Last updated 26. February 2021.

Class relationships in Java define a special relationship between different class types.

For example, there is a special relationship between a class called car and another called car : A car is a type of car.

There are several relations between the classes Shape, Circle, Rectangle and Square. A circle is a kind of shape. The rectangle is a kind of shape.

When we develop a large-scale application or program, we need to look at the relationships between classes. He helps us in many ways.

Assuming we have classes in the application with normal behavior (methods), we can save ourselves the trouble of putting the normal behavior (methods) in a superclass.

Assuming that some classes are unrelated, you can assign different programmers to implement each of them without worrying about one having to wait for the other.

To do this, we need to learn the different types of relationships between classes in Java. Relationships between classes help you understand how the objects in a program interact and communicate with each other.

Types of relationships between classes in Java

There are three common relationships between Java classes, which are as follows:

a. Dependency (Use-A)
b. Association (Has-A)
c. Heritage (Is-A)

The association is divided into two categories: aggregation and composition, which will be discussed in detail in the next tutorial.

Let us decompose these three relations between classes one by one.

Dependency relationship (Usage-A) in Java

When we create an object of one class within a method of another class, this relationship is called dependency relationship or simply Uses-A relationship in Java.

In other words: In Java, when a class method uses an object of another class, it is called a dependency. These are the most obvious and common relationships in Java.

Consider the following illustration, in which the display() method of class B uses an object of class A. So we can say that class A depends on another class B when it uses an object of class A.

When different classes of an application program depend on each other, connectivity between classes is said to be high.

It is good programming practice to minimize dependencies between classes (i.e., communications) because too many dependencies make it difficult to manage the application program.

On the other hand, if there are only a few dependencies between classes, we say that the relationship between classes is weak.

Why is clutching important?

If a class changes its behavior in the next version of the application program, all classes that depend on it may also be affected. In this situation, we need to update all related classes.

So if the communication between classes is weak, we can easily manage that. In this way, we should avoid unnecessary communication between classes.

Association relationship (Has-A) with Java

Association is another fundamental relationship between classes, informally referred to as the Has-A relationship.

In Java, when an object in one class is created as a data element in another class, it is called an associative relationship or simply a Has-A relationship.

Look at the illustration above where an object of class A is created as a data member in another class B. This relationship is called a Has-A relationship. This is easy to understand and makes it a stronger form of addiction.

We will learn more about the Has-A relationship in the next lesson.

Heredity relations (Is-A) with Java

Inheritance represents a relationship with Java. It establishes a connection between a more general class (called superclass) and a more specific class (called subclass).

In other words: The Is-A-Relation defines a relationship between two classes, where one class is an extension of the other class.

Look at the image above where class B connects to class A through the extension keyword and inherits data elements from class A.

For more information, see this tutorial : Inheritance in Java | Example, use, advantage

UML notation in Java

UML stands for Unified Modeling Language. This is an international standard classification. Many programmers use this notation to draw a class diagram that explains the relationship between classes.

There are a number of tools for creating UML diagrams. The table in the following figure shows the UML notation for class relations in Java.

I hope that with the following diagrams, this tutorial has covered almost all the important points about class relation types in Java. I hope you understand this simple matter.

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Thanks for reading.

frequently asked questions

Does a Java class diagram have a relationship?

Class diagrams are the cornerstone of object-oriented modeling. They are used to represent the different objects in the system, their attributes, operations and the relationships between them. The example shows a class called Loan Account. … The central part contains the class properties.

How many types of class relations are there?

This reusability is made possible by the interconnection of the classes used. Object-oriented programming generally supports 4 types of relationships, namely inheritance, association, composition and aggregation. All of these relationships are based on relationships, relationship being and relationship being.

In Java, the Has-A relationship is also called composition. In Java, the has-a relationship simply means that an instance of a class refers to an instance of another class or to another instance of the same class. … For example, a car has an engine, a dog has a tail, etc.

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