1. Basic Text File Input/Output
In Java
28 April 2015

2. CSC 102 Computer Science Principles
Introduction
The class File
Defines platform-independent methods for manipulating a native file.
Does not allow access to file contents.
I.e., it allows you to get info about a file, not to read its content.
See Java API info for class File in java.io package.
Note some of the methods.
CSC 102 Computer Science Principles

3. CSC 102 Computer Science Principles
Terminology
Stream-An object that allows for the flow of data between your program and some I/O device or some file.
Text file-A file containing ASCII data.
Binary File-A file handled as sequences of binary digits.
CSC 102 Computer Science Principles

4. Text File Input Using Scanner
Basically the same as for keyboard input except
replace System.in in the scanner constructor with the stream that is connected to the text file, or with the new File instance of the text file itself.
CSC 102 Computer Science Principles

5. Text File Input Using Scanner
What do we need to do to read a text file on disk using Scanner?
Create a File object for the text file.
Create a Scanner object that allows Scanner methods to be used on the File object created above.
Use Scanner methods as appropriate.
Test for end of a text file with hasNextInt, hasNextLine, etc., to see whether there is more input.
CSC 102 Computer Science Principles

6. Example: Read text file “any.dat”
SeeTextFileScannerExample1A.java
CSC 102 Computer Science Principles

7. Example Text File Input Using Scanner
Create a File object named “anyFile” whose physical file name is “Any.dat”.
anyFile is the logical file name, sometimes called the internal file name. This is the name used inside the program.
Any.dat is the physical file name, sometimes called the external file name. This is the name that the operating system uses for the file.
CSC 102 Computer Science Principles

8. Example Text File Input Using Scanner
Create a stream of the class Scanner. Scanner inputStream = null;
Connect the stream to a text file by creating a new Scanner object that uses the File object we already created.
inputStream= new Scanner(anyFile);
CSC 102 Computer Science Principles

9. Example: Read from a file named at execution time.
See TextFileScannerExample2.java
Notice that this program allows the programmer to specify what happens if the file can’t be opened; in this case, the programmer is going to “handle” the error. When the throws clause appears on the main header, this is called “declaring” the error.
Java requires that we pay attention to certain possible errors; this is one of them that Java requires we either “handle or declare”.
CSC 102 Computer Science Principles

10. CSC 102 Computer Science Principles
Text File Output
Construct a PrintWriter stream object. PrintWriter pw = new PrintWriter(filename);
Now, Use the print() and println() methods to write to the file.
Example: pw.println(“This will be a line of text in the file.”);
Finally, use close() to disconnect the file from the program.
CSC 102 Computer Science Principles

11. CSC 102 Computer Science Principles
Text File Output
See TextFileOutDemoSimpler.java
CSC 102 Computer Science Principles

12. CSC 102 Computer Science Principles
Text File Output
Exercises:
Use a text editor to create a file and save it, then write a program to read it and display its contents.
Convert TextFileOutDemoSimpler.java to copy one file’s contents to another.
CSC 102 Computer Science Principles

13. CSC 102 Computer Science Principles
File I/O
There are several variations on file I/O besides what is shown here . . .
CSC 102 Computer Science Principles