CheckBox.java

public class CheckBox {
  
  /* ... */
  
  // Provide the view interface for the Controller.
  public MutableCheckbox getMutable () {
    return isChecked -> CheckBox.this.setState(isChecked);
  }
  
  // Provide the read-only view interface.
  public ReadOnlyCheckbox getReadOnlyView () {
    return () -> Checkbox.this.getState();
  }
}