Home > Articles

  • Print
  • + Share This
This chapter is from the book

2.2 Reading and Writing Binary Data

Text format is convenient for testing and debugging because it is humanly readable, but it is not as efficient as transmitting data in binary format. In the following sections, you will learn how to perform input and output with binary data.

2.2.1 The DataInput and DataOutput interfaces

The DataOutput interface defines the following methods for writing a number, a character, a boolean value, or a string in binary format:

writeChars          writeFloat
writeByte           writeDouble
writeInt            writeChar
writeShort          writeBoolean
writeLong           writeUTF

For example, writeInt always writes an integer as a 4-byte binary quantity regardless of the number of digits, and writeDouble always writes a double as an 8-byte binary quantity. The resulting output is not human-readable, but it will use the same space for each value of a given type and reading it back in will be faster than parsing text.

The writeUTF method writes string data using a modified version of the 8-bit Unicode Transformation Format. Instead of simply using the standard UTF-8 encoding, sequences of Unicode code units are first represented in UTF-16, and then the result is encoded using the UTF-8 rules. This modified encoding is different for characters with codes higher than 0xFFFF. It is used for backward compatibility with virtual machines that were built when Unicode had not yet grown beyond 16 bits.

Since nobody else uses this modification of UTF-8, you should only use the writeUTF method to write strings intended for a Java virtual machine—for example, in a program that generates bytecodes. Use the writeChars method for other purposes.

To read the data back in, use the following methods defined in the DataInput interface:

readInt            readDouble
readShort          readChar
readLong           readBoolean
readFloat          readUTF

The DataInputStream class implements the DataInput interface. To read binary data from a file, combine a DataInputStream with a source of bytes such as a FileInputStream:

var in = new DataInputStream(new FileInputStream("employee.dat"));

Similarly, to write binary data, use the DataOutputStream class that implements the DataOutput interface:

var out = new DataOutputStream(new FileOutputStream("employee.dat"));

2.2.2 Random-Access Files

The RandomAccessFile class lets you read or write data anywhere in a file. Disk files are random-access, but input/output streams that communicate with a network socket are not. You can open a random-access file either for reading only or for both reading and writing; specify the option by using the string "r" (for read access) or "rw" (for read/write access) as the second argument in the constructor.

var in = new RandomAccessFile("employee.dat", "r");
var inOut = new RandomAccessFile("employee.dat", "rw");

When you open an existing file as a RandomAccessFile, it does not get deleted.

A random-access file has a file pointer that indicates the position of the next byte to be read or written. The seek method can be used to set the file pointer to an arbitrary byte position within the file. The argument to seek is a long integer between zero and the length of the file in bytes.

The getFilePointer method returns the current position of the file pointer.

The RandomAccessFile class implements both the DataInput and DataOutput interfaces. To read and write from a random-access file, use methods such as readInt/writeInt and readChar/writeChar that we discussed in the preceding section.

Let’s walk through an example program that stores employee records in a random-access file. Each record will have the same size. This makes it easy to read an arbitrary record. Suppose you want to position the file pointer to the third record. Simply set the file pointer to the appropriate byte position and start reading.

long n = 3;
in.seek((n - 1) * RECORD_SIZE);
var e = new Employee();
e.readData(in);

If you want to modify the record and save it back into the same location, remember to set the file pointer back to the beginning of the record:

in.seek((n - 1) * RECORD_SIZE);
e.writeData(out);

To determine the total number of bytes in a file, use the length method. The total number of records is the length divided by the size of each record.

long nbytes = in.length(); // length in bytes
int nrecords = (int) (nbytes / RECORD_SIZE);

Integers and floating-point values have a fixed size in binary format, but we have to work harder for strings. We provide two helper methods to write and read strings of a fixed size.

The writeFixedString writes the specified number of code units, starting at the beginning of the string. If there are too few code units, the method pads the string, using zero values.

public static void writeFixedString(String s, int size, DataOutput out)
      throws IOException
{
   for (int i = 0; i < size; i++)
   {
      char ch = 0;
      if (i < s.length()) ch = s.charAt(i);
      out.writeChar(ch);
   }
}

The readFixedString method reads characters from the input stream until it has consumed size code units or until it encounters a character with a zero value. Then, it skips past the remaining zero values in the input field. For added efficiency, this method uses the StringBuilder class to read in a string.

public static String readFixedString(int size, DataInput in)
      throws IOException
{
   var b = new StringBuilder(size);
   int i = 0;
   var done = false;
   while (!done && i < size)
   {
      char ch = in.readChar();
      i++;
      if (ch == 0) done = true;
      else b.append(ch);
   }
   in.skipBytes(2 * (size - i));
   return b.toString();
}

We placed the writeFixedString and readFixedString methods inside the DataIO helper class.

To write a fixed-size record, we simply write all fields in binary.

DataIO.writeFixedString(e.getName(), Employee.NAME_SIZE, out);
out.writeDouble(e.getSalary());
LocalDate hireDay = e.getHireDay();
out.writeInt(hireDay.getYear());
out.writeInt(hireDay.getMonthValue());
out.writeInt(hireDay.getDayOfMonth());

Reading the data back is just as simple.

String name = DataIO.readFixedString(Employee.NAME_SIZE, in);
double salary = in.readDouble();
int y = in.readInt();
int m = in.readInt();
int d = in.readInt();

Let us compute the size of each record. We will use 40 characters for the name strings. Therefore, each record will contain 100 bytes:

  • 40 characters = 80 bytes for the name

  • 1 double = 8 bytes for the salary

  • 3 int = 12 bytes for the date

The program shown in Listing 2.2 writes three records into a data file and then reads them from the file in reverse order. To do this efficiently requires random access—we need to get to the last record first.

Listing 2.2 randomAccess/RandomAccessTest.java


 1  package randomAccess;
 2  
 3  import java.io.*;
 4  import java.time.*;
 5  
 6  /**
 7  * @version 1.14 2018-05-01
 8  * @author Cay Horstmann
 9  */
10  public class RandomAccessTest
11  {
12    public static void main(String[] args) throws IOException
13    {
14        var staff = new Employee[3];
15    
16        staff[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15);
17        staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
18        staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15);
19    
20        try (var out = new DataOutputStream(new FileOutputStream("employee.dat")))
21        {
22          // save all employee records to the file employee.dat
23          for (Employee e : staff)
24              writeData(out, e);
25        }
26    
27        try (var in = new RandomAccessFile("employee.dat", "r"))
28        {
29          // retrieve all records into a new array
30    
31          // compute the array size
32          int n = (int)(in.length() / Employee.RECORD_SIZE);
33          var newStaff = new Employee[n];
34          
35          // read employees in reverse order
36          for (int i = n - 1; i >= 0; i--)
37          {
38              newStaff[i] = new Employee();
39              in.seek(i * Employee.RECORD_SIZE);
40              newStaff[i] = readData(in);
41          }
42          
43          // print the newly read employee records
44          for (Employee e : newStaff)
45              System.out.println(e);
46        }
47    }
48  
49      /**
50      * Writes employee data to a data output
51      * @param out the data output
52      * @param e the employee
53      */
54      public static void writeData(DataOutput out, Employee e) throws IOException
55      {
56        DataIO.writeFixedString(e.getName(), Employee.NAME_SIZE, out);
57        out.writeDouble(e.getSalary());
58        
59        LocalDate hireDay = e.getHireDay();
60        out.writeInt(hireDay.getYear());
61        out.writeInt(hireDay.getMonthValue());
62        out.writeInt(hireDay.getDayOfMonth());
63      }
64  
65      /**
66      * Reads employee data from a data input
67      * @param in the data input
68      * @return the employee
69      */
70      public static Employee readData(DataInput in) throws IOException
71      {
72        String name = DataIO.readFixedString(Employee.NAME_SIZE, in);
73        double salary = in.readDouble();
74        int y = in.readInt();
75        int m = in.readInt();
76        int d = in.readInt();
77        return new Employee(name, salary, y, m - 1, d);
78      }
79  }

2.2.3 ZIP Archives

ZIP archives store one or more files in a (usually) compressed format. Each ZIP archive has a header with information such as the name of each file and the compression method that was used. In Java, you can use a ZipInputStream to read a ZIP archive. You need to look at the individual entries in the archive. The getNextEntry method returns an object of type ZipEntry that describes the entry. Read from the stream until the end, which is actually the end of the current entry. Then call closeEntry to read the next entry. Do not close zin until you read the last entry. Here is a typical code sequence to read through a ZIP file:

var zin = new ZipInputStream(new FileInputStream(zipname));
ZipEntry entry;
while ((entry = zin.getNextEntry()) != null)
{
   read the contents of zin
   zin.closeEntry();
}
zin.close();

To write a ZIP file, use a ZipOutputStream. For each entry that you want to place into the ZIP file, create a ZipEntry object. Pass the file name to the ZipEntry constructor; it sets the other parameters such as file date and decompression method. You can override these settings if you like. Then, call the putNextEntry method of the ZipOutputStream to begin writing a new file. Send the file data to the ZIP output stream. When done, call closeEntry. Repeat for all the files you want to store. Here is a code skeleton:

var fout = new FileOutputStream("test.zip");
var zout = new ZipOutputStream(fout);
for all files
{
   var ze = new ZipEntry(filename);
   zout.putNextEntry(ze);
   send data to zout
   zout.closeEntry();
}
zout.close();

ZIP input streams are a good example of the power of the stream abstraction. When you read data stored in compressed form, you don’t need to worry that the data are being decompressed as they are being requested. Moreover, the source of the bytes in a ZIP stream need not be a file—the ZIP data can come from a network connection.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020