Main.java

public Main {
  public void init(Object o) {
    theme = UIManager.initFirstTheme("/theme");
    
    // IMPORTANT: Notice we don't use MyClass.class.getName()! This won't work due to obfuscation!
    Util.register("MyClass", MyClass.class);
  }
  
  public void start() {
    //...
  }
  
  public void stop() {
    //...
  }
  
  public void destroy() {
    //...
  }
}

MyClass.java

public MyClass implements Externalizable {
  // allows us to manipulate the version, in this case we are demonstrating a data change between the initial release
  // and the current state of object data
  private static final int VERSION = 2;
  
  private String name;
  private Map<String, Object> data;
  
  // this field was added after version 1
  private Date startedAt;
  
  public int getVersion() {
    return VERSION;
  }
  
  public void externalize(DataOutputStream out) throws IOException {
    Util.writeUTF(name, out);
    Util.writeObject(data, out);
    if(startedAt != null) {
        out.writeBoolean(true);
        out.writeLong(startedAt.getTime());
    } else {
        out.writeBoolean(false);
    }
  }
  public void internalize(int version, DataInputStream in) throws IOException {
    name = Util.readUTF(in);
    data = (Map<String, Object>)Util.readObject(in);
    if(version > 1) {
        boolean hasDate = in.readBoolean();
        if(hasDate) {
            startedAt = new Date(in.readLong());
        }
    }
  }
  public String getObjectId() {
    // IMPORTANT: Notice we don't use getClass().getName()! This won't work due to obfuscation!
    return "MyClass";
  }
}

ReadAndWrite.java

// will read the file or return null if failed
MyClass object = (MyClass)Storage.getInstance().readObject("NameOfFile");

// write the object back to storage
Storage.getInstance().writeObject("NameOfFile", object);

Sample usage of Externalizable interface.

From the Codename One project