Aggregation in Java with Example

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OOPS Tutorial

In this article, we will learn the important object-oriented concept of Aggregation.
Aggregation is an association that represents a part of a whole relationship where a part can exist without a whole. It has a weaker relationship.

1. Intent/Definition

Aggregation is an association that represents a part of a whole relationship where a part can exist without a whole. It has a weaker relationship.
It is a specialized form of Association where all object has their own lifecycle but there is ownership. This represents a “whole-part or a-part-of” relationship.
Let’s take an example of the relationship between the Department and the Teacher. A Teacher may belong to multiple departments. Hence Teacher is a part of multiple departments. But if we delete a Department, the Teacher Object will not destroy.

Key Points

  • It represents the Has-A relationship.
  • It is a unidirectional association i.e. a one-way relationship. For example, the department can have students but vice versa is not possible and thus unidirectional in nature.
  • In Aggregation, both the entries can survive individually which means ending one entity will not affect the other entity.

2. Implementation

Let's take the example of LineItem and its Products. If line-item HAS-A product, then a line item is a whole and product is a part.
If a line item is deleted, then the corresponding product needs not to be deleted.
Step 1: Create a Product class.
class Product {
    private int id;
    private String name;
    private String description;

    public Product(int id, String name, String description) {
         super();
         this.id = id;
         this.name = name;
         this.description = description;
    }
    public int getId() {
         return id;
    }
    public void setId(int id) {
         this.id = id;
    }
    public String getName() {
         return name;
    }
    public void setName(String name) {
         this.name = name;
    }
    public String getDescription() {
        return description;
    }
    public void setDescription(String description) {
         this.description = description;
    }
    @Override
    public String toString() {
        return "Product [id=" + id + ", name=" + name + ", description=" + description + "]";
    }
}
Step 2: This is LineItem class, which HAS-A aggregation associated with the Product class. That means, if you delete LineItem, then the associated Product can exist.
class LineItem {
    private int id;
    private int quantity;
    private Product p;

    public LineItem(int id, int quantity, Product p) {
         super();
         this.id = id;
         this.quantity = quantity;
         this.p = p;
    }
    public int getId() {
         return id;
    }
    public void setId(int id) {
         this.id = id;
    }
    public int getQuantity() {
         return quantity;
    }
    public void setQuantity(int quantity) {
         this.quantity = quantity;
    }
    public Product getP() {
         return p;
    }
    public void setP(Product p) {
         this.p = p;
    }
    @Override
    public String toString() {
         return "LineItem [id=" + id + ", quantity=" + quantity + ", p=" + p + "]";
    }
}
Step 3: Let's test an Aggregation.
public class Aggregation {
    public static void main(String[] args) {
        // Create Products
        Product p1 = new Product(1, "Pen", "This is red pen");
        Product p2 = new Product(2, "Pencil", "This is pencil");
        Product p3 = new Product(3, "ColorBox", "This is color box");

        // Create lineItem and add quntity of the products
        LineItem item1 = new LineItem(1, 2, p1);
        LineItem item2 = new LineItem(1, 2, p2);
        LineItem item3 = new LineItem(1, 2, p3);
  
        // Before deleting line item 1 
        System.out.println(item1.getId());
        System.out.println(item1.getQuantity());
        System.out.println(item1.getP());
        item1 = null;

        // Still product exist and not deleted
        System.out.println(p1);
    }
}

3. Aggregation vs Composition

GitHub Repository

The source code of this post is available on GitHub: Object-Oriented Design Guide.
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