protected access

`protected` access

Figure 494. protected access

package model;
public abstract class Shape ❶{

  final protected long creationTime ❷= System.nanoTime();
...
}
------------------------------------------------
package model.sub;
public class Rectangle ❸extends Shape {
  static final Logger log = LogManager.getLogger(Rectangle.class);

  @Override public double getArea() {
    log.info("Rectangle creation time:" + creationTime ❹);
    return width * height;
  } ...
}

❶	Defining superclass `Shape` in package `model`.
❷	Defining a `protected` instance attribute creationTime in superclass `Shape`.
❸	Deriving class `Rectangle` in different package `model.sub` from superclass `Shape`.
❹	Accessing superclass attribute creationTime across package boundary.

exercise No. 174

`protected` vs. “package private”

Implement Figure 494, “protected access ” in your IDE but exchanging Rectangle by Circle accordingly. Then execute Circle.getArea() and watch the logging outcome.

Now apply the following two changes:

Remove creationTime's protected modifier.
Move class Circle to package model.

Are you still able to run your code? Explain the result.

Implementing class Circle is straightforward:

public class Circle extends Shape {
  static final Logger log = LogManager.getLogger(Circle.class);

  @Override public double getArea() {
    log.info("Circle creation time:" + creationTime );
    return Math.PI * radius * radius;
  }
  private double radius;
}

Executing new Circle().getArea() results in:

2018-01-03 08:31:18,811 INFO  [main] sup.Circle (Circle.java:11) - Circle creation time:3340216708709

Removing protected yields a compile time error:

public class Circle extends Shape {
  static final Logger log = LogManager.getLogger(Circle.class);

  @Override public double getArea() {

// Error: 'creationTime' is not public in 'testprotect.model.Shape'.
// Cannot be accessed from outside package
    log.info("Circle creation time:" + creationTime );
...

Moving class Circle to package model resolves the issue: Having both classes Shape and Circle in a common package model implies “package private” access. Thus every method in Circle has full access to all fields and methods of Shape.

Note

In a production environment this may or may not be desired and is thus a design choice.

exercise No. 175