Composition and aggregation are two important and common techniques for combining related classes in the Java programming language. Though similar, composition and aggregation differ in subtle ways. In this article, we will break down composition versus aggregation in Java and how they differ from each other. Additionally, we’ll take a look at the advantages and disadvantages of using each of these techniques.
What is Composition in Java?
Composition is a type of relationship between two objects in which one class owns another class and can access all its data and behavior. In other words, the class that owns the other class is considered to be composed of the other class. The most common example of composition is the relationship between a car and its tires. A car is usually composed of four tires, so when there is a change in the state of the car (for example, when the car starts moving), each tire is also affected accordingly.
When you declare a member field of another type, then you are using composition. The instance of one class is created as part of the instance of a second class. E.g., class A {B obj = new B()}. Through composition, an object contains another object as a part of itself.
Composition is a powerful concept in Java, as it allows for the creation of complex objects from simpler ones. It also allows for the reuse of code, as the same object can be used in multiple classes. This makes it easier to maintain and debug code, as changes to the object only need to be made in one place. Additionally, composition can be used to create objects with different levels of abstraction, allowing for more flexibility in the design of an application.
What is Aggregation in Java?
In Java, Aggregation is a type of relationship where one class is used by another class but does not own it. In other words, the class that does not own it still has access to all of its data and behavior. It’s a weaker association than composition because the lifetime of the class being used is not dependent on the lifetime of the class using it. A classic example of aggregation is the relationship between a book and an author. A book has only one author, while an author may have written many books. Thus, the relationship between a book and an author is an example of aggregation.
When we declare a reference field of another type then we are using aggregation. The instance of one class can exist independently without an instance of the second class being present. For example, class A {B obj;}. Through aggregation, an object holds a reference to another object.
Aggregation is a useful concept in Java because it allows for the reuse of code and the ability to create complex relationships between objects. It also allows for the creation of more efficient and maintainable code. By using aggregation, developers can create relationships between objects that are more flexible and easier to maintain than if they were to use composition.
Differences Between Composition and Aggregation in Java
Composition and aggregation are two very similar concepts. However, there are some distinct differences between them. The main difference between composition and aggregation is that composition defines a strong ownership relationship between two classes while aggregation defines a weak ownership relationship. In composition, if the lifetime of one object ends then the lifetime of the other object also ends, but in aggregation the lifetime of one object is not necessarily dependent on the other object.
In addition, in composition there is no meaningful existence of an object without its container object, but in aggregation there is meaningful existence of an object even without its container object. Finally, in composition, a parent is responsible for creating and managing its child objects, however, in aggregation it is not necessary for a parent to create or manage its child objects.
Composition is a more tightly coupled relationship than aggregation, as the child object cannot exist without the parent object. This means that the parent object has full control over the child object, and can access and modify its data. On the other hand, aggregation is a looser relationship, as the child object can exist independently of the parent object. This means that the parent object does not have full control over the child object, and cannot access or modify its data.
Practical Examples of Composition and Aggregation
Here are some practical examples of composition and aggregation in Java:
- Composition: Room Class – A Room has members like wall and windows which are made up of other separate parts like cement, paint etc.
- Aggregation: Library Class – A Library contains several books; each book exists independently.
Another example of composition is a Car class. A car is composed of several parts such as an engine, wheels, and a steering wheel. Each of these parts is made up of smaller components such as screws, bolts, and nuts. Aggregation can be seen in a University class. A university contains several departments, each of which exists independently.
Advantages of Using Composition and Aggregation
Composition and aggregation have their advantages. Composition helps to reduce code size by increasing the code reusability; it also provides better control over the objects at run time. Aggregation also helps to reduce code size and increase code reusability; it also increases encapsulation. Additionally, Aggregation also enables you to model “has-a” relationships.
Composition and aggregation also help to improve the maintainability of the code. By using composition and aggregation, developers can easily identify and modify the code that needs to be changed. This makes it easier to debug and maintain the code. Furthermore, composition and aggregation can help to reduce the complexity of the code, making it easier to understand and maintain.
Choosing Between Composition and Aggregation
While both composition and aggregation offer methods to establish relationships between classes, the right choice hinges upon the type of relationship you aim to model:
- Lifespan Dependency: If a ‘Part’ object should cease to exist when the ‘Whole’ object is destroyed, use composition. For example, a car’s engine ceases to be useful once the car is scrapped.
- Reusable Components: If you find that a component can exist independently and might be shared across multiple objects, aggregation is the way to go. For instance, a student and a library book; even if the student graduates or drops out, the book remains a part of the library.
- Modeling Real-World Relationships: Sometimes, nature of the real-world relationship between entities might be the deciding factor. For example, a ‘Person’ might be associated with a ‘Country’ by birth (an aggregation, as both can exist independently). However, a ‘Heart’ is in composition with a ‘Person’, as it cannot exist independently.
Java Code Example:
Composition:
class Engine {
// Engine-specific attributes and methods
}
class Car {
private final Engine carEngine; // This signifies that Car "owns" Engine.
Car() {
carEngine = new Engine();
}
// Other Car methods
}
Aggregation:
class Department {
// Department-specific attributes and methods
}
class University {
private Department[] departments; // University has departments, but doesn't exclusively "own" them.
University(Department[] departments) {
this.departments = departments;
}
// Other University methods
}
Disadvantages of Using Composition and Aggregation
Despite their advantages, composition and aggregation have some drawbacks. Composition can be more difficult to model in some situations, while object memory usage may increase due to longer object life spans. Aggregation can make programming tasks more complicated as it introduces more than one class to model the same behavior.
In addition, composition and aggregation can lead to a complex object structure, making it difficult to maintain and debug. Furthermore, the use of composition and aggregation can lead to tight coupling between objects, making it difficult to modify or extend the code in the future.
Deep Dive: Aggregation Types
Beyond basic aggregation, there’s also the concept of “shared” and “composite” aggregation. Shared aggregation implies that the child entity can be shared among multiple parent entities. Composite aggregation suggests a whole-part hierarchy where the child can belong to only one parent at a time.
Conclusion
Composition and aggregation are important techniques used in Object Oriented Programming languages like Java. Though similar, they have subtle differences that make them ideal for modeling certain types of relationships among classes. Though they have their advantages, they also come with certain drawbacks that should be weighed when deciding which technique is best for any given situation. Knowing when to use each technique can help developers write better code with fewer bugs.
Composition is a strong form of relationship between classes, where one class is a part of another class. This type of relationship is often used when one class is dependent on another class, and the lifetime of the dependent class is dependent on the lifetime of the parent class. Aggregation, on the other hand, is a weaker form of relationship between classes, where one class is associated with another class. This type of relationship is often used when one class is not dependent on another class, and the lifetime of the dependent class is not dependent on the lifetime of the parent class.
by the Bito team. 
