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    <p>A clean way to approach this is to use a <strong>visitor</strong> pattern to <strong>keep your encoding implementation separate from your business objects</strong>. Some people will argue that you can simply implement the <code>Externalizable</code> interface along with <code>readExternal</code> / <code>writeExternal</code> but this has the problems that:</p> <ul> <li>Your protocol is embededded within your business object, meaning it is distributed across your codebase rather than being in one place.</li> <li>Your application cannot support multiple protocols (as offered by Google Protocol Buffers).</li> </ul> <p><strong>Example</strong></p> <pre><code>/** * Our visitor definition. Includes a visit method for each * object it is capable of encoding. */ public interface Encoder { void visitAnimal(Animal a); void visitVegetable(Vegetable v); void visitMineral(Mineral m); } /** * Interface to be implemented by each class that can be encoded. */ public interface Encodable { void applyEncoder(Encoder e); } public class Animal implements Encodable { public void applyEncoder(Encoder e) { // Make call back to encoder to encode this particular Animal. // Different encoder implementations can be passed to an Animal // *without* it caring. e.visitAnimal(this); } } </code></pre> <p>Typically one would then define a stateful <code>Encoder</code> implementation that would "push" each object to an <code>OutputStream</code> when its <code>visitXXX</code> method is called; e.g.</p> <pre><code>public class EncoderImpl implements Encoder { private final DataOutputStream daos; public EncoderImpl(File file) throws IOException { this.daos = new DataOutputStream(new BufferedOutputStream(new FileOutputStream(file))); } public void visitAnimal(Animal a) { daos.writeInt(a.getWeight()); daos.writeUTF(a.getName()); // Write the number of children followed by an encoding of each child animal. // This allows for easy decoding. daos.writeInt(a.getChildren().size()); for (Animal child : a.getChildren()) { visitAnimal(child); } } // TODO: Implement other visitXXX methods. /** * Called after visiting each object that requires serializing. */ public void done() { daos.close(); } } </code></pre>
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