Welcome!

Java Authors: Paige Leidig, Carmen Gonzalez, Liz Dickinson, Pat Romanski, Bob Gourley

Related Topics: Java

Java: Article

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).

Comments (13)

Share your thoughts on this story.

Add your comment
You must be signed in to add a comment. Sign-in | Register

In accordance with our Comment Policy, we encourage comments that are on topic, relevant and to-the-point. We will remove comments that include profanity, personal attacks, racial slurs, threats of violence, or other inappropriate material that violates our Terms and Conditions, and will block users who make repeated violations. We ask all readers to expect diversity of opinion and to treat one another with dignity and respect.


@ThingsExpo Stories
SYS-CON Events announced today that SOA Software, an API management leader, will exhibit at SYS-CON's 15th International Cloud Expo®, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. SOA Software is a leading provider of API Management and SOA Governance products that equip business to deliver APIs and SOA together to drive their company to meet its business strategy quickly and effectively. SOA Software’s technology helps businesses to accelerate their digital channels with APIs, drive partner adoption, monetize their assets, and achieve a...
SYS-CON Events announced today that Aria Systems, the recurring revenue expert, has been named "Bronze Sponsor" of SYS-CON's 15th International Cloud Expo®, which will take place on November 4-6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Aria Systems helps leading businesses connect their customers with the products and services they love. Industry leaders like Pitney Bowes, Experian, AAA NCNU, VMware, HootSuite and many others choose Aria to power their recurring revenue business and deliver exceptional experiences to their customers.
SYS-CON Events announced today that AgilePoint, the leading provider of Microsoft-centric Business Process Management software, will exhibit at SYS-CON's 2nd International @ThingsExpo which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. AgilePoint is the leading provider of Microsoft-based Business Process Management (BPM) software products, has 1,300+ on-premise and cloud deployments in 25+ countries and provides the same advanced BPM feature set as J2EE vendors like IBM and Appian for the Microsoft .NET native environment. AgilePoint customer...
The Internet of Things (IoT) promises to evolve the way the world does business; however, understanding how to apply it to your company can be a mystery. Most people struggle with understanding the potential business uses or tend to get caught up in the technology, resulting in solutions that fail to meet even minimum business goals. In his session at Internet of @ThingsExpo, Jesse Shiah, CEO / President / Co-Founder of AgilePoint Inc., will show what is needed to leverage the IoT to transform your business. He will discuss opportunities and challenges ahead for the IoT from a market and tec...
SYS-CON Events announced today that Utimaco will exhibit at SYS-CON's 15th International Cloud Expo®, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Utimaco is a leading manufacturer of hardware based security solutions that provide the root of trust to keep cryptographic keys safe, secure critical digital infrastructures and protect high value data assets. Only Utimaco delivers a general-purpose hardware security module (HSM) as a customizable platform to easily integrate into existing software solutions, embed business logic and build s...
One of the biggest challenges when developing connected devices is identifying user value and delivering it through successful user experiences. In his session at Internet of @ThingsExpo, Mike Kuniavsky, Principal Scientist, Innovation Services at PARC, will describe an IoT-specific approach to user experience design that combines approaches from interaction design, industrial design and service design to create experiences that go beyond simple connected gadgets to create lasting, multi-device experiences grounded in people’s real needs and desires.
SYS-CON Events announced today that TeleStax, the main sponsor of Mobicents, will exhibit at Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. TeleStax provides Open Source Communications software and services that facilitate the shift from legacy SS7 based IN networks to IP based LTE and IMS networks hosted on private (on-premise), hybrid or public clouds. TeleStax products include Restcomm, JSLEE, SMSC Gateway, USSD Gateway, SS7 Resource Adaptors, SIP Servlets, Rich Multimedia Services, Presence Services/RCS, Diame...
Samsung VP Jacopo Lenzi, who headed the company's recent SmartThings acquisition under the auspices of Samsung's Open Innovaction Center (OIC), answered a few questions we had about the deal. This interview was in conjunction with our interview with SmartThings CEO Alex Hawkinson. IoT Journal: SmartThings was developed in an open, standards-agnostic platform, and will now be part of Samsung's Open Innovation Center. Can you elaborate on your commitment to keep the platform open? Jacopo Lenzi: Samsung recognizes that true, accelerated innovation cannot be driven from one source, but requires a...
The Internet of Things will greatly expand the opportunities for data collection and new business models driven off of that data. In her session at Internet of @ThingsExpo, Esmeralda Swartz, CMO of MetraTech, will discuss how for this to be effective you not only need to have infrastructure and operational models capable of utilizing this new phenomenon, but increasingly service providers will need to convince a skeptical public to participate. Get ready to show them the money! Speaker Bio: Esmeralda Swartz, CMO of MetraTech, has spent 16 years as a marketing, product management, and busin...
Things are being built upon cloud foundations to transform organizations. This CEO Power Panel at 15th Cloud Expo, moderated by Roger Strukhoff, Cloud Expo and @ThingsExpo conference chair, will address the big issues involving these technologies and, more important, the results they will achieve. How important are public, private, and hybrid cloud to the enterprise? How does one define Big Data? And how is the IoT tying all this together?
We certainly live in interesting technological times. And no more interesting than the current competing IoT standards for connectivity. Various standards bodies, approaches, and ecosystems are vying for mindshare and positioning for a competitive edge. It is clear that when the dust settles, we will have new protocols, evolved protocols, that will change the way we interact with devices and infrastructure. We will also have evolved web protocols, like HTTP/2, that will be changing the very core of our infrastructures. At the same time, we have old approaches made new again like micro-services...
SYS-CON Events announces a new pavilion on the Cloud Expo floor where WebRTC converges with the Internet of Things. Pavilion will showcase WebRTC and the Internet of Things. The Internet of Things (IoT) is the most profound change in personal and enterprise IT since the creation of the Worldwide Web more than 20 years ago. All major researchers estimate there will be tens of billions devices--computers, smartphones, tablets, and sensors – connected to the Internet by 2020. This number will continue to grow at a rapid pace for the next several decades.
The only place to be June 9-11 is Cloud Expo & @ThingsExpo 2015 East at the Javits Center in New York City. Join us there as delegates from all over the world come to listen to and engage with speakers & sponsors from the leading Cloud Computing, IoT & Big Data companies. Cloud Expo & @ThingsExpo are the leading events covering the booming market of Cloud Computing, IoT & Big Data for the enterprise. Speakers from all over the world will be hand-picked for their ability to explore the economic strategies that utility/cloud computing provides. Whether public, private, or in a hybrid form, clo...
SYS-CON Events announced today that Gridstore™, the leader in software-defined storage (SDS) purpose-built for Windows Servers and Hyper-V, will exhibit at SYS-CON's 15th International Cloud Expo®, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Gridstore™ is the leader in software-defined storage purpose built for virtualization that is designed to accelerate applications in virtualized environments. Using its patented Server-Side Virtual Controller™ Technology (SVCT) to eliminate the I/O blender effect and accelerate applications Gridsto...
SYS-CON Events announced today that Red Hat, the world's leading provider of open source solutions, will exhibit at Internet of @ThingsExpo, which will take place on November 4–6, 2014, at the Santa Clara Convention Center in Santa Clara, CA. Red Hat is the world's leading provider of open source software solutions, using a community-powered approach to reliable and high-performing cloud, Linux, middleware, storage and virtualization technologies. Red Hat also offers award-winning support, training, and consulting services. As the connective hub in a global network of enterprises, partners, a...
As the Internet of Things unfolds, mobile and wearable devices are blurring the line between physical and digital, integrating ever more closely with our interests, our routines, our daily lives. Contextual computing and smart, sensor-equipped spaces bring the potential to walk through a world that recognizes us and responds accordingly. We become continuous transmitters and receivers of data. In his session at Internet of @ThingsExpo, Andrew Bolwell, Director of Innovation for HP’s Printing and Personal Systems Group, will discuss how key attributes of mobile technology – touch input, senso...
The Internet of Things (IoT) is making everything it touches smarter – smart devices, smart cars and smart cities. And lucky us, we’re just beginning to reap the benefits as we work toward a networked society. However, this technology-driven innovation is impacting more than just individuals. The IoT has an environmental impact as well, which brings us to the theme of this month’s #IoTuesday Twitter chat. The ability to remove inefficiencies through connected objects is driving change throughout every sector, including waste management. BigBelly Solar, located just outside of Boston, is trans...
Connected devices and the Internet of Things are getting significant momentum in 2014. In his session at Internet of @ThingsExpo, Jim Hunter, Chief Scientist & Technology Evangelist at Greenwave Systems, will examine three key elements that together will drive mass adoption of the IoT before the end of 2015. The first element is the recent advent of robust open source protocols (like AllJoyn and WebRTC) that facilitate M2M communication. The second is broad availability of flexible, cost-effective storage designed to handle the massive surge in back-end data in a world where timely analytics...
Internet of @ThingsExpo Silicon Valley announced on Thursday its first 12 all-star speakers and sessions for its upcoming event, which will take place November 4-6, 2014, at the Santa Clara Convention Center in California. @ThingsExpo, the first and largest IoT event in the world, debuted at the Javits Center in New York City in June 10-12, 2014 with over 6,000 delegates attending the conference. Among the first 12 announced world class speakers, IBM will present two highly popular IoT sessions, which will take place November 4-6, 2014 at the Santa Clara Convention Center in Santa Clara, Calif...
From a software development perspective IoT is about programming "things," about connecting them with each other or integrating them with existing applications. In his session at @ThingsExpo, Yakov Fain, co-founder of Farata Systems and SuranceBay, will show you how small IoT-enabled devices from multiple manufacturers can be integrated into the workflow of an enterprise application. This is a practical demo of building a framework and components in HTML/Java/Mobile technologies to serve as a platform that can integrate new devices as they become available on the market.