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Java Aggregation Vs Composition: Java Explained

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Java is an incredibly vast and complex programming language, due to its many features and possibilities necessary for coding. As such, gaining a deep understanding of Java requires delving into the various intricacies of its structure and composition. Two important and fundamental concepts within Java to understand are aggregation and composition. Both concepts play a vital role within Java and it is important to understand how they differ, how they are used and the benefits of utilising them. This article will cover all of these components, so that you can gain a full understanding of aggregation and composition in Java and how best to use them.

What is Aggregation?

Aggregation is a special case of the Association relationship in object-oriented programming, which is a term for the relationship between two or more objects. In particular, aggregation involves a relationship between two classes that involves one class having ownership or part ownership over the other class. A common metaphorical example of this would be a university, which has ownership over its departments, students and staff members.

In terms of its functionality, aggregation occurs when one object contains a reference to another object. This reference can be directly or indirectly taken from there. Direct references are those where a class holds a direct reference to an object of another class, while indirect references are those where a class holds a reference to an object through another class.

Aggregation is a powerful tool for object-oriented programming, as it allows for the creation of complex relationships between objects. It also allows for the reuse of code, as the same object can be used in multiple classes. This can help to reduce the amount of code that needs to be written, as well as making it easier to maintain and debug.

What is Composition?

Composition is similar to aggregation in that it also refers to building up a complex object from simpler components. However, the difference with composition is that the components are part of the composite. We can think again of our university example. In this case, the university is made up of various departments, students and staff members, which form essential components parts of the university and without which it would not exist. In other words, the composite cannot exist without its components.

In terms of its functionality, composition also involves one object containing a reference to another object. In this case however, the object references are fundamental and necessary for the object’s existence, meaning a change or removal of a component would make the object invalid. Removing or changing the component would also directly affect the object’s behaviour.

Composition is a powerful tool for creating complex objects from simpler components. It allows us to create objects that are more than the sum of their parts, and can be used to create objects that are highly customisable and reusable. By using composition, we can create objects that are more efficient and easier to maintain than if we had created them from scratch.

Key Differences Between Aggregation and Composition

The key distinctions between aggregation and composition arise from the fact that aggregation involves ownerships rather than part ownership; it is in essence like acquiring a new class whilst composition represents a new class alone. As per the examples given earlier, we can see this difference in action.

In the case of aggregation, the objects represent separate entities which are related only by their association; with composition however, the component represents an essential part which forms part of a whole entity.

On a more technical level, aggregation deals with references while composition deals with objects; this is due to the fact that composition requires that changes made to the component objects reflect on the composite parts as well as on their own at the same time.

How Does Aggregation Work in Java?

In Java, aggregation works in quite simple terms when compared to other programming languages. To explain this further we can refer once again to our university example. In this scenario, we have three classes; University, Staff Member and Student. University would be the composite class of aggregation, since it holds references to each of its employees – both Staff member and Student classes – which are in turn its component classes. An example of this code within Java would be as follows:

public class University {     private StaffMember[] staffMembers;     private Student[] students;      public University(StaffMember[] staffMembers, Student[] students) {         this.staffMembers = staffMembers;         this.students = students;     }      //Getters and Setters … }

How Does Composition Work in Java?

In terms of how it works within Java, composition is quite similar to aggregation but with a few key differences. For example, for composition there is only ever one composite part and each component is made up of its composite part. As such, references to components from other classes are generally excluded. An example of such code within Java would be as follows:

public class University {     private Department department;     private Student student;      public University(Department department, Student student) {         this.department = department;         this.student = student;     }      //Getters and Setters … }

Benefits of Using Aggregation and Composition in Java

There are both advantages and disadvantages of using either aggregation or composition in Java. The advantages include flexibility since there is no need to change the code when changing from one type to another; this allows for easy extensibility and reuse of code. Additionally, it allows for better code organization since associated classes can be separated into different files for easier readability and better understanding.

As for disadvantages, one potential issue is that of complexity; when using either aggregation or composition it can be difficult to understand the relationship between different classes and their interactions.

Examples of Aggregation and Composition in Java Code

Below are two examples that demonstrate both composition and aggregation in action:

//An example of aggregation  public class University {     //Declare aggregated classes as fields     private Department[] departments;     private Student[] students;           public University(Department departments[],  Student[] students) {         this.departments = departments;         this.students = students;     }           //Getters & Setters for departments & students … }
//An example of composition  public class University {     //Declare composed classes as fields     private Department department;     private Student student;          public University(Department department, Student student) {         this.department = department;         this.student = student;     }          //Getters & Setters for department & student … }

Conclusion

In conclusion, we can see that both aggregation and composition are important features within Java which allow us to create complex objects from simpler structures. Though sometimes difficult to understand due to their complexity and subtle differences, gaining a firm grasp of both of these concepts will assist you greatly as you progress in your understanding of Java.

Anand Das

Anand Das

Anand is Co-founder and CTO of Bito. He leads technical strategy and engineering, and is our biggest user! Formerly, Anand was CTO of Eyeota, a data company acquired by Dun & Bradstreet. He is co-founder of PubMatic, where he led the building of an ad exchange system that handles over 1 Trillion bids per day.

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