Simple Java I O Part I General Principles Jan 14 2019 Prologue They say you can hold seven plus or minus two pieces of information in your mind I can t remember how to open files in Java I ve written chapters on it I ve done it a bunch of times but it s too many steps And when I actually analyze it I realize these are just silly design decisions that they made Even if they insisted on using the Decorator pattern in java io they should have had a convenience constructor for opening files simply Because we open files all the time but nobody can remember how It is too much information to hold in your mind Bruce Eckel http www artima com intv aboutme2 html 2 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 3 How to do I O import java io Open the stream Use the stream read write or both Close the stream 4 Why Java I O is hard open use close 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 5 Opening a stream open use close 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 6 Example of opening a stream open use close 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 7 Using a stream open use close 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 8 Example of using a stream open use close 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 9 Manipulating the input data open use close 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 10 Reading lines open use close String s s bufferedReader readLine A BufferedReader will return null if there is nothing more to read 11 Closing open use close 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 12 Simple Java I O Part II LineReader and LineWriter Jan 14 2019 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 14 My LineReader class class LineReader BufferedReader bufferedReader LineReader String fileName String readLine void close 15 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 16 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 17 readLine String readLine try return bufferedReader readLine catch IOException e e printStackTrace return null 18 close void close try bufferedReader close catch IOException e 19 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 20 The LineWriter class class LineWriter PrintWriter printWriter LineWriter String fileName void writeLine String line void close 21 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 22 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 23 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 24 writeLine void writeLine String line printWriter println line 25 close void close printWriter flush try printWriter close catch Exception e 26 Simple Java I O Part III FileDialogs Jan 14 2019 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 28 Typical FileDialog window 29 FileDialog constructors FileDialog Frame f FileDialog Frame f String title Creates a FileDialog attached to Frame f 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
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