File IO and Recursion
CS0007: Introduction to Computer Programming
Review
 The steps taken when reading or writing to a file:
 Open the file.
 Data is written or read from a file.
 When the program is finished with the file, the file must be closed.
 Two kinds of files in input and output:
 Input File – File in which data is read from.
 Output File – File in which data is written to.
 The Java API provides a class for writing to a file called
 PrintWriter.
 The parameter supplied to the PrintWriter constructor is…
 A filename
Review
 Two methods used to write to a file:
 print
 println
 Two classes needed to read from a file:
 File
 Scanner
 Method that reads a single line from a file:
 readLine
 Method that detects if there is anything left in a file:
 hasNext
Appending to a File
 When you create a PrintWriter object by passing the file’s name as an argument to the
constructor, it will erase the contents of the file if it exists.
 What can I do if I want to retain the text already in the file and add to the end of it?
 Answer: You can append to the file
 Appending means adding to the end.
 You can append text to the end of a file by creating a FileWriter object and passing
it to the PrintWriter’s constructor:
FileWriter fwriter = new FileWriter("names.txt", true);
PrintWriter outputFile = new PrintWriter(fwriter);
 Notes:
 FileWriter throws an IOException, if you do the same steps for handling the
FileNotFoundException, but with IOException instead, you can handle
both the FileNotFoundException and the IOException.
 The second argument for the FileWriter’s constructor indicates that you want to append
to the file
 Example: Append.java
Specifying the File Path
 When you give a file’s name as an argument, you can specify
the entire path to the file if you want it to be read or written
to a speciffic location.
 For instance, if the path to a file on my desktop on the windows
machine in my office would be:
 \\ad.cs.pitt.edu\Users\Users1\eth13\Desktop\myFile.txt

If I had to supply this as a parameter, what would be the problem?
o Answer: Windows uses backslashes as file separators. Strings in
Java uses backslashes to begin escape sequences.
o Solution: Java gives you the ability to replace all backslashes with
forward slashes.
• //ad.cs.pitt.edu/Users/Users1/eth13/Desktop/myFile.txt
 Example: FilePath.java
Detecting Whether a File Exists
 The File class has a method called exists that checks to see if
the file specified by the path already exists.
 This can be used in both reading and writing to a file:
 In reading, instead of having your program end abnormally with a
runtime error, you can display an error message.
 Example: ReadExists.java
 In writing, you can warn the user if she is about to overwrite an existing
file.
 Example: WriteExists.java
Recursion
 Recursion is the process of repeating something in a self-similar way.
 The way this appears in programming is recursive methods.
 A recursive method is a method that calls itself.
 What is the problem with this: BadRecursion.java?
 This method has no way of stopping the calls to itself, so it runs forever!
o What it needs is a base case.
• A base case is a condition in which the method stops calling itself.
 So a recursive method has two parts:



A base case.
A recursive case (A case in which it calls itself)
Example: GoodRecursion.java
 Usually, the number of recursive calls (depth of recursion) is dependent on
an argument passed to a method.
 Each time a method is called, a new copy of the parameter is made for that method
call.
Recursion

Recursion is confusing…why does anyone use it?
 Good Question! The fact of the matter is that all recursive problems can be solved iteratively (with
loops), and iterative solutions are often more efficient than recursive ones.


Answer: Some problems are easily defined a solved recursively.
 If a problem can be broken down into successive smaller parts that are identical to the original problem, it is
often easy to think of their solutions as recursive ones.
o For example, consider factorial.
In general, a recursive method works like this:
 If the problem can be solved now, without recursion, then the method solves it and returns the value.
 If the problem, cannot be solved now, then the method reduces it to a smaller but similar problem
ands calls itself to solve the smaller problem.
The way you design a recursive method is to identify the recursive and base cases.
 Now you try: Factorial.java
 Many problems can be solved recursively:

 Fibonacci Numbers
 Fibonacci.java
 Towers of Hanoi
 Hanoi.java
 HanoiDemo.java