1. Very Brief Introduction to Java I/O with Buffered Reader and Buffered Writer

2. Introductory Points Java I/O is primarily “stream-oriented.”  Data flows into the program and out of the program, like a ‘stream.’  We have ‘input’ streams and ‘output’ streams. “Standard” I/O streams:  System.in – defaults to the keyboard  System.out – defaults to the monitor  System.err – defaults to where System.out is: monitor. These are all public and static and found in the System class of the Java API.

3. Standard I/O We’ve used standard I/O in creating Scanner objects to process data normally associated with standard input (keyboard) and standard output (monitor). “I/O exceptions” are handled for us by Scanner.  Many ‘opportunity for problems in doing any kind of input / output: Examples include files not defined; file not found; formats not what were expected, EOF early, etc. and other exceptions.  I/O in Java can be very complicated. We oftentimes need to be able to read text data or pure binary data. These can come from external files, memory, or from strings.  Here, we will emphasize input/output as pertains to external files.

4. The java I/O package The java.io package provides us classes that let us define desired input and output streams.  Because we use the classes themselves (as in class.method) the methods we use are public and static. Recall: static methods don’t need to be part of an object to use them, as in the Math class – recall: Math.sqrt() (Math is the class; sqrt() is method and we simply say, Math.sqrt()…) We did not have to instantiate an object of type Math in order to access the method! (note capitalized class name followed by member name) Recall: in die1.roll(), die1 was an object; hence object.method() call…. Some classes provide for buffering (manipulate data in stream itself.)  Like, we can change format and more (ahead) What we do is to combine a number of these classes to lock in to exactly what we want to do our input/output. The general topic of java I/O is huge and we will cover only what we need here.

5. The java.io package To restate: so many I/O operations can cause problems and `throw’ an “I/O Exception” when we are trying to do I/O operations such as read() or write() operations. We must either Use a try…catch combination:  ‘catch’ the exception in a ‘try’ block and process it (use a try() … catch() sequence) or Recognize that any method that may catch a problem must have a ‘throws’ clause in the header of the method where the I/O is attempted. For now, we will elect this approach. At this time, it is simpler…

6. Very Simplified I/O – for now We will just use: throws IOException clause for File I/O. For much more, see Chapter 10 slides.

7. Code – for Input import java.io.*; // You need this to access the classes cited below. public static void main (String[ ] args) throws IOException. //you need the “throws.” Why?? You must also // 1. download (right click, Save Target As) your input file (from my web page), and //2. put it (drag it) into your project folder so the program can find it and read lines from it. FileInputStream fis1 = new FileInputStream(“Countries.txt"); // much more in Chapter 10 BufferedReader br1 = new BufferedReader(new InputStreamReader(fis1)); // for an input file to be read// // or // FileReader fr = new FileReader(“Countries.txt”); // BufferedReader br = new BufferedReader (fr); String inputString; String countryName String countryWhatever…. // additional attributes here… inputString = br1.readLine(); //reads one complete line (record) from the input file into inputString. // Now you need to get to the parts of the inputString. We call this ‘parsing’ the input. Consider: while (inputString != null) // looks at entire string you just read into inputString { countryName = inputString.substring(0, 15).trim() //read country name countryWhatever = inputString.substring(15, 30), //read country capital countryPopulatiion = Integer.parseInt(inputString.substring(55, 60)); //read country code # // Note: these are not the precise character positions in ‘your’ file. // You will need to check appropriate attribute field lengths. // Look in your book about substring method. Echo print everything to verify!!! // inputString = br1.readLine(); // read next input line. } //end while loop br1.close(); //Close input file being read

8. FileReader fr = new FileReader(“Countries.Small.txt”); BufferedReader br = new BufferedReader (fr); // Sample names made up to show how the parsing works: String inputString; String someName, someCapital; // but declare one attribute per line. Int somePop; // Note: this has nothing to do with creating objects or the array. // Only ensures you are able to access the individual attributes from each input line. // Once you verify all this, you can comment out the print lines. inputString = br1.readLine(); //reads one complete line (record) from the input file into inputString. // Now you need to get to the parts of the inputString. We call this ‘parsing’ the input. Consider: while (inputString != null) // looks at entire string you just read into inputString { someName = inputString.substring(0, 15).trim() //read country name // echo print to verify you ‘read in and parse what you think you did. System.out.println (“ some Name read in was: “ + someName); someCapital = inputString.substring(15, 30), //read country capital System.out.println (“some Capital read in was: “ + someCapital); somePop = Integer.parseInt(inputString.substring(55, 62)); //read country code # // Statements like these are used to extract specific positions from the input record. // Note: these are not the precise character positions in ‘your’ file. // Look in your book about substring method. Echo print everything to verify!!! …. Other code (later: create object and move into array….Later!) // inputString = br1.readLine(); // read next input line. } //end while loop With a few more comments:

9. Code – for Output Must include up top: import java.io.*; // this lets us use the classes below. Alter your method header as follows: //much more in Chapter 10…. public static void main (String[ ] args) throws IOException. // you need this! Sample code: (from book) String file = “test.dat”; // name your output file FileWriter fw = new FileWriter (file); // creates object fw of type FileWriter. BufferedWriter bw = new BufferedWriter (fw); // creates object bw of type... PrintWriter outFile = new PrintWriter (bw); // creates object outFile of type.. …. outFile.print (value + “ “);// writes this as a stream. Try it! outFile.println (); // prints a blank line in the file … outFile.close();// at end, close the file This writes to an output file.

10. More Later In using a standard IDE such as NetBeans, your input file needs to be in your project package so your program can find it. Be certain to move it (drag it) into your package and test accessing the file from your package before you zip up your project and send it to me.