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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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