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In software engineering, a class diagram [1] in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects.
A singleton implementation may use lazy initialization in which the instance is created when the static method is first invoked. In multithreaded programs, this can cause race conditions that result in the creation of multiple instances. The following Java 5+ example [6] is a thread-safe implementation, using lazy initialization with double ...
Software reuse is also a major benefit of using Object classes. Classes facilitate re-use via inheritance and interfaces. When a new behavior is required it can often be achieved by creating a new class and having that class inherit the default behaviors and data of its superclass and then tailoring some aspect of the behavior or data accordingly.
While the library classes java.util.Observer and java.util.Observable exist, they have been deprecated in Java 9 because the model implemented was quite limited. Below is an example written in Java that takes keyboard input and handles each input line as an event.
A sample UML class and sequence diagram for the Decorator design pattern. [7] In the above UML class diagram, the abstract Decorator class maintains a reference (component) to the decorated object (Component) and forwards all requests to it (component.operation()). This makes Decorator transparent (invisible) to clients of Component.
In the above UML class diagram, the Creator class that requires a Product object does not instantiate the Product1 class directly. Instead, the Creator refers to a separate factoryMethod() to create a product object, which makes the Creator independent of the exact concrete class that is instantiated.
Composition over inheritance (or composite reuse principle) in object-oriented programming (OOP) is the principle that classes should favor polymorphic behavior and code reuse by their composition (by containing instances of other classes that implement the desired functionality) over inheritance from a base or parent class. [2]
They can be overridden by using a different metaclass instead of type. The above example contains some redundant code to do with the four attributes make, model, year, and color. It is possible to eliminate some of this redundancy using a custom metaclass. In Python, a metaclass is most easily defined as a subclass of type.