UML relationships – dashed line vs solid line

I’m trying to give simple examples of the two types of lines.

In the first diagram, the solid line shows an association:

If the classes were declared in Java, this would be like ClassA storing a reference to ClassB as an attribute (it could be passed in to the constructor, created, etc.). So, you might see something like:

public class ClassA {
    ClassB theClassB = ...
    ...
}

In the second diagram, it shows a dependency:

A dependency is much weaker than an association. To quote from UML Distilled:

With classes, dependencies exist for various reasons: One class sends a message to another; one class has another as part of its data; one
class mentions another as a parameter to an operation. […] You use dependencies
whenever you want to show how changes in one element might alter other elements.

Again, using Java, a couple of examples exist: an argument of type ClassB is passed to a method, or a method declares a local variable of type ClassB:

public class ClassA {
    ...
    public void someMethod(ClassB arg1) {...}
    ...
    public void someOtherMethod() {
        ClassB localReferenceToClassB = ...
    }
    ...
}

Other ways ClassA could depend on ClassB without having an association (not an exhaustive list):

• ClassB has a static method that ClassA calls
• ClassA catches exceptions of type ClassB
• Whenever ClassB is modified, ClassA must also be modified (e.g., some logic is shared)