928 Files and Streams Chapter 16 16.7 Random-Access (Geocities web hosting)
928 Files and Streams Chapter 16 16.7 Random-Access Files So far, we have seen how to create sequential-access files and to search through them to locate particular information. Sequential-access files are inappropriate for so-called instant-access applications, in which a particular record of information must be located immediately. Some popular instant-access applications are airline reservation systems, banking systems, point-of-sale systems, automated-teller machines and other kinds of transaction-processing systems that require rapid access to specific data. The bank at which you have your account might have hundreds of thousands or even millions of other customers, yet, when you use an automated teller machine, the bank determines in seconds whether your account has sufficient funds for the transaction. This kind of instant access is possible with random-access files. A program can access individual records of a random- access file directly (and quickly) without searching through other records. Random-access files are sometimes called direct-access files. As we have said, Java does not impose structure on a file, so an application that wants to use random-access files must create them. Several techniques can be used to create random-access files. Perhaps the simplest is to require that all records in a file be of the same fixed length. Using fixed-length records makes it easy for a program to calculate (as a function of the record size and the record key) the exact location of any record relative to the beginning of the file. We will soon see how this facilitates immediate access to specific records, even in large files. Figure 16.10 illustrates Java s view of a random-access file composed of fixed-length records (each record in this figure is 100 bytes long). A random-access file is like a railroad train with many cars some empty, some with contents. A program can insert data in a random-access file without destroying other data in the file. Also, a program can update or delete data stored previously without rewriting the entire file. In the following sections, we explain how to create a random-access file, enter data, read the data both sequentially and randomly, update the data and delete data no longer needed. 16.8 Creating a Random-Access File RandomAccessFile objects have all the capabilities of DataInputStream and DataOutputStream objects discussed earlier. When a program associates an object of class RandomAccessFile with a file, the program reads or writes data beginning at the location in the file specified by the file-position pointer and manipulates all data as primitive data types. When writing an int value, 4 bytes are output to the file. When reading a double value, 8 bytes are input from the file. The size of the data types is guaranteed, because Java has fixed sizes for all primitive data types regardless of the computing platform. Random-access file-processing programs rarely write a single field to a file. Normally, they write one object at a time, as we show in the following examples. Consider the following problem statement: Create a transaction-processing program capable of storing up to 100 fixed-length records for a company that can have up to 100 customers. Each record should consist of an account number that will be used as the record key, a last name, a first name and a balance. The program should be able to update an account, insert a new account and delete an account.
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