
    ....
}

For the text output, there are several overloaded methods write() that allow you to write one character, one String or an array of characters at a time.

To append data to an existing file while writing, use the 2-arguments constructor (the second argument toggles the append mode):

FileWriter fOut = new FileWriter("xyz.txt", true);

Below is yet another version of the tax calculation program (see the lesson Intro to Object-Oriented Programming with Java). This is a Swing version of the program and it populates the populate the dropdown box chState with the data from the text file states.txt.

import java.awt.event.*;
import java.awt.*;
import java.io.FileReader;
import java.io.BufferedReader;
import java.io.IOException;

public class TaxFrameFile extends java.awt.Frame implements ActionListener {
  Label lblGrIncome;
  TextField txtGrossIncome = new TextField(15);
  Label lblDependents=new Label("Number of Dependents:");
  TextField txtDependents = new TextField(2);
  Label lblState = new Label("State: ");
  Choice chState = new Choice();

  Label lblTax = new Label("State Tax: ");
  TextField txtStateTax = new TextField(10);
  Button bGo = new Button("Go");
  Button bReset = new Button("Reset");

  TaxFrameFile() {
    lblGrIncome = new Label("Gross Income: ");
    GridLayout gr = new GridLayout(5,2,1,1);
    setLayout(gr);

    add(lblGrIncome);
    add(txtGrossIncome);
    add(lblDependents);
    add(txtDependents);
    add(lblState);
    add(chState);
    add(lblTax);
    add(txtStateTax);
    add(bGo);
    add(bReset);

    // Populate states from a file
    populateStates();
    txtStateTax.setEditable(false);

    bGo.addActionListener(this);
    bReset.addActionListener(this);

    // Define, instantiate and register a WindowAdapter
    // to process windowClosing Event of this frame

       this.addWindowListener(new WindowAdapter() {
        public void windowClosing(WindowEvent e) {
                           System.out.println("Good bye!");
            System.exit(0);
        }});
    }

    public void actionPerformed(ActionEvent evt) {
       Object source = evt.getSource();
        if (source == bGo ){
           // The Button Go processing
             try{
               int grossInc =
                Integer.parseInt(txtGrossIncome.getText());
               int dependents   =
                 Integer.parseInt(txtDependents.getText());
               String state = chState.getSelectedItem();

               Tax tax=new Tax(dependents,state,grossInc);
               String sTax =
                       Double.toString(tax.calcStateTax());
               txtStateTax.setText(sTax);
             }catch(NumberFormatException e){
                 txtStateTax.setText("Non-Numeric Data");
             }catch (Exception e){
                txtStateTax.setText(e.getMessage());
             }
         }
         else if (source == bReset ){
            // The Button Reset processing
        txtGrossIncome.setText("");
        txtDependents.setText("");
                          chState.select("  ");
        txtStateTax.setText("");
             }
    }
   // This method will read the file states.txt and  
   // populate the dropdown chStates
    private void populateStates(){
      FileReader myFile = null;
      BufferedReader buff = null;
        try {
            myFile = new FileReader("states.txt");
            buff = new BufferedReader(myFile);

            boolean eof = false;
            while (!eof) {
                String line = buff.readLine();
                if (line == null)
                   eof = true;
                else
                   chState.add(line);
         }
        }catch (IOException e){
           txtStateTax.setText("Can't read states.txt");
       }
       finally{
         // Closing the streams
         try{
            buff.close();
            myFile.close();
         }catch(IOException e){
            e.printStackTrace();
         }
       }
    }

    public static void main(String args[]){
       TaxFrameFile taxFrame = new TaxFrameFile();
       taxFrame.setSize(400,150);
       taxFrame.setVisible(true);
    }
}

Data Streams

If you are expecting to work with a stream of a known data structure, i.e. two integers, three floats and a double, use either the DataInputStream or the DataOutputStream. A method call readInt() will read the whole integer number (4 bytes ) at once, and the readLong() will get you a long number (8 bytes).

The DataInput stream is just a filter. We are building a "pipe" from the following fragments:

FileInputStream --> BufferedInputStream --> DataInputStream

FileInputStream myFile = new FileInputStream("myData.dat");
BufferedInputStream buff = new BufferedInputStream(myFile);
DataInputStream data = new  DataInputStream(buff);

try {
   int num1 = data.readInt();
   int num2 = data.readInt();
   float num2 = data.readFloat();
   float num3 = data.readFloat();
   float num4 = data.readFloat();
   double num5 = data.readDouble();
} catch (EOFException eof) {...}

Class StreamTokenizer

Sometimes you need to parse a stream without knowing in advance what data types you are getting. In this case you want to get each "piece of data" (token) based on the fact that a delimiter such as a space, comma, etc separates the data elements.

The class java.io.StreamTokenizer reads tokens one at a time. It can recognize identifiers, numbers, quoted strings, etc. Typically an application creates an instance of this class, sets up the rules for parsing, and then repeatedly calls the method nextToken() until it returns the value TT_EOF (end of file).

Let's write a program that will read and parse the file customers.txt distinguishing strings from numbers.

Suppose we have a file customers.txt with the following content:

John Smith  50.24
Mary Lou  234.29
Alexander Popandopula  456.11

Here is the program that parses it:

import java.io.StreamTokenizer;
import java.io.FileReader;

public class CustomerTokenizer{
  public static void main(String args[]){

  StreamTokenizer stream =null;
  try{
    stream = new StreamTokenizer( new
                            FileReader("customers.txt"));
    while (true) {

         int token = stream.nextToken();
         if (token == StreamTokenizer.TT_EOF)
             break;
         if (token == StreamTokenizer.TT_WORD) {
             System.out.println("Got the string: " +
                                         stream.sval);
             }
         if (token == StreamTokenizer.TT_NUMBER) {
             System.out.println("Got the number: " +
                                         stream.nval);
             }
        }
      }catch (Exception e){
         System.out.println("Can't read Customers.txt: " +
                                             e.toString());
      }
      finally{
          try{
             stream.close();
          }catch(Exception e){e.printStackTrace();}          
      }
   }
}

After compiling and running the program CustomerTokenizer, the system console will look like this:

javac CustomerTokenizer.java

java CustomerTokenizer

Got the string: John
Got the string: Smith
Got the number: 50.24
Got the string: Mary
Got the string: Lou
Got the number: 234.29
Got the string: Alexander
Got the string: Popandopula
Got the number: 456.11

When a StreamTokenizer finds a word, it places the value into the sval member variable, and the numbers are placed into the variable nval.

You can specify characters that should be treated as delimiters by calling the method whitespaceChars(). The characters that represent quotes in the stream are set by calling the method quoteChar().

To make sure that certain characters are not misinterpreted, call a method ordinaryChar(), for example ordinaryChar('/');

Class StringTokenizer

The class java.util.StringTokenizer is a simpler version of a class StreamTokenizer, but it works only with strings. The set of delimiters could be specified at the creation time, i.e. comma and angle brackets:

StringTokenizer st = new StringTokenizer(
              "Yakov, 12 Main St., New York", ",<>");
while (st.hasMoreTokens()) {
    System.out.println(st.nextToken());
}

The above code fragment would print the following:

HTML
Yakov
12 Main St.
New York

The previous sample would not return the value of a delimiter - it just returned the tokens. But sometimes, in case of multiple delimiters, you may want to know what's the current delimiter. The 3-argument constructor will provide this information. The following example defines 4 delimiters: greater and less then signs, comma and a white space:

StringTokenizer st=new StringTokenizer( "...IBM...price<...>86.3", "<>, ", true);

If the third argument is true, delimiter characters are also considered to be tokens and will be returned to the calling method, so a program may apply different logic based on the delimiter. If you decide to parse HTML file, you'll need to know what's the current delimiter to apply the proper logic.

Class File

This class has a number of useful file maintenance methods that allow rename, delete, perform existence check, etc. First you have to create an instance of this class:

File myFile = new File("abc.txt");

The line above does not actually create a file - it just creates an instance of the class File that is ready to perform some manipulations with the file abc.txt.The method createNewFile() should be used for the actual creation of the file.

Below are some useful methods of the class File:

   1. createNewFile()creates a new, empty file named according to the file name used during the File instantiation. It creates a new file only if a file with this name does not exist
   2. delete() deletes file or directory
   3. renameTo() renames a file
   4. length() returns the length of the file in bytes
   5. exists() tests whether the file with specified name exists
   6. list() returns an array of strings naming the files and directories in the specified directory
   7. lastModified() returns the time that the file was last modified
   8. mkDir() creates a directory

The code below creates a renames a file customers.txt to customers.txt.bak. If a file with such name already exists, it will be overwritten.

File file = new File("customers.txt");
File backup = new File("customers.txt.bak");
if (backup.exists()){
 backup.delete();
}
file.renameTo(backup);

More Stories By Yakov Fain

Yakov Fain is a co-founder of two software companies: Farata Systems and SuranceBay. He authored several technical books and lots of articles on software development. Yakov is Java Champion (https://java-champions.java.net). He leads leads Princeton Java Users Group. Two of Yakov's books will go in print this year: "Enterprise Web Development" (O'Reilly) and "Java For Kids" (No Starch Press).

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