Stub, mock, and proxy are three common types of objects used in testing, and they all serve different purposes. Here's a brief overview of each:

1. Stub: A stub is a simplified version of a class that provides a predefined set of responses to method calls. Stubs are often used to replace real objects in order to isolate and test specific functionality in a controlled environment. Stubs are typically created manually or using a framework like Mockito.

2. Mock: A mock is a special type of object that allows you to verify interactions between the system under test and other objects. Mocks are often used to test the behavior of a system when interacting with other objects, such as a database or network service. Like stubs, mocks are typically created using a mocking framework.

3. Proxy: A proxy is an object that wraps an existing object and intercepts method calls to that object. Proxies are often used for performance reasons, such as caching or lazy loading, or to implement cross-cutting concerns like security or logging. In testing, proxies can be used to create test doubles of objects and verify that certain methods are called.

When it comes to testing in Java, stubs, mocks, and proxies are often used together to provide a complete testing solution. For example, you might use a stub to isolate a specific part of your code for testing, a mock to verify interactions with external systems, and a proxy to intercept and test cross-cutting concerns.

Overall, understanding the differences between stubs, mocks, and proxies can help you write more effective tests and ensure that your code is behaving correctly in a variety of scenarios.