Module 2: Class Design

Key Concepts Explained

Class design is about creating effective structures for your Java applications. Good class design follows object-oriented principles and patterns.

Class Diagrams and Implementation

UML class diagrams represent the structure of classes in a visual way. When implementing a class diagram:

• Create classes based on the entities in the diagram
• Implement attributes as instance variables
• Implement operations as methods
• Create relationships between classes (composition, inheritance, etc.)

// Example of implementing a class from UML
public class Book {
    // Attributes
    private String title;
    private String author;
    private int pageCount;
    
    // Constructor
    public Book(String title, String author, int pageCount) {
        this.title = title;
        this.author = author;
        this.pageCount = pageCount;
    }
    
    // Methods
    public String getTitle() {
        return title;
    }
    
    public void setTitle(String title) {
        this.title = title;
    }
    
    // Other getters and setters...
}
            

Class Composition

Composition is a "has-a" relationship where a class contains objects of other classes as instance variables.

// Example of composition
public class Library {
    private String name;
    private Book[] books; // Library has-a collection of Books
    
    public Library(String name, int capacity) {
        this.name = name;
        this.books = new Book[capacity];
    }
    
    public void addBook(Book book, int index) {
        if (index >= 0 && index < books.length) {
            books[index] = book;
        }
    }
}
            

Access Control

Deciding on the right access modifiers is crucial for encapsulation:

• Instance variables - Usually private to protect data integrity
• Methods - Public for those that form the class's interface, private for internal implementation
• Constructors - Usually public, but may be private for singleton pattern

Compare private and public access:

• Private access: Encapsulates implementation details, prevents unwanted modification
• Public access: Exposes functionality to other classes, creates a stable interface