2.4 Encapsulation

Learning Objectives

•Mark fields private to hide internal state.
•Write getter and setter methods that control access.
•Explain the benefits of encapsulation for maintainability and validation.

Introduction

Encapsulation hides an object's internal state behind a controlled interface. In Java, this typically means making fields `private` and exposing controlled access through public "getter" and "setter" methods, which can validate input or compute derived values instead of allowing direct, unchecked access.

Key Concepts

`private` restricts a field or method to the declaring class only.

Getters (`getX()`) return a field's value; setters (`setX(value)`) update it, optionally validating first.

Encapsulation lets internal representation change later without breaking code that only uses the public getters/setters.

Java Syntax

Code Examples

`balance` cannot be modified directly from outside the class. `deposit` is the only way to increase it, and it validates that the amount is positive first.

Common Mistakes

⚠Making fields `public`, which removes all control over how they're read or changed.
⚠Writing setters that don't validate input, defeating the purpose of encapsulation.
⚠Returning a mutable field directly from a getter, allowing outside code to bypass validation by mutating it after the fact (matters more for objects/arrays than primitives).

Practice

Practice 1

Programming Exercisemedium

Add a `withdraw(double amount)` method to BankAccount that only succeeds if `amount` is positive and no greater than the current balance.

Summary

Encapsulation hides fields behind `private` and exposes controlled access via getters/setters, enabling validation and letting internal representation evolve safely.

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