What follows is a contrived example just trying to illustrate the point. My main questions: FileWithSave handles its saving itself, but delegates it to a repository object. This allows implementation details to be available without being present in the API when saving (transpose this to a Mail sending API as well, available for send but not public). Is this OK in terms of OOP practices? Are there any maintainence gotchas I'm missing? Does it need reorganization?

public interface File {
 /**
 * Get the unique identifier.
 *
 * @return unique identifier.
 */
 String getId();
 /**
 * Get the file name.
 *
 * @return file name.
 */
 String getName();
 /**
 * Sets name for the file.
 *
 * @param name file name.
 */
 void setName(String name);
 /**
 * Get the file size.
 *
 * @return file size.
 */
 Long getSize();
}

public class FileImpl implements File {
 // I don't want it in my public signature but I want it when I'm saving for some reason :(
 private Something somethingPrivate;
}

public interface FilingCabinet {
 /**
 * Get a file from the cabinet.
 *
 * @param id unique identifier.
 * @return file.
 */
 File get(String id);
 /**
 * Store a file in the cabinet.
 *
 * @param file.
 */
 void store(File file);
}

Without save sample code:

public interface FileWithSave extends File {
 /**
 * Save the changes.
 */
 File store();
}

public class FileWithSaveImpl implements FileWithSave {
 private final PrivateFilingCabinet repo;
 private Something somethingPrivate;
 public FileWithSaveImpl(PrivateFilingCabinet repo) {
 this.repo = repo;
 }
 // those other methods
 @Override
 public void store() {
 repo.save(this);
 }
}

/**
 * Wraps PrivateFilingCabinet and injects PrivateFilingCabinet in FileWithSave object.
 */
public interface Secretary {
 /**
 * Get the file for someone.
 *
 * @param unique identifier.
 * @return file.
 */
 FileWithSave getFile(String id);
}

public interface PrivateFilingCabinet {
 /**
 * Get a file from the cabinet.
 *
 * @param id unique identifier.
 * @return file.
 */
 FileWithSaveImpl get(String id);
 /**
 * Store a file in the cabinet.
 *
 * @param file.
 */
 void store(FileWithSaveImpl file);
}

With save sample code:

My main questions: FileWithSave handles its saving itself, but delegates it to a repository object. This allows implementation details to be available without being present in the API when saving (transpose this to a Mail sending API as well, available for send but not public). Is this OK in terms of OOP practices? Are there any maintainence gotchas I'm missing? Does it need reorganization?

package com.example.external;
public interface File {
 /**
 * Get the unique identifier.
 *
 * @return unique identifier.
 */
 String getId();
 /**
 * Get the file name.
 *
 * @return file name.
 */
 String getName();
 /**
 * Sets name for the file.
 *
 * @param name file name.
 */
 void setName(String name);
 /**
 * Get the file size.
 *
 * @return file size.
 */
 Long getSize();
}

package com.example.internal;
public class FileImpl implements File {
 // I don't want it in my public signature but I want it when I'm saving for some reason :(
 private Something somethingPrivate;
 private final String id;
 private final String name;
 private final Long size;
 public FileImpl(String id, Long size) {
 this.id = id;
 this.size = size;
 }
 public String getId() {
 return id;
 }
 public String getName() {
 return name;
 }
 public void setName(String name) {
 this.name = name;
 }
 public Long getSize() {
 return size;
}
}

package com.example.external;
public interface FilingCabinet {
 /**
 * Get a file from the cabinet.
 *
 * @param id unique identifier.
 * @return file.
 */
 File get(String id);
 /**
 * Store a file in the cabinet.
 *
 * @param file.
 */
 void store(File file);
}

Without save client sample code:

package com.example.external;
public interface FileWithSave extends File {
 /**
 * Save the changes.
 */
 File store();
}

package com.example.internal;
public class FileWithSaveImpl implements FileWithSave {
 private Something somethingPrivate;
 private final String id;
 private final String name;
 private final Long size;
 private final PrivateFilingCabinet repo;
 public FileWithSaveImpl(String id, Long size, PrivateFilingCabinet repo) {
 this.id = id;
 this.size = size;
 this.repo = repo;
 }
 @Override
  public String getId() {
 return id;
 }
 @Override
 public String getName() {
 return name;
 }
 @Override
 public void setName(String name) {
 this.name = name;
 }
  @Override
 public Long getSize() {
 return size;
 }
 @Override
 public void store() {
 repo.save(this);
 }
}

package com.example.external;
/**
 * Wraps PrivateFilingCabinet and injects PrivateFilingCabinet in FileWithSave object.
 */
public interface Secretary {
 /**
 * Get the file for someone.
 *
 * @param unique identifier.
 * @return file.
 */
 FileWithSave getFile(String id);
}

package com.example.internal;
public interface PrivateFilingCabinet {
 /**
 * Get a file from the cabinet.
 *
 * @param id unique identifier.
 * @return file.
 */
 FileWithSaveImpl get(String id);
 /**
 * Store a file in the cabinet.
 *
 * @param file.
 */
 void store(FileWithSaveImpl file);
}

With save client sample code: