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Hi guys, if you are preparing for Java interview and looking for difference between Proxy and State design pattern, then you are at the right place. In the past, I have explained several important object-oriented design patterns like State, Strategy, Adapter, Facade, Factory, Observer, Decorator, Command, and Composite patterns and also shared frequently asked questions about design patterns. In this post, I will share the key difference between Proxy and State design pattern in Java and OOP. The difference between Proxy and State pattern lies in the intention and structure, which is the key difference between Proxy and State design pattern in object-oriented programming.

Difference between Proxy and State design patterns in Java

1. Structurally, the main difference between them is that the Proxy pattern has only one implementation, while the State pattern has more than one implementation to represent different states.
2. Proxies are used to control access to the real class, whereas the state pattern allows us to change the state dynamically and allows the class to behave differently in different states.
3. The State pattern not only adds more implementations to the proxy, but also enables switching from one implementation (state) to another during the lifecycle of the proxy (the object whose state is changing).
4. It is not necessary for the actual class to have the same interface as the proxy, as long as the proxy represents the actual class in some way, but this goes against the definition of the Proxy pattern given by GOF. However, it is best to have both the Proxy and the real class implement the same interface so that the actual class has all the methods that the proxy needs to call.
5. Another difference between the Proxy and State patterns is the problems they solve. The Proxy pattern can be used in different styles, such as

• Remote proxy : Acts as a proxy for an object in a different address space, such as in a separate JVM. If we have used RMI in Java, then we know that the RMI compiler (rmic) automatically creates a remote proxy for us when it creates the stub and skeleton.
• Virtual Proxy : can be used to facilitate "lazy initialization" of expensive objects. The proxy initially returned has no heavy lifting and is only created when needed.
• Protective Proxy : Used when we don’t want to expose the full functionality of an object to its clients. This is a real-world access control restriction example of the Proxy pattern .

Similarities between Proxy and State design patterns

Now that we understand the difference between the Proxy and State patterns , let’s look at the similarities between them to understand them better:

1. The Proxy pattern and the State pattern are both examples of the proxy pattern, i.e. they both provide a front-end class that delegates work to the real class hidden behind the proxy.
2. In both cases, the proxy class is derived from a base class along with other implementations, albeit for different reasons. In the case of the Proxy pattern, the proxy class implements the same interface as the actual class so that it can be used in place of it. This way we can pass the proxy to a method that accepts an actual class, or we can return an instance of Proxy instead of the actual class.

This is the difference between State and Proxy patterns in Java or any other object-oriented programming language.