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The Java classes that read Zip files, e.g. ZipInputStream, do not support decrypting password-protected Zip files. I don't know of any publicly-available code in Java which will do this, either. But this functionality is straightforward to add yourself. The decryption algorithm needed is described in ftp://ftp.pkware.com/appnote.zip. You can subclass the existing Java classes to add support for decryption; you will also have to define a mechanism for a user of your subclass to enter the appropriate password, using a dialog box for example. (And if you do so, please make that code publically available and submit a link to that code as feedback to this question!)
Java Developer, Thailand