preface
Let time accumulate real value
I’ve been seeing some class diagrams lately, probably because I don’t see enough of them, and I don’t know what they mean. You need to go back and forth to Baidu. In order to save time looking at UML class diagrams, I made this note for my own reference.
How do you distinguish between classes
The lines between class diagrams represent the relationships between classes. There are basically 6 kinds of connections.
For simplicity, instead of using a real class diagram, use rectangles instead.
Generalization relation — solid line hollow triangle arrow
To put it bluntly, class B inherits class A, and B and A have A generalization relationship
In Java, inheritance relationships are often represented by the extends keyword
interface A{
.
}
class B extends A{
.
}
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Realize the relationship — dotted line hollow triangle arrow
Simply put, a class implements an interface for another class.
Implementing relationships in Java is often represented by the implements keyword
class A{
.
}
class B implements A{
.
}
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Dependencies — dotted arrow:
Represents dependencies, with arrows pointing to dependent entities (dependencies, where one node is part of another node)
In Java, dependencies often take the form of local variables, method parameters, and so on
class A{
.
}
class B {
// Method A depends on class A
public void methodA(A a){
.
}
}
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Association relation — solid arrow:
Association, representing connections between objects. Associations enable one object to know the properties and methods of another object.
In Java, associations are often represented using instance variables (class attributes)
class A{
.
}
class B {
private A a;
.
}
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Aggregation relationship — hollow diamond + solid line + arrow
Aggregation relation is a special case of correlation relation, strong correlation relation, and the relation between individual and whole.
Note: the individual and the whole are separable. For example, departments and employees (employees can be independent from departments)
In Java, aggregate relationships are also represented using instance variables and typically use a collection to load individuals (one-to-many relationships).
class A{
.
}
class B {
private List<A> aArray;
.
}
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Combination relation – solid diamond + solid line + arrow
Aggregative relation can be regarded as an enhanced version of aggregative relation. The individual cannot exist independently from the whole, just like the relationship between man and hand.
Syntactic relation and combinatorial relation cannot be distinguished directly, but only logical relation between objects can be distinguished.
added
Generalization and realization relationships are common and easy to recognize. The coupling degree among dependency relation, association relation, aggregation relation and combination relation increases successively.
How to draw UML class entities
Generally speaking, there are two kinds of commonly used class diagrams, one is the interface class diagram, the other is the ordinary class diagram
Generic class class diagram
The class diagram is divided into three squares, and different squares represent different contents.
The first box — the class name
The second square — attributes of the class
There are many rows, and each row corresponds to a property
+ attribute1:type = defaultValue
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The position of the first “+” is used to indicate the visibility of the property. Among them:
-
“+” for public
-
“-” corresponds to private
-
The “#” corresponds to protected
-
Attribute1 represents the name of the attribute
-
Type corresponds to the type of the attribute
The third square — the method of the class
Like properties, a line represents a method, and method visibility is also represented by “+”, “-“, and “#”.
The argument list needs to be enclosed in parentheses for the method
Interface class diagram
There are no attributes in an interface class. In contrast to a normal class diagram, the interface class diagram removes the second square.