More eyes please :(

[Ryan Waggoner]

Heres the code and the error, I am not sure how to deal with. Can anyone see what is going wrong?

    
public class PersistantArray {
// Fields----
public static File file;
public static ByteBuffer bbuf;
public static FileChannel wChannel;
public static Header header;
public static void create(String fn, int blockSize, long payLoadSize) {
// Create a new header object for the new file
header = new Header(blockSize, payLoadSize);
// Create a byte buffer from the header's properties
bbuf = header.pack();
// Setting the file to the file with the name passed in
file = new File(fn);
try {
// Create a writable file channel
wChannel = new FileOutputStream(file, true).getChannel();
// Write the ByteBuffer contents
wChannel.write(bbuf);
// Set the buffer position to the begining of the buffer
bbuf.rewind();
} catch (Exception e) {
}
}
/**
 * Method used to retrieve a block from a file
 * 
 * @param id -
 *            Long used to identify which block to retrieve
 * @return a bytebuffer
 */
public ByteBuffer getBlock(int id) {
bbuf.rewind();
// ByteBuffer to hold information extracted
byte[] block = new byte[header.getBlockSize()];
// offset of where to start writing to the file
// headersize + (id * blocksize)
int position = (Header.headerSize + (id * Header.blockSize));
try {
FileInputStream fin = new FileInputStream(file.toString());
fin.read(block, position, header.getBlockSize());
 
bbuf = ByteBuffer.wrap(block);
return bbuf;
} catch (Exception exc) {
exc.printStackTrace();
}
return null;
}
 
/**
 * 
 * @param bbuf -
 *            a buffer containing the data to be entered
 * @param id -
 *            the block id to be inserted
 * @return 1 if successful, 0 if not
 */
public int putBlock(ByteBuffer bbuf, int id) {
bbuf.rewind();
try {// offset of where to start writing to the file
// headersize + (id * blocksize)
int position = (Header.headerSize + (id * header.getBlockSize()));
// Create a writable file channel
wChannel = new FileOutputStream(file, true).getChannel();
// Write the ByteBuffer contents
wChannel.write(bbuf, position);
// Set the buffer position to the begining of the buffer
bbuf.rewind();
} catch (Exception e) {
return 0;
}
return 1;
}

And the error...

java.lang.IndexOutOfBoundsException
at java.io.FileInputStream.readBytes(Native Method)
at java.io.FileInputStream.read(FileInputStream.java:194)
at neumont.edu.blocks.PersistantArray.getBlock(PersistantArray.java:94)
at neumont.edu.junit.PersistantArrayTest.testGetBlock(PersistantArrayTest.java:43)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at org.eclipse.jdt.internal.junit.runner.junit3.JUnit3TestReference.run(JUnit3TestReference.java:130)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:460)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:673)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:386)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:196)
 

I am doing the create, then running putBlock, and then when I try to do the getBlock, I am getting that error.

[Tony Docherty]

fin.read(block, position, header.getBlockSize());

This line reads 'header.getBlockSize()' number of bytes into the 'block' byte array starting at index 'position'. Now given that the 'block' array was created at the size of 'header.getBlockSize()' you should be able to see that if the starting index is anything other than 0 the 'block' array isn't going to be big enough for all the values.

[Ryan Waggoner]

Tony,
I'm sorry I don't quite see what you mean.

I was trying to create the block array to the block size.
Then i was trying to read in the amount of a block size, into the block array, starting at that position.

[Joanne Neal]

If you try and read 10 bytes into a byte array of size 10 starting at position zero, then you will fill positions 0 to 9. No problem

If you try and read 10 bytes into a byte array of size 10 starting at position 5, then you will fill positions 5 to 14. Unfortunately you don't have positions 10 to 15, so you get an out of bounds exception.

Your array is header.getBlockSize() bytes long. You are trying to read 'header.getBlockSize() bytes into it. So, unless Header.headerSize + (id * Header.blockSize) evaluates to zero, you will get an out of bounds exception.

[Ryan Waggoner]

Ah!

I think I understand that, thanks a lot to the both of you!