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Mr. Wortzman.  So far, we have gotten all our input and written all our output to the console  In reality, this is somewhat uncommon  Instead, we often.

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Presentation on theme: "Mr. Wortzman.  So far, we have gotten all our input and written all our output to the console  In reality, this is somewhat uncommon  Instead, we often."— Presentation transcript:

1 Mr. Wortzman

2  So far, we have gotten all our input and written all our output to the console  In reality, this is somewhat uncommon  Instead, we often use files for input and output  This has the advantage of requiring much less user interaction

3  Most of this is done in Java through the File class  File f = new File("myFile.txt");  This class has some useful methods:  exists() - returns true if the named file exists  getName() - returns the file's name  renameTo() - changes the name of the file  delete() - delete's the file from the disk  See the Java API for more

4  None of these methods can work with the contents of the file however  For that, we need to use either a Scanner (for input) or a PrintStream (for output) Scanner input = new Scanner(new File("input.txt")); PrintStream output = new PrintStream(new File("output.txt"));

5  What happens when you try to compile with the code on the previous slide?  Certain types of errors must be dealt with somehow in the program  These are considered dangerous, so Java wants to make sure you know they might occur  Many types of file errors are considered checked exceptions

6  The easiest way to "deal with" these errors is to use a throws clause public static void readInput() throws FileNotFoundException {... }  This tells Java "I know something bad might happen, and I accept the consequences if so"  Any method that calls a method that throws must also throw

7  Once we've hooked up the file, we can use Scanner and PrintStream as usual Scanner input = new Scanner(new File("input.txt")); while (input.hasNext()) { System.out.print(input.next() + " "); } String[] words = {"cat", "dog", "bird", "hedgehog"}; PrintStream output = new PrintStream("output.txt"); for (int i = 0; i < words.length; i++) { output.println(words[i]); }

8  Important notes:  Opening an existing file for output will overwrite the file ▪ You can use the exists() method of File to check for this  Never open the same file for input and output at the same time-- everything will get erased  You can walk off the end of an input file if you're not careful-- use the has methods to look before you leap

9  Files are typically written such that each line is a separate entity  Therefore, we usually read files one line at a time while(file.hasNextLine()) {... }  But each line might contain multiple tokens/values

10  Remember that we can make a Scanner from a String, so we can do something like: Scanner file = new Scanner(new File("myFile.txt")); while (file.hasNextLine()) { Scanner tokens = new Scanner(file.nextLine()); while (tokens.hasNext()) {... }

11  Exercise 1: Write a Java program to prompt the user for an input and output file, and copy the contents of the input file to the output file.  Exercise 2: Write a Java program to read a file (specified by the user) containing a list of names and scores, and print out each person's total. ▪ Wortzman ▪ Hawker ▪ K  Should print ▪ Wortzman 32 ▪ Hawker 30 ▪ K 21


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