File IO and Recursion

1. File IO and Recursion CS0007: Introduction to Computer Programming

2. 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

3. 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

4. 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: FileWriterfwriter = newFileWriter("names.txt", true); PrintWriteroutputFile = newPrintWriter(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

5. 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? • Answer: Windows uses backslashes as file separators. Strings in Java uses backslashes to begin escape sequences. • 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

6. 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

7. 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! • 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.

8. 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. • 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