Reading and writing files

Reading and writing files

To read from a file:

• Create a FileReader passing in the name of a file. A FileReader can only read one character at a time, so not very useful!
• Create a BufferedReader passing in the file reader created in step 1.
• Loop through the file reading each line by calling readLine(). You are done reading the file when readLine returns null.
• Close the file by calling close().

Here is some code from the Short words demo that does all of these steps:

// Open the file
wordReader = new BufferedReader(new FileReader(DICTIONARY_FILE));

// get the first line of the file
words[numWords] = wordReader.readLine();
			
// Read words until we run out of words or the array becomes full
while (words[numWords] != null && numWords < MAX_WORDS) {
   // Display the word
   allWordsArea.append(words[numWords] + "\ n");
   numWords++;

   // get the next line from the file
   words[numWords] = wordReader.readLine();
}

wordReader.close();

Lots of things can go wrong working with files. For example,

• You might try to read from a file that doesn't exist
• You might try to read or write a file that you don't have the right permissions for
• You might try to write to a file when the disk is full.

Because of these problems, file operations are generally surrounded by a try statement so these exceptions (IOException) can be handled. These exceptions are a little different than other exceptions we have seen this semester. These are checked exceptions, meaning that Java requires us to handle them. If we don't we will get a compiler error.

Writing a file requires similar steps:

• Create a FileWriter passing in the name of the file
• Write a string to the file by calling write passing the string as an argument.
• Close the file by calling close().

Here is code from the ShortWords demo to write a file:

// open the stream to write to the file
FileWriter wordWriter = new FileWriter(SHORT_WORDS_FILE);

// write the fixed length words
wordWriter.write(shortWordsArea.getText());
			
// Close the file
wordWriter.close();

This code would again be within a try statement to handle the IOExceptions that might occur. See the complete example for details.

Reading and writing files