# 钻石标志在UML类图中表示什么?[英] What does a diamond sign signify in UML class diagrams?

### 问题描述

Customer类是在两种情况下的任何一个中都负责Order类的生命周期吗?

## 其他推荐答案

http://en.wikipedia.org/wiki/wiki/class_diagram

### 问题描述

Consider the below two diagrams. The top one contains a plain arrow at the right end, and the bottom one contains an arrow with a diamond at the left end and a plain arrow at the right end. The great book GoF has usages of both of these two kinds of diagrams.

I want to understand what does the diamond signify in the second case, and which one of these two gives a stronger association between Customer and Order.

Is the Customer class responsible for the lifecycle of Order class in either of the two cases?

## 推荐答案

If there is no diamond, then we have a simple association.

If the diamond is left empty, it signifies it is an aggregation. This relation is stronger than a simple association. In this case a Customer aggregates Orders.

If the diamond is black, this means it is a composition, which is even stronger than an aggregation because the composite class cannot be composite by other classes. Its "life" depends on the container.

I think it's explained a little bit more clearly, at:

## 其他推荐答案

The customer and order is a "has-a" composition relationship because if the customer ceases to exist so does his/her order. Therefore, there is a life cycle dependency between the container (customer) and the instances within that container (order). When the container ceases to exist so does all its instances.

On the other hand, if we say a customer has a book. In this case, we have a "has-a" aggregation relationship. Because if the customer ceases to exist, the book is still available.

## 其他推荐答案

It signifies aggregation. From wikipedia:

In UML, it is graphically represented as a hollow diamond shape on the containing class end of the tree with a single line that connects the contained class to the containing class.The aggregate is semantically an extended object that is treated as a unit in many operations,although physically it is made of several lesser objects.

http://en.wikipedia.org/wiki/Class_diagram