CS 111A Lecture Notes - File Input and Output - Sections 8.1 - 8.2

File Output

All of the classes we use for file i/o require that we import java.io.*
File - a built-in Java class for accessing computer files. Has methods "exists", "delete" and "renameTo"
PrintWriter - a built-in Java class for outputting text to a file. When you call the PrintWriter constructor with a File object, you are creating that file on the disk for output (replacing any file of the same name which may already exist).

The above 2 classes are put together to send output to a file, for example:

    // open the file "outfile.txt" and connect pStream to it for output:
    PrintWriter pStream = new PrintWriter(
                            new File("outfile.txt"));
    // Now we can send output to the file:
    pStream.println("Hello File");
    pStream.close();  // and close it.

To help you understand how these 2 classes work together, think of the analogy of a pen writing on paper. The File object is like the piece of paper. The PrintWriter object is like the pen - it connects with the paper to write, or can write on other surfaces (such as the screen or a network connection).
The code to open and work with files may generate run-time errors (Exceptions) so we need to put the following at the end of the method header: throws IOException
To increase efficiency, output to the file isn't written with every print and println call - it's saved up and output in larger chunks. To force all output to be completed, the output needs to be flushed, using the flush() method. The method close() also flushes the output.
Example: GradeInput.java: demonstrates file output with formatting

File Input

To read input from a file, we link a Scanner to the File object. Here is how it looks:

    String lineIn;
    // open the file "infile.txt" and connect iStream to it for output:
    Scanner iStream = new Scanner(new File("infile.txt"));

    // Now we can get input from the file:
    lineIn = iStream.nextLine();

    iStream.close();  // and close it.

As before, we need to write: import java.io.* and: throws IOException
When the end of a file is reached, the hasNext() method returns false.

Examples

JunkMail.java: demonstrates file i/o
In-class exercise: Write a program to average all numbers from a file, and output the resulting average into a new file.
Average.java: solution to above exercise.

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