Simple Java I O Part I General Principles Streams All modern I O is stream based A stream is a connection to a source of data or to a destination for data sometimes both An input stream may be associated with the keyboard An input stream or an output stream may be associated with a file Different streams have different characteristics A file has a definite length and therefore an end Keyboard input has no specific end How to do I O import java io Open the stream Use the stream read write or both Close the stream open use close Why Java I O is hard Java I O is very powerful with an overwhelming number of options Any given kind of I O is not particularly difficult The trick is to find your way through the maze of possibilities open use close Opening a stream There is data external to your program that you want to get or you want to put data somewhere outside your program When you open a stream you are making a connection to that external place Once the connection is made you forget about the external place and just use the stream open use close Example of opening a stream A FileReader is a used to connect to a file that will be used for input FileReader fileReader new FileReader fileName The fileName specifies where the external file is to be found You never use fileName again instead you use fileReader open use close Using a stream Some streams can be used only for input others only for output still others for both Using a stream means doing input from it or output to it But it s not usually that simple you need to manipulate the data in some way as it comes in or goes out open use close Example of using a stream int ch ch fileReader read The fileReader read method reads one character and returns it as an integer or 1 if there are no more characters to read The meaning of the integer depends on the file encoding ASCII Unicode other open use close Manipulating the input data Reading characters as integers isn t usually what you want to do A BufferedReader will convert integers to characters it can also read whole lines The constructor for BufferedReader takes a FileReader parameter BufferedReader bufferedReader new BufferedReader fileReader open use close Reading lines String s s bufferedReader readLine A BufferedReader will return null if there is nothing more to read open use close Closing A stream is an expensive resource There is a limit on the number of streams that you can have open at one time You should not have more than one stream open on the same file You must close a stream before you can open it again Always close your streams Simple Java I O Part II LineReader and LineWriter Text files Text txt files are the simplest kind of files text files can be used by many different programs Formatted text files such as doc files also contain binary formatting information Only programs that know the secret code can make sense of formatted text files Compilers in general work only with text My LineReader class class LineReader BufferedReader bufferedReader LineReader String fileName String readLine void close Basics of the LineReader constructor Create a FileReader for the named file FileReader fileReader new FileReader fileName Use it as input to a BufferedReader BufferedReader bufferedReader new BufferedReader fileReader Use the BufferedReader but first we need to catch possible Exceptions The full LineReader constructor LineReader String fileName FileReader fileReader null try fileReader new FileReader fileName catch FileNotFoundException e System err println LineReader can t find input file fileName e printStackTrace bufferedReader new BufferedReader fileReader readLine String readLine try return bufferedReader readLine catch IOException e e printStackTrace return null close void close try bufferedReader close catch IOException e How did I figure that out I wanted to read lines from a file I found a readLine method in the BufferedReader class The constructor for BufferedReader takes a Reader as an argument An InputStreamReader is a kind of Reader A FileReader is a kind of InputStreamReader The LineWriter class class LineWriter PrintWriter printWriter LineWriter String fileName void writeLine String line void close The constructor for LineWriter LineWriter String fileName try printWriter new PrintWriter new FileOutputStream fileName true catch Exception e System err println LineWriter can t use output file fileName Flushing the buffer When you put information into a buffered output stream it goes into a buffer The buffer may not be written out right away If your program crashes you may not know how far it got before it crashed Flushing the buffer is forcing the information to be written out PrintWriter Buffers are automatically flushed when the program ends normally Usually it is your responsibility to flush buffers if the program does not end normally PrintWriter can do the flushing for you public PrintWriter OutputStream out boolean autoFlush writeLine void writeLine String line printWriter println line close void close printWriter flush try printWriter close catch Exception e Simple Java I O Part III FileDialogs About FileDialogs The FileDialog class displays a window from which the user can select a file The FileDialog window is modal the application cannot continue until it is closed Only applications not applets can use a FileDialog only applications can access files Every FileDialog window is associated with a Frame Typical FileDialog window FileDialog constructors FileDialog Frame f Creates a FileDialog attached to Frame f FileDialog Frame f String title Creates a FileDialog attached to Frame f with the given title FileDialog Frame f String title int type Creates a FileDialog attached to Frame f with the given title the type can be either FileDialog LOAD or FileDialog SAVE Useful FileDialog methods I String getDirectory Returns the selected directory String getFile Returns the name of the currently selected file or null if no file is selected int getMode Returns either FileDialog LOAD or FileDialog SAVE depending on what the dialog is being used for Useful FileDialog methods II void setDirectory String directory Changes the current directory to directory void setFile String fileName Changes the current file to fileName void setMode int mode Sets the mode to either FileDialog LOAD or FileDialog SAVE Using a FileDialog Using a FileDialog isn t difficult but it is lengthy See my LineReader class in the Encryption assignment for a complete example Simple Java I O Part IV
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