File IO can't handle Strings?

[Kenneth Kim]

I created a method called readFile as shown below:

public static String readFile(File file) throws IOException{
long length = file.length();
byte[] bytes = new byte[(int) length];
InputStream is = new FileInputStream(file);
is.read(bytes);
is.close();
return new String(bytes);
}

And in my GUI, the pertinent code is like this:

public void actionPerformed(ActionEvent event) {
fileName = fileTextField.getText();
if (event.getSource()==readButton)
readFile(fileName);
outputText.setText(fileString);

My question is:

a) Is my method for reading a file and converting it into a string correct?
b) Why do I get the error message readFile(java.io.File) cannot be applied to java.lang.String?

[Ernest Friedman-Hill]

a) InputStreams are for reading raw data, while Readers are for reading text. Although what you've written works OK for ASCII files, it won't work for other character sets, limiting the portability of your code. You can instead write something like

public String readFileIntoString(String filename) throws IOException {
char[] buffer = new char[100];
FileReader reader = new FileReader(filename);
try {
StringBuffer result = new StringBuffer();
int count;
while ((count = reader.read(buffer)) > 0)
result.append(buffer, 0, count);
} finally {
reader.close();
}
return result.toString();
}

b) You've declared your method to accept a java.io.File object as an argument, but you're passing a String. You need to say

readFile(new File(fileName));

[marc weber]

 

Originally posted by Kenneth Kim:
... Why do I get the error message readFile(java.io.File) cannot be applied to java.lang.String?

Your readFile method is expecting a reference to an instance of File -- not String. But you should be able to use that Sting to make a new instance of File, and then pass the File reference to your method.

[Jim Yingst]

While we're at it, the read(byte[]) method has a return value. You should not assume that the byte[] array has been filled after one read - there are several reasons why a read may be incomplete. If you want to get a complete read from an InputStream, you need to check the return value and keep reading until you're done.

Or more simply, you can use a RandomAccessFile which has a readFully() method - somewhat simpler to use.

Also, I strongly recommend putting putting the close() statement in a close() block. If an error occurs, you generally want to make sure the file gets closed anyway. This is particularly important if you try to do something else with the file afterwards, such as moving it or deleting it.

[Ilja Preuss]

 

Originally posted by Jim Yingst:

Also, I strongly recommend putting putting the close() statement in a close() block.

Jim probably meant a *finally block* here...

[Jim Yingst]

Something like that, yes... Thanks, Ilja.

[Jeff Albertson]

FYI, java.util.Scanner can be used to read the file in one step:
http://java.sun.com/developer/JDCTechTips/2004/tt1201.html#1

[Jim Yingst]

Slightly more than one step, Jeff, but it's simpler, that's true. Note that Scanner became available in JDK 5.

I also note that I overlooked the fact that EFH's code already incorporated checking the return value and using a finally block. Oh, well.