Data Hierarchy

Ultimately, a computer processes all data items as combinations of zeros and ones, because it is simple and economical for engineers to build electronic devices that can assume two stable statesone representing 0 and the other representing 1. It is remarkable that the impressive functions performed by computers involve only the most fundamental manipulations of 0s and 1s.

The smallest data item in a computer can assume the value 0 or the value 1. Such a data item is called a bit (short for "binary digit"a digit that can assume one of two values). Computer circuitry performs various simple bit manipulations, such as examining the value of a bit, setting the value of a bit and reversing the value of a bit (from 1 to 0 or from 0 to 1).

It is cumbersome for programmers to work with data in the low-level form of bits. Instead, programmers prefer to work with data in such forms as decimal digits (09), letters (AZ and az), and special symbols (e.g., $, @, %, &, *, (, ), , +, ", :, ? and / ). Digits, letters and special symbols are known as characters. The computer's character set is the set of all the characters used to write programs and represent data items. Computers process only 1s and 0s, so a computer's character set represents every character as a pattern of 1s and 0s. Characters in Java are Unicode characters composed of two bytes, each composed of eight bits. Java contains a data type, byte, that can be used to represent byte data. The Unicode character set contains characters for many of the world's languages. See Appendix F for more information on this character set. See Appendix B, ASCII Character Set for more information on the ASCII (American Standard Code for Information Interchange) character set, a subset of the Unicode character set that represents uppercase and lowercase letters, digits and various common special characters.

Just as characters are composed of bits, fields are composed of characters or bytes. A field is a group of characters or bytes that conveys meaning. For example, a field consisting of uppercase and lowercase letters can be used to represent a person's name.

Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress from bits to characters to fields, and so on.

Typically, several fields compose a record (implemented as a class in Java). In a payroll system, for example, the record for an employee might consist of the following fields (possible types for these fields are shown in parentheses):

• Employee identification number (int)
• Name (String)
• Address (String)
• Hourly pay rate (double)
• Number of exemptions claimed (int)
• Year-to-date earnings (int or double)
• Amount of taxes withheld (int or double)

Thus, a record is a group of related fields. In the preceding example, all the fields belong to the same employee. Of course, a company might have many employees and thus have a payroll record for each employee. A file is a group of related records. [Note: More generally, a file contains arbitrary data in arbitrary formats. In some operating systems, a file is viewed as nothing more than a collection of bytesany organization of the bytes in a file (e.g., organizing the data into records) is a view created by the applications programmer.] A company's payroll file normally contains one record for each employee. Thus, a payroll file for a small company might contain only 22 records, whereas one for a large company might contain 100,000 records. It is not unusual for a company to have many files, some containing billions, or even trillions, of characters of information. Figure 14.1 illustrates a portion of the data hierarchy.

Figure 14.1. Data hierarchy.

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To facilitate the retrieval of specific records from a file, at least one field in each record is chosen as a record key. A record key identifies a record as belonging to a particular person or entity and is unique to each record. This field typically is used to search and sort records. In the payroll record described previously, the employee identification number normally would be chosen as the record key.

There are many ways to organize records in a file. The most common is called a sequential file, in which records are stored in order by the record-key field. In a payroll file, records are placed in ascending order by employee identification number.

Most businesses store data in many different files. For example, companies might have payroll files, accounts receivable files (listing money due from clients), accounts payable files (listing money due to suppliers), inventory files (listing facts about all the items handled by the business) and many others. Often, a group of related files is called a database. A collection of programs designed to create and manage databases is called a database management system (DBMS). We discuss this topic in Chapter 25, Accessing Databases with JDBC