There are several common relationships between classes

1. USES-A: Dependency. Type A will use Type B. This relationship is accidental and temporary. However, the changes in category B will affect category A. This is reflected in the code as: The parameters in the A method include the B class.

2. Association relationship: Type A will use type B. This is a strong dependency, which is long-term and not accidental. The performance in the code is: A class member variable contains B class.

3. HAS-A: aggregation relationship, ownership relationship, is a special case of association relationship, it is the relationship between whole and part. For example, the relationship between a flock of birds and birds is an aggregation relationship, and every part of the flock is a bird.

4. IS-A: Represents inheritance. The parent category and sub category, this will not be explained.

5. contains-a: Combination relationship is also a special case of association relationship. It reflects a contains-a relationship, which is stronger than aggregation and is also called strong aggregation. It also reflects the relationship between the whole and the part, but the whole and the part are inseparable.