Terminology
Client Application: An application that uses a class.
Interface: An interface utilizes public methods. It is not an entire class, it is a list of methods used by a class.
Encapsulation: Hides a program’s implementation details from the client application using private classes method and fields. If an object contains only private methods and object then it has no interface, very rarely would anything be designed like this but it is possible.
Byte Offset: A single records length. Must be 0 or greater, and must be of a long data type for precision.
Record Structure: The unchanging details that are generic to a set of records so that they are all the same which is integral when utilizing random access.
Binary File: Cannot be read in a text editor, requires specific file reading application.
Character File: Can be read in a simple text editor. Each char is encoded with a character set. The character set used by java is Unicode. Unicode is a 16 bit character set. The implication here is that one character is two bytes of storage in memory. Storing ‘1’ uses 2 bytes, storing ‘’35876” uses 10 bytes.
Super Class: java.lang.Object – java.io.RandomAccessFile

Code
- The StringBuffer is a useful container for a field in a record.
o For example:
o StringBuffer sb = new StringBuffer(“john”);
o sb.setLength(15);
o (Note: The four chars are in the container, aswell as 11 nulls)
o StringBuffer sb = new StringBuffer(“aaaaaaaaaaaaaaaaaaaaa”);
o Sb.setLength(15);
o (Notes: There are more chars than there is space in the StringBuffer so the first 15 chars are stored and the rest are lost)
- A StringBuffer object can be directly written to a file
o For example:
o File.writeChars(“sb.ToString());
o (Note: uses writeChar method of RandomAccessFileClass to write a StringBuffer (being changed to a String) to a file.
- The RandomAccessFile class has an interface which supports dataInput and dataOutput.
o It’s constructor is RandomAccessFile(File file, String mode)
o (Note: mode “r “, “rw”, “rws”, or “rwd”)
o Useful dataInput interface:
o RandomAccessFileClassName.writeInt(integer)
o RandomAccessFileClassName.writeDouble(double)
o RandomAccessFileClassName.writeChars(string)
- The try block is used to handle exceptions. A try block is minimally at least a try statement with catch statements, a finally statement extends the try blocks capabilities.
o Try{//Statements of concern}
o Catch(exceptionClass paramVar){//Statements to deal with exception}
o Finally{Statements that will always run, error or none}
o (Note: The only way to stop a finally block from running is to add an explicit System.exit(0); because it will tell the system to stop running this code right now.)

Information
- Java imposes no restrictions on file structure. File structure is entirely created by the programmer.
- Character fields of a record must be fixed length for precise accessibility.
o To Make a Field a Fixed Length
o Decide how many chars you will need
o (Note: Memory requires 2 bytes per char)
o Create a container for the field. The best way to do this is with the String Buffer.
o StringBuffer sb = new StringBuffer(15);
o (Note: This containers length is still 0, it’s capacity is 15)
o sb.setLength(15);
o (Note: By default nulls have been written into the file, now this container has 15 nulls in it)
- An int takes up 4 bytes in memory, a double requires 8 bytes, and a char uses 2 bytes.
- The RandomAccessFile class is not like any other file reader or file writer classes. It’s superclass is object. It has a file input stream and a file output stream. This class is unique in that it uses a filePointer to point to a location in a file. Similarly, when we open a file the filePointer automatically seeks to 0, and would read fro 0 until we told it to stop. Aswell, if we wanted to read the third record, we would move the filePointer to the zone that contains the bytes we would like to edit. It is critical to place the filePointer where we want to get our data from or the data extracted will be useless. The space to write data must be initialised or there will be leftover bytes that will ruin our file reading capabilities.
- The first line of a constructor must call to the superclass.