UML (Unified Model Language)
to document object-oriented programs

It can be used to graphically depict the BASIC STRUCTURES of object-oriented programming, such as CLASSES and their INHERITANCE relationships. 
These diagrams are usually referred to as CLASS DIAGRAMS.








Class
In UML, a class is represented by a frame consisting of 3 parts. 
At the top is the class NAME, 
in the middle are the class ATTRIBUTES, and 
at the bottom are the class METHODS.

ATTRIBUTES are presented in the form
name: type


A + or - is written in front of the attribute depending on whether the attribute is 'public' or 'private'. 
The sign for a 'protected' attribute is (usually) #.

So for example a protected, double type attribute balance would be marked:
#balance: double

...and a private Student type attribute tutor:
-tutor: Student





METHODS are represented in a similar form, but also the PARAMS of the methods are announced in the diagram. 
The 'method type' is reported after the parameter list.

So, for example, the set method 'setName' could be marked as follows:
+setName(name: String): void




=================================================


 

Let's look at the class 'Exercise' as an example:

class Exercise {
    // PRIVATE ATTRIBUTES
    private String correctAnswer;
    private String exerciseDescription;
    private double points;

    // CONSTRUCTOR
    public Exercise(String correctAnswer, String exerciseDescription, double points) {
        this.correctAnswer = correctAnswer;
        this.exerciseDescription = exerciseDescription;
        this.points = points;
    }

    public String getExerciseDescription() {
        return exerciseDescription;
    }

    public double getPoints() {
        return points;
    }

    public void setPoints(double points) {
        this.points = points;
    }
    
    public void checkAnswer(String answer) {
        if (answer.equals(correctAnswer)) {
            points = 10;
        }
    }
}
 

The UML diagram drawn from the class would look like this:
https://ville.utu.fi/APP/connector/0/219/source/9c79f234-86e5-43c5-821a-eaee0895bdeb.png


Exercise
-correctAnswer: String
-exercise Description: String
-points: double

+Exercise (correctAnswer: String, exercise Description: String, points: double)
+getExerciseDescription(): String
+getPoints(): String
+setPoints(points: double): void
+checkAnswer(answer: string): void







====================================


Modeling Class Hierarchies

Inheritance is marked with an ARROW that STARTS FROM the INHERITING CLASS. 
The head of the arrow is a triangle. 
If, for example, we have a class 'Teacher' that inherits from the class 'Person', the diagram could look like this:
https://ville.utu.fi/APP/connector/0/227/source/bf884576-16f4-4ef0-a9e5-a4e69cf1faa3.png



Person
+email: String
-name: String
+Person(email: String, name: String)
+getName(): String
+setName(): String

^
-
|
|
|
|
|

Teacher
-teachingSubject: String
+Teacher(email: String, name: String, teachingSubject: String)
+getTeaching Subject(): String








Interface classes are marked by writing the word <interface> above the class name. 
The implementation of the 'interface' is marked with an arrow similar to inheritance.

For example, the 'interface' class 'Priced' and its implementing class 'Product':

 

interface Priced {
    double getPrice();
    void setPrice(double price);
}


class Product implements Priced {
    
    private String name;
    private double price;
    private String code;
    
    public Product(String name, double price, String code) {
        this.name = name;
        this.price = price;
        this.code = code;
    }

    @Override
    public double getPrice() {
        return price;
    }

    @Override
    public void setPrice(double price) {
        this.price = price;
    }
}
 

...could be represented with a diagram like this:
https://ville.utu.fi/APP/connector/0/234/source/9b1af723-9438-4cb5-96bc-d20e586f2239.png



<interface>
Priced
-getPrice(): double
+setPrice(price: double): void


^
-
|
|
|
|
|


Product
-code: String
-name: String
-price: double
+Product(code: String, name: String, price: double)
+getPrice(): double
+setPrice(price: double): void












====================================




More Information on Class Diagrams

There is plenty of additional information about UML and class diagrams on the internet.

Diagrams can be drawn with almost any drawing program. 
A good free environment for drawing diagrams is DIA, for example:
http://dia-installer.de/

The online draw.io is also well suited for drawing UML diagrams:
https://app.diagrams.net/

There are also various plugins for Eclipse and other editors 
that can automatically generate a UML diagram from your code (or even code from a UML diagram).