Understanding the difference between composition and aggregation in Java can be difficult. That is why it is important to know when to use one method over the other. This article will explain composition and aggregation in Java, the differences between them and explain when you should use each for better coding outcomes.

What is Composition in Java?

Composition in Java is a form of association between two objects, where one class has a reference to another. This can be a property, method, or constructor in the composed class, and they can refer to another object or a group of objects. For example, a Car class may have a knob object, which would be an instance of a Knob class. The Knob object is a part of the Car class and can be accessed by all other classes within the Car.

What is Aggregation in Java?

Aggregation in Java is essentially a more “loose” form of composition. Rather than one class having strict references to other classes, aggregation allows an instance of one class to refer to any number of instances of another class. For example, a Project class may have instances of Task objects associated with it, but each Task does not have to be related to the Project class itself. This type of relation between classes is loosely coupled and allows for more flexible access.

Aggregation is a useful tool for creating complex data structures in Java. By using aggregation, developers can create objects that contain references to other objects, allowing for a more dynamic and flexible approach to programming. Additionally, aggregation can be used to create relationships between objects that are not necessarily related, such as a Project class and a Task class. This allows for a more efficient way of managing data and relationships between objects.

Advantages of Using Composition and Aggregation

Composition and aggregation in Java are powerful tools that can allow you to create more robust and maintainable applications. By utilizing composition or aggregation in your Java code, you can declare explicit relationships between classes, allowing you to maximize encapsulation and make future changes easier.

Composition and aggregation also allow you to create more flexible and extensible applications. By using composition and aggregation, you can easily add new features and functionality to your application without having to rewrite existing code. This makes it easier to maintain and update your application over time.

What Are the Differences Between Composition and Aggregation?

The main difference between composition and aggregation is that composition requires an explicit relationship between classes, while aggregation requires a more loosely coupled relationship. Composition implies that an object has ownership over another object (e.g., Car owns the Knob object), while aggregation implies that an object may have access to other instances of another class without taking ownership (e.g., Project may have access to multiple Task objects without taking actively ownership of them).

When to Use Composition Over Aggregation?

Composition should be used when one class needs to take explicit ownership over another. This is useful when creating applications with complex relationships between objects, or when you are looking to maximize encapsulation in your code. Composition is also often used when you are creating applications that are single-instance oriented, as opposed to object-oriented.

When to Use Aggregation Over Composition?

Aggregation should be used when you want to ensure a more loosely coupled relationship between two classes. This allows for greater flexibility for applications that need to function independently of each other, but still access data from one another. Aggregation is also beneficial for applications that are designed for multiple instances, as opposed to single-instance designs.

In addition, aggregation is useful when you need to create a relationship between two classes that is not necessarily permanent. For example, if you have a class that needs to access data from another class, but the data is not always needed, aggregation can be used to create a temporary relationship between the two classes. This allows for greater flexibility and scalability in applications.

Examples of Composition and Aggregation in Java

One common example of composition in Java is when a Vehicle class has an instance of an Engine class inside it. The Engine object is an explicit part of the Vehicle class, which means that the Vehicle owns the Engine object and is responsible for it. Another example is when an Employee class has a List of LeaveRequest objects associated with it. In this case, LeaveRequests are explicitly associated with an Employee, so the Employee owns the List of LeaveRequests.

An example of aggregation in Java is when an Organization class has access to a List of Employee objects, but does not take explicit ownership of them. The Organization class can loop through and access each Employee object, but does not have direct control over them (the Employee objects are owned by an external entity). Similarly, an Order class may have access to multiple Item objects, where it can loop through and access data from each Item without taking direct ownership of them.

In addition to composition and aggregation, there is also a concept of inheritance in Java. This is when a class inherits the properties and methods of another class. This allows for code reuse and helps to reduce the amount of code that needs to be written. Inheritance is a powerful concept in object-oriented programming and is used extensively in Java.

Summary

Composition and aggregation in Java are two powerful forms of association between classes that can be used in your code for better flexibility and maintainability. Composition implies that one class takes explicit ownership over another class’s instance, while aggregation allows for a more loosely coupled relationship between two classes. When deciding between composition and aggregation, consider how complex the relationship needs to be, what kind of application you are designing (single vs. multiple instance), and the level of encapsulation that you need.