Unit 4: Interfaces, Implementations, and Component Boundaries

A class tells Java how one kind of object works. An interface lets several classes promise the same public behavior without requiring them to share the same internal design. That becomes especially useful in robotics when one algorithm should work with a real component model, a simulator, or fixed test data.

This chapter also introduces a second meaning of the word interface. ROS 2 message, service, and action interfaces describe communication data, not Java method contracts. You will learn both ideas without merging them together.

Chapter question

How can a Java program depend on a clear behavior contract while keeping implementation details, invalid-state handling, and ROS 2 communication boundaries explicit?

What you will understand

You will learn how Java interfaces, implements, @Override, polymorphism, and exceptions work. You will also compare a Java interface with the ROS 2 sensor_msgs/msg/Range message interface and identify the information a future integration layer would need to translate between them.

What you will do

You will create several implementations of one RangeSource interface, store them together in a List<RangeSource>, apply the same decision logic to each, deliberately reject an invalid update, and inspect a controlled ROS 2 message definition and sample message.

What you will produce

Your final artifact is a Component Boundary Review showing the Java contract, two implementations, one preserved exception case, post-failure state, the ROS 2 message fields, and a translation-gap analysis.

How the week connects

Week 6 introduced classes and encapsulation. Week 7 used collections to manage several objects. Week 8 combines both ideas: a collection can now hold different concrete classes through one shared Java interface. The testing work in Unit 5 will use those clean boundaries to exercise component rules with known inputs.

Key vocabulary

interface, implements, @Override, implementation, polymorphism, contract, exception, throw, try, catch, ROS 2 interface, message interface

Approximate time

About 210 minutes for the five lessons, lab, and wrap-up. Spend extra time on the code examples if interfaces or exception handling are new to you.

Final evidence required

A complete Component Boundary Review that demonstrates two Java implementations of one interface, an invalid request rejected without corrupting valid state, a controlled inspection of sensor_msgs/msg/Range, and a technically accurate statement of what is still missing before Java data could participate in ROS 2 communication.