implements Closeable or implements AutoCloseable

AutoCloseable (introduced in Java 7) makes it possible to use the try-with-resources idiom:

public class MyResource implements AutoCloseable {    public void close() throws Exception {        System.out.println("Closing!");    }}

Now you can say:

try (MyResource res = new MyResource()) {    // use resource here}

and JVM will call close() automatically for you.

Closeable is an older interface. For some reason To preserve backward compatibility, language designers decided to create a separate one. This allows not only all Closeable classes (like streams throwing IOException) to be used in try-with-resources, but also allows throwing more general checked exceptions from close().

When in doubt, use AutoCloseable, users of your class will be grateful.

Closeable extends AutoCloseable, and is specifically dedicated to IO streams: it throws IOException instead of Exception, and is idempotent, whereas AutoCloseable doesn't provide this guarantee.

This is all explained in the javadoc of both interfaces.

Implementing AutoCloseable (or Closeable) allows a class to be used as a resource of the try-with-resources construct introduced in Java 7, which allows closing such resources automatically at the end of a block, without having to add a finally block which closes the resource explicitly.

Your class doesn't represent a closeable resource, and there's absolutely no point in implementing this interface: an IOTest can't be closed. It shouldn't even be possible to instantiate it, since it doesn't have any instance method. Remember that implementing an interface means that there is a is-a relationship between the class and the interface. You have no such relationship here.

It seems to me that you are not very familiar with interfaces. In the code you have posted, you don't need to implement AutoCloseable.

You only have to (or should) implement Closeable or AutoCloseable if you are about to implement your own PrintWriter, which handles files or any other resources which needs to be closed.

In your implementation, it is enough to call pw.close(). You should do this in a finally block:

PrintWriter pw = null;try {   File file = new File("C:\\test.txt");   pw = new PrintWriter(file);} catch (IOException e) {   System.out.println("bad things happen");} finally {   if (pw != null) {      try {         pw.close();      } catch (IOException e) {      }   }}

The code above is Java 6 related. In Java 7 this can be done more elegantly (see this answer).