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copied!<p>Message is one thing. A container of data.</p> <p>Validation is a <em>separate</em> thing. Validation is separate from the Message because validation depends on non-Message data. Validation includes one or more algorithms that establishes if a valid Message could be built from some data.</p> <p>You want Messages to be built <strong>only</strong> when they're valid. You want all of the the methods of a message to require and depend on validity. That gives you easy-to-understand responsibility.</p> <p>Therefore, a Message should not contain any validation logic. That logic lives just outside the boundaries of a valid Message. It's part of a <code>MessageFactory</code> or <code>MessageValiadtor</code> that determines if some given puddle of data could be used to build a valid Message.</p> <p>You have two things: <code>MessageValidator</code> (or <code>MessageFactory</code>) and <code>Message</code>.</p> <p>There is no <code>isValid</code> in a Message. It must be valid or it would not exist.</p> <p>MessageValidator builds Messages from source data. If it cannot build a message can can throw an exception. It should also accumulate the list of errors it finds so it can be interrogated to determine what the various problems were.</p> <p>Consider the possible mutations: Multiple variant subclasses of Message. The Validator may identify a specific subclass. No application changes are required to implement this.</p> <p>Multiple Validators for special situations, exceptions, extensions, etc. It's just a subclass of MessageValidator with the same interface all validators have and emits an instance of Message (or one of it's subclasses.)</p> <p>Since the validation rules are separate from a valid message, the Validator can provide different assumed defaults or derived data, also.</p>

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