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This article shows you how to distinguish the relationship between relevance, composition and aggregation in UML. The content is concise and easy to understand, which will definitely brighten your eyes. I hope you can get something through the detailed introduction of this article.

In the past, when learning object-oriented, I often heard about the various relationships between objects, common associations, composition and aggregation.

Association:

Relevance is one of the most common and common forms of relationship. It generally means that one object can send a message to another object. A typical implementation means that an object has a pointer or reference to an entity variable, which can also be called an association when it is accessed by passing or creating a local variable through the parameters of the method.

The typical code is as follows:

Class A {private B itemB;}

It may also take the following form:

Class A {void test (BB) {...}}

In general, we can call the relationship between two objects in the form of reference, parameter transfer and so on.

Aggregation (aggregation):

Aggregation represents a kind of has-a relationship, at the same time, it is also a whole-part relationship. Its characteristic is that the life cycle of this part is not managed by the whole. In other words, when the whole object no longer exists, some objects may continue to exist. Its uml diagram is represented as follows:

We use a hollow arrow to represent the aggregation relationship.

Generally speaking, the management of declaration cycle is still relatively vague. Let's explain it further with the Person and Address classes shown in the figure. Suppose we want to define these two objects, and for each person, he has an associated address. The relationship between person and address is the relationship of has-a. However, we cannot say that this address is an integral part of this person. At the same time, we establish that address objects and human objects can exist relatively independently.

In terms of code, the typical code style is as follows:

Public class Address {. . . } public class Person {private Address address; public Person (Address address) {this.address = address;}. . . }

We usually use Person objects in the following ways:

Address address = new Address (); Person person = new Person (address)

Or:

Person person = new Person (new Address ())

We can see that we created a separate Address object and then passed this object into the constructor of Person. When the Person object declaration cycle ends, the Address object may continue to exist if there are other references to it. In other words, their declaration cycle is relatively independent.

Combination (Composition):

After understanding the relationship of aggregation, the relationship of combination is much better. Composition is a stricter has-a relationship than aggregation. It represents a strict constituent relationship. Take the car and the engine as an example, the engine is an integral part of the car. They are a strict partial composition relationship, so their declaration cycle should also be consistent. In other words, the declaration cycle of the engine is managed by the car object.

The uml diagram of the combination is shown as follows:

A solid arrow is usually used to represent a combination.

Typical examples of combined code are as follows:

Public class Engine {. . . } public class Car {Engine e = new Engine ();. }

The Engine object is created inside the Car object, so at the end of the life cycle of the Car object, the life cycle of the Engine object also ends.

The above content is how to distinguish the relationship among relevance, combination and aggregation in UML. Have you learned the knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

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