CheckBox.java

public class CheckBox {
  
  private MutableCheckbox mutable = null;
  private ReadOnlyCheckbox readOnlyView = null;
  
  /* ... */
  
  // Provide the view interface for the Controller.
  public MutableCheckbox getMutable () {
    if (mutable == null) {
      mutable = this::setState;
    }
    return mutable;
  }
  
  // Provide the read-only view interface.
  public ReadOnlyCheckbox getReadOnlyView () {
    if (readOnlyView == null) {
      readOnlyView = this::getState;
    }
    return readOnlyView;
  }
}