Aggregation in Java with Example

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OOPS Tutorial

1. Overview

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

2. Intent/Definition

Aggregation is an association 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 “whole-part or a-part-of” relationship.
Let’s take an example of the relationship between Department and Teacher. A Teacher may belong to multiple departments. Hence Teacher is a part of multiple departments. But if we delete a Department, 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.

3. Implementation

Let's take an example of Line item and their 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 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 association with Product class. That means, if you delete LineItem, then 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);
    }
}

Aggregation vs Composition

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

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