I would like to check if a key exists in a given bucket using Java. I looked at the API but there aren't any methods that are useful. I tried to use getObject but it threw an exception.
Use the jets3t library. Its a lot more easier and robust than the AWS sdk. Using this library you can call, s3service.getObjectDetails(). This will check and retrieve only the details of the object (not the contents) of the object. It will throw a 404 if the object is missing. So you can catch that exception and deal with it in your app.
But in order for this to work, you will need to have ListBucket access for the user on that bucket. Just GetObject access will not work. The reason being, Amazon will prevent you from checking for the presence of the key if you dont have ListBucket access. Just knowing whether a key is present or not, will also suffice for malicious users in some cases. Hence unless they have ListBucket access they will not be able to do so.