Let's imagine a situation where we want to write a pure Java application that must download files from a remote computer running an FTP server. We also want to filter downloads on the basis of remote file information like name, date, or size.

Although it is possible, and maybe fun, to write a protocol handler for FTP from scratch, doing so is also hard, long, and potentially risky. Since we'd rather not spend the time, effort, or money writing a handler on our own, we prefer instead reusing an existing software component. And plenty of libraries are available on the World Wide Web. With an FTP client library, downloading a file can be written in Java as simply as:

FTPClient ftpClient = new FTPClient();
ftpClient.connect("ftp.foo.com", "user01", "pass1234");
ftpClient.download("C:\\Temp\\", "README.txt");
// Eventually other operations here ...