Java · Beginner level

Beginner level · Module 4 of 5 · 11 guided hours

Classes and Objects in Java

Model real things as classes with fields, constructors and methods, and protect data with encapsulation.

Learning objectives

  • Write classes with fields, constructors and instance methods
  • Create and use objects, including lists of objects
  • Apply encapsulation with private fields and validated setters

Topics

  • class vs object
  • fields and constructors
  • this keyword
  • instance vs static
  • private fields, getters and setters
  • toString

Lessons

  1. Lesson 1 Your First Class

    Define a class with fields and methods, create objects with new, and call methods on them.

    classobjectfieldinstance method

  2. Lesson 2 Constructors and the this Keyword

    Give objects valid starting values with constructors, use this to refer to the current object, and know when to use static.

    constructorthisconstructor overloadingstatic members

  3. Lesson 3 Encapsulation: Protecting an Object's Data

    Make fields private and control changes through methods that check the rules, so objects can never hold invalid data.

    privategetter and settervalidationinvariant

Practical activity

Model a mobile-money wallet as a class with deposit, withdraw and balance methods that never allow a negative balance, and test it from main.

Expected outcomes

  • Working classes that keep their own data valid
  • Programs that create and use several objects

Assessment

Code review of the wallet class against a checklist (private fields, validation, clear method names) plus a short oral explanation of how an object differs from a class.

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