Chapter 12 Using data files

The concept of file

Logically cohesive data stored on a permanent storage, such as hard disk (most cases), CD, USB.

Types of data files

• object (.obj)

• executable (.exe)

• document (.doc)

• text (.txt): A sequence of characters, including special characters such as line break, end of file. A (Java) source code can be viewed as a text file.

Text files vs. strings

• Although text files and strings both contain character data, it is important to keep in mind the following differences:

1. The information stored in a file is permanent. The value of a string variable persists only as long as the variable does. Local variables disappear when the method returns, and instance variables disappear when the object goes away. Information stored in a file exists until the file is deleted.

2. Files are usually read/written sequentially. When you read/write data from/to a file, you usually start at the beginning and read/write the characters in order.

Reading a text file

• Include Java.io package

import java.io.*;• Open a file by constructing an object that is

ultimately an instance of the FileReader class

FileReader rd = new FileReader(“Hamlet.txt”);Object rd is associated with the file Hamlet.txt.

The FileReader class offers a low-level set of methods that supports reading characters from the file one at a time. (inconvenient)

Reading a text file (cont.)

• Construct a higher level class from any reader. Usually you call both constructors in the same declaration.BufferedReader rd = new BufferedReader (new FileReader(“Hamlet.txt”));

• Read a line into a buffer (string variable)

String line = rd.readLine();A file is accessed sequentially. After a read, the current

position is updated. The next read/write starts from the new position.

• Close the file. Finish reading.rd.close();

Writing a text file

• Include Java.io package

import java.io.*;

• Open a file for writingPrintWriter wr = = new PrintWriter(new FileWriter(“Hello.txt”));

Object wr is associated with the file Hello.txt.

• Write a line to the file

wr.println(“Hello, world”);A file is accessed sequentially. After a write, the current position is

updated. Next read/write starts from the new position.

• Close the file. Finish writing.

wr.close();

Exception handling

• Unfortunately, the process of reading a file is not as simple as the previous slides suggest. When you work with the classes in the java.io package, you must ordinarily indicate what to do if an operation fails. In the case of opening a file, for example, you need to specify what the program should do if the requested file does not exist.

• Java’s library classes often respond to such conditions by throwing an exception. If the FileReader constructor, for example, cannot find the requested file, it throws an IOException to signal that fact.

• When Java throws an exception, it stops whatever it is doing and looks back through its execution history if any method has indicated an interest in “catching” that exception by including a try statement.

The try statement

• Java uses the try statement to indicate an interest in catching an exception. In its simplest form, the try statement syntax is

try { code in which an exception might occur } catch (type identifier) { code to respond to the exception }where type is the name of some exception class and identifier is the name of avariable used to hold the exception itself.• The range of statements in which the exception can be caught includes not

only the statements explicitly enclosed in the try body but also any methods those statements call. If an exception occurs inside some other method, any subsequent stack frames are removed until control returns to the try statement itself.

The End