It is very important to verify if the file is downloaded successful or not. Most of the cases we just concentrate on clicking the downloaded button. But at the same time it is also very important to confirm that file is downloaded successfully without any errors or if some other file is getting downloaded.
In most of the cases we know which file is getting downloaded after clicking on download button / link. Now when we know the file name, we can verify using java for the 'File Exists' in a downloaded folder location which we specify.