File Input output using Loops Frequently a program will require more input data than can simply be input by a user at the keyboard Data files can be prepared ahead of time using a text editor stored on disk and accessed via an executing program Also instead of writing output to the screen a program can direct that that output be stored in a file and saved to disk There are many classes provided by Java that help us read from write to files The simplest are classes Scanner and PrintWriter Before we cover these classes in detail we need to review some terms and concepts involved with files Binary and Text Files A binary file stores data in the format that it is stored in memory in an internal format A text file stores data as ASCII values in a human readable format When data is stored in binary format it does not need to be translated to or from ASCII and therefore data transfer is more efficient and optimized Stream I O This refers to the manner in which data is stored in or output to a file With stream I O data is read as a series of continuous characters or bytes This is how input from the keyboard is read Two classes FileInputStream and FileOutputStream are used to access streams of bytes Classes Reader and Writer and their descendents FileReader and PrintWriter are used to access streams of characters We will learn to use class PrintWriter to write data to an output file and class FileReader to allow Scanner to read input from a file Buffered Stream rather than read one character byte at a time from the input stream or write one character byte at a time to the output stream we can create a buffered stream in memory whereby a block of stream data is written to or read from the buffer before any actual access to the input output device occurs We can think of the buffer as a container in memory into which our data is written When the buffer is full it s contents are written to the device This allows multiple reads writes to occur from the buffer between actual accesses to the device thus speeding up the process of I O again Reading input from a File To read data from a file the programmer needs to have an understanding of the format of the data stored in the file and its meaning Often each physical line will contain multiple values which together form a logical record A logical record is a set of values which together describe one unique entity in our problem space For example if we have a data file containing information about employees and their hours worked it might look like the following Smith John 37 5 7 35 s b Lewis Larry 45 0 5 50 s n Jones James 55 0 8 20 m b In this example the first string represents the employee s last name followed by first name number of hours worked hourly wage whether single or married and whether or not they are benefits eligible So the first line would translate into John Smith who worked 37 5 hours at 7 35 per hour single and is benefits eligible We would write our program to read and process the data one line at a time Class Scanner offers many methods that you can use with loops to read multiple lines of data from a data file In addition to the methods that accomplish input nextInt nextFloat etcetera that would be used to access individual values Class Scanner offers others that manipulate and test the contents of a file so that we do not attempt to read beyond the end of the physical file these include o hasNext This method returns true if the data file has more data of any type that has not yet been read o hasNextInt returns true if the next value in the data file can be interpreted as an Integer o hasNexttFloat returns true if the next value in the data file can be interpreted as an floating point number o hasNextBoolean returns true if the next value in the data file can be interpreted as a Boolean value o hasNextLine returns true if there is another line of input in the file Using one of these methods as we can create a loop to read data from a file until all of that data is exhausted For example if we have defined a Scanner instance called console the logic of our program would look like while console hasNext READ THE DATA FROM THAT LINE PROCESS THE DATA We could also use the method hasNextLine To allow Scanner to access a physical file we must use class FileReader Instances of this class are used to create a connection between out program and a physical file Scanner infile new Scanner new FileReader c mydata txt Our Program mydata txt Any methods from Scanner that are invoked for infile are now applied to the data file rather than the console In the previous example the instance of FileReader is created when we create the instance of scanner there are other ways FileReader name new FileReader path and file name We can define instance of FileReader separately the filename given may or may not include the full path if only a file name is given ie mydata txt then the file is assumed to be in the current working directory You will notice something unusual in the first example which included a path name The character is a control character in Java as in n which is the new line character If it is part of a path name then we must use two So for example c cs110 inputdata mydata txt Once the instance of FileReader is declared we can then use it in the declaration of our scanner FileReader inputFile new FileReader mydata txt Scanner infile new Scanner inputFile Please Note when declaring an instance of FileReader the file name can also be specified using a string variable instead of a quoted literal Which means it is possible to prompt the user for the name of the input file read it into a string variable and then create our connection to the file String filename Scanner console new Scanner System in System out println Please enter the name of the file to be read filename console nextLine FileReader inputFile new FileReader filename Scanner input new Scanner inputFile There is a possible error condition for which we must account in case the file we are accessing does not exist This is a file not found exception and this causes Java to raise or throw an IOException What does that mean anyway There are two types of exceptions thrown by Java methods checked and unchecked An exception is an error that occurs during the execution of a program When you call a method that throws a checked exception the compiler checks to see that you don t ignore it Checked exceptions are caused by external circumstances that the programmer can not control or prevent