A class diagram is a diagram used in designing and modeling software to describe classes and their relationships.

Class diagrams enable us to model software in a high level of abstraction and without having to look at the source code.

Classes in a class diagram correspond with classes in the source code.

The diagram shows the names and attributes of the classes, connections between the classes, and sometimes also the methods of the classes.

Describing class and class attributes

In a class diagram, a class is represented by a rectangle with the name of the class written on top. A line below the name of the class divides the name from the list of attributes (names and types of the class variables). The attributes are written one attribute per line.

In a class diagram, class attributes are written "attributeName: attributeType". A + before the attribute name means the attribute is public, and a - means the attribute is private.

Describing class constructor

In a class diagram, we list the constructor (and all other methods) below the attributes. A line below the attributes list separates it from the method list.

Methods are written with +/- (depending on the visibility of the method), method name, parameters, and their types. The constructor is written +Person(initialName:String)

Describing class methods

In a class diagram, we list all class methods including the constructors; constructors are listed first and then all class methods

We also write the return type of a method in the class diagram.

However a class diagram tells us nothing about the implementation of the constructors or the methods. Therefore a class diagram describes the structure of an object but not its functionality.

Connections between classes

public class Book {
    private String name;
    private String publisher;

    // constructors and methods
}