14

Since ByteArrayOutputStream simply writes to memory, an IOException should never occur. However, because of the contract of the OutputStream interface, all stream operations define IOException in their throws clause.

What is the correct way to "handle" this never-occurring IOException? Simply wrap operations in an empty try-catch block?

Or are there any actual situations where ByteArrayOutputStream could throw an exception?

(See also: How can I handle an IOException which I know can never be thrown, in a safe and readable manner?)

EDIT

As Jon points out, ByteArrayOutputStream doesn't declare a throws clause on the write methods it defines -- however, it inherits write(byte[]) from OutputStream, and that one does throw IOEXception (quite odd that BAOS wouldn't override this method, as it could replace the superclass version -- which writes one byte at a time -- with a far more efficient arraycopy call)

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6 Answers 6

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12

Well, ByteArrayOutputStream doesn't declare that any of its methods throw IOException except writeTo and close. (I don't know why close still declares it, to be honest.)

If you've got a reference of type OutputStream though, you would still see the throws declarations from that, of course.

I wouldn't use an empty catch block - I'd throw something like IllegalStateException or a similar unchecked exception: it means you're in a situation you really don't expect, and something's gone badly wrong.

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11 Comments

the IOException on close() must be a mistake - especially when the javadoc says it "has no effect"
+1 Thanks! I just noticed that ByteArrayOutputStream.write doesn't in fact declare IOException -- but Eclipse complains about an unhandled exception whenever I use it... strange.
@Jen: Are you sure you're calling methods on a variable declared as ByteArrayOutputStream?
@Jon: I'm calling write(byte[]) from within a subclass of ByteArrayOutputStream. BAOS does not override this method, so I'm actually the method defined in OutputStream (which does throw an IOE)
@Jen: Ah, right. That makes sense. You could always just call write(data, 0, data.length) yourself.
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4

Since Java 11, there is also a new method ByteArrayOutputStream.writeBytes(byte[]) which does not throw an IOException as well:

/**
 * Writes the complete contents of the specified byte array
 * to this {@code ByteArrayOutputStream}.
 *
 * ...
 *
 * @since   11
 */
public void writeBytes(byte b[]) {
    write(b, 0, b.length);
}

You could use this method if you don't want to handle an IOException which is never thrown.

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1 Comment

I could see in Java 8 a method write(byte b[], int off, int len) which is what the newly introduced syntax also makes a call to.. Point being, the existing method also didn't throw an IOException as well.
2

I just noticed that ByteArrayOutputStream.write doesn't in fact declare IOException -- but Eclipse complains about an unhandled exception whenever I use it... strange.

That's easy to explain. You've probably done something like this:

    OutputStream os = new ByteArrayOutputStream();
    ...
    os.write();

The "problem" is that you are calling the method as OutputStream.write() rather than as ByteArrayOutputStream.write(). So the compiler says:

"Ah ... write() on an OutputStream can throw an IOException, so you gotta deal with it."

It cannot say:

"This particular OutputStream is really a ByteArrayOutputStream ... so we'll let you off."

because the JLS doesn't allow it.

This is one of those edge cases where following "best practice" by coding to the interface rather than the implementation class comes back to bite you.

OK so ...

  • its a gentle nip, not a full-on bite.
  • OutputStream is implemented as a Java class not a Java interface, but that is beside the point.
  • most compilers don't actually hold conversations with you while compiling your code :-)

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2 Comments

In general you're right. However ByteArrayOutputStream unintelligently redeclares close() doing nothing and throwing IOException so in this case the reference type doesn't matter (OpenJDK).
Thanks, great explanation. I found out what's going on. I'm calling write(byte[]) from within a subclass of ByteArrayOutputStream. BAOS does not override this method, so I'm actually the method defined in OutputStream (which does throw an IOE)
1

A typical cliche is to throw new RuntimeException(theIOException) in the catch block. If the impossible happens, you at least find out about it.

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1

Exception chaining is the best practice in this situation. i.e throw a RuntimeException.

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1

There is an enhancement ticket for this issue since 2002. The reason why this does not get fixed, is that it would impact compatibility with previous java versions.

Here follow 2 workarounds that I would consider.

Workaround 1

The write(byte[], int, int) method does not throw checked exceptions. It is a bit more verbose to specify the 2 additional parameters. But all by all the footprint is smaller without the try-catch.

baos.write(array, 0, array.length);

Workaround 2

Another possible solution, is to write your own ByteUtil class, which catches the exception internally.

public final class ByteUtil
{
  public static void write(ByteArrayOutputStream baos, byte[] bytes)
  {
    try
    {
      baos.write(bytes);
    }
    catch (IOException e)
    {
      // impossible
    }
  }
}

// usage
ByteUtil.write(baos, bytes);
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