cs ch4

Chapter 4: Writing Classes

• Focuses on developing foundational skills in Java programming by creating classes from scratch instead of relying solely on existing Java API classes.

Key Concepts

Introduction to Classes

• Programs utilize predefined classes from the Java API.

• Transition to designing programs with self-defined classes, starting from the main method.

• True object-oriented programming involves creating classes that represent real-world objects with defined characteristics and behaviors.

Anatomy of a Class

• A class serves as a blueprint for objects, containing:

  • Data Declarations: Variables that store data relevant to the class (e.g., int size, weight).

  • Method Declarations: Functions that define the behaviors of the objects.

• Instance data differs for each object created from a class, enabling unique state representation.

Encapsulation

• Two perspectives of an object:

  • Internal: Details of the variables and methods.

  • External: Services provided to other parts of the system.

• Encapsulation keeps the internal workings hidden, allowing interaction only through defined methods.

• Visibility modifiers such as public, private, and protected enforce data access levels and control encapsulation.

Methods in Classes

• Method Declaration:

  • Includes method name, return type, and parameter list.

  • Control flow jumps to called methods and returns upon completion.

• Accessors and Mutators: Methods allowing controlled access to private instance variables, often named getX for access and setX for modification.

• Mutators can impose restrictions, ensuring valid state for the object.

Example Classes

• Die Class:

  • Represents a die with attributes and methods to manipulate its state (e.g., roll method).

• Account Class:

  • Manages bank account operations: deposit, withdraw, and add interest.

  • Demonstrates encapsulation, using private variables and public methods for operations.

Structure of Programs

• Driver Programs: These display or test functionality of classes, such as Transactions that manages multiple Account objects.

Graphical User Interfaces (GUI)

• Created using controls, events, and event handlers:

  • Controls: Elements like buttons and text fields interact with users.

  • Events: Represent user actions (e.g., clicking a button).

  • Event Handlers: Methods that respond to events, modified to handle specific outcomes based on user interactions.

• Layouts: JavaFX provides different layout options for arranging controls (e.g., StackPane, GridPane).

JavaFX and Arcs

• The Arc class represents a segment of an ellipse with params for center, radius, start angle, and arc length.

• Demonstrates dynamic visualizations using JavaFX.

Important Definitions

Constructors

• Special methods for initializing object state, sharing the name with the class, and having no return type.

Instance Data vs. Local Data

• Instance data: Variable scoped to the class, permitting multi-instance data retention (e.g., faceValue in Die).

• Local data: Exists only within the method context and is discarded after execution.

UML Diagrams

• Unified Modeling Language (UML) diagrams illustrate class structures, relationships, and attributes, serving as visual documents for class design.

Summary of Topics

• Writing classes: class definitions, instance data, encapsulation, Java modifiers, method declarations, parameter passing, constructors, and GUI elements including events and event handlers.