Objects, Encapsulation and Abstraction

Starter: think before typing

Before running this object-oriented design example, find the line where the main idea becomes active. Write a prediction: what must already be true for that line to work, and what should be different after it runs? The checked run ends with `Ada`; predict how the focus line helps produce that evidence.

Learning Objectives

• Create classes with fields, constructors and methods.
• Use encapsulation to protect object state.
• Explain abstraction as purposeful simplification.
• Design a small object model from a brief.

Learning Outcomes

• By the end of the lesson, you can create classes with fields, constructors and methods.
• By the end of the lesson, you can use encapsulation to protect object state.
• By the end of the lesson, you can explain abstraction as purposeful simplification.
• By the end of the lesson, you can design a small object model from a brief.

Why this idea exists

Object-oriented programming grew from attempts to model software as interacting entities with state and behaviour. Languages such as Simula and Smalltalk shaped ideas that later influenced Java.

Encapsulation protects an object from being put into an invalid state by random outside code. Abstraction lets programmers work with a useful public idea while hiding representation details that may change.

The point is not to create classes for everything. The point is to find concepts in the problem domain that deserve stable names, rules and responsibilities.

Deep dive

Lesson visual

Type this and run it

Create ObjectsEncapsulationAndAbstractionDemo.java, type the program, and run it before changing anything. This section is about reproducing the checked baseline.

public class ObjectsEncapsulationAndAbstractionDemo {
  public static void main(String[] args) {
    Student student = new Student("Ada");
    System.out.println(student.displayName());
  }
}

class Student {
  private final String name;

  Student(String name) {
    if (name.isBlank()) throw new IllegalArgumentException("Name required");
    this.name = name;
  }

  String displayName() {
    return name;
  }
}

Build and run it with:

javac ObjectsEncapsulationAndAbstractionDemo.java && java ObjectsEncapsulationAndAbstractionDemo

Expected baseline: Ada

Line-by-line explanation

Read the code as a sequence of responsibilities. The focus line for this lesson is Student student = new Student("Ada");; the surrounding lines prepare it, use its result or make the behaviour observable.

Worked example

Trace the program

Common misconception

A common mistake in objects, encapsulation and abstraction is treating the example as a finished answer. For object-oriented design, the important question is narrower: which operation carries the idea, what does it make possible, and what would break if you changed it carelessly?

Quick checks

How to study this lesson

For Objects, Encapsulation and Abstraction, predict how object-oriented design changes the run before you press Run.

Use the first portfolio task as your main edit: Model a library loan with three classes.

Use the second task as your variation: Add private fields and constructor checks.

Finish with evidence, not a diary entry: Explain what details the public methods hide.

Portfolio Practice

1. Model a library loan with three classes.
2. Add private fields and constructor checks.
3. Explain what details the public methods hide.

Final self-check

Study route

Practise object-oriented design by predicting the Java example, typing it, running it in the browser, tracing the result and saving portfolio evidence.

Next, move into Inheritance for Reuse and carry forward one improvement from this lesson into the next program.