Classes and objects

Java objects combine state and behaviour behind a typed class.

Beginner 13 min read Lesson 6 of 300 Challenge included

What you will be able to do

Keep fields private and expose intention-revealing methods.
Validate constructor arguments immediately.
Use records for small immutable value types.

In plain English

Fields hold state, methods define behaviour and constructors establish valid initial state. Prefer immutability when possible. Access modifiers are part of the design: expose only what callers need.

Course.javaJava

public record Course(String slug, String title, int lessons) {
    public Course {
        if (lessons < 1) throw new IllegalArgumentException("lessons");
    }
}

public final class Progress {
    private int completed;

    public void markComplete() {
        completed += 1;
    }

    public int completed() {
        return completed;
    }
}

Your turn

Validate a record

Easy

Write a record Lesson with title and minutes that rejects a negative minute value.

JAVA · starter

public record Lesson(String title, int minutes) {}

Check your understanding before moving on.

Knowledge checkQuestion 1

Which access modifier hides a field from all other classes?

Select an answer to continue

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