File Handling in Java

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File Handling
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Read Characters from a File
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Classes javaioFileReader Methods int read( ) void close( )
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Although byte streams are technically sufficient to handle all file input tasks (because all files can be treated as a stream of bytes), Java offers a better approach when operating on character data: the character streams The character-based streams operate directly on objects of type char (rather than bytes) Therefore, when operating on files that contain text, the character-based streams are often the best choice To read characters from a file, you can use FileReader FileReader is derived from InputStreamReader and Reader (InputStreamReader provides the mechanism that translates bytes into characters) FileReader implements the Closeable and Readable interfaces (Readable is packaged in javalang and defines an object that supplies characters via the read( ) method) When reading a file via FileReader, the translation of bytes into characters is handled automatically
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To read a file using FileReader involves three main steps: 1 Open the file using FileReader 2 Read from the file using the read( ) method 3 Close the file by calling close( )
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To open a file, you simply create a FileReader object FileReader defines three constructors The one we will use is FileReader(String leName) throws FileNotFoundException Here, fileName is the name of a file It throws a FileNotFoundException if the file does not exist To read a character from the file, you can use this version of read( ) (inherited from Reader): int read( ) throws IOException Each time that it is called, it reads a single character from the file and returns the character as an integer value read( ) returns 1 when the end of the file is encountered It will throw an IOException if an I/O error occurs Other versions of read( ) can input multiple characters at a time and put them into an array When you are done with the file, you must close it by calling close( ) It is shown here: void close( ) throws IOException If an error occurs when attempting to close the file, an IOException is thrown
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Example
The following program uses FileReader to input and display the contents of a text file, one character at a time
// // // // // // // // // Use a FileReader to display a text file To use this program, specify the name of the file that you want to see For example, to see a file called Testtxt, use the following command line java ShowFile Testtxt
import javaio*; class ShowFile { public static void main(String args[]) { FileReader fr; // First make sure that a file has been specified if(argslength != 1) { Systemoutprintln("Usage: ShowFile file"); return; } try { // Open the file fr = new FileReader(args[0]); } catch(FileNotFoundException exc) { Systemoutprintln("File Not Found"); return; } // At this point, the file is open and // its contents can be read try { int ch; // Read the file one character at a time do { ch = frread(); if(ch != -1) Systemoutprint((char)ch); } while(ch != -1); } catch(IOException exc) { Systemoutprintln("Error Reading File"); }
3:
File Handling
try { frclose(); } catch(IOException exc) { Systemoutprintln("Error closing File"); } } }
Options and Alternatives
There are three other forms of read( ) available to FileReader The first is int read(char buf [ ]) throws IOException This method fills the array referred to by buf with characters read from the file It returns the number of characters actually read, which might be less than buflength if the end of the file is encountered Attempting to read at end-of-file causes read( ) to return 1 For example, in the preceding program, here is another way to write the portion of code that reads and displays the characters in the file:
try { int count; char chrs[] = new char[80]; // Read the file one buffer at a time do { count = frread(chrs); for(int i=0; i < count; i++) Systemoutprint(chrs[i]); } while(count != -1); } catch(IOException exc) { Systemoutprintln("Error Reading File"); }
This approach creates a 80-character array and uses it to read up to 80 characters with each call to read( ) This is more efficient than performing 80 separate read operations, as the example does The next form of read( ) is shown here: int read(char buf[ ], int startIdx, int num) throws IOException This version reads num characters from the file and stores them in buf, beginning at the index specified by startIdx It returns the number of characters actually read, which might be less than num if the end of the file is encountered Attempting to read at end-of-file causes read( ) to return 1 The final version of read( ) is shown here: int read(CharBuffer buf) throws IOException
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