Designing a Java application for testability means making it easy to write tests for the application. The goal is to create an application that can be easily tested, with minimal dependencies and a clear separation of concerns. This helps to reduce the complexity of testing, while also making it easier to identify and fix bugs.

Here are some design principles and practices that can help to make a Java application more testable:

1. Use dependency injection: By using dependency injection, you can make it easy to replace dependencies with mock objects during testing. This makes it possible to isolate the code under test and focus on testing specific behaviors.

2. Use interfaces: By defining interfaces for your classes, you can create a clear separation of concerns and reduce dependencies. This makes it easier to test your code, since you can easily replace implementations with mock objects.

3. Write small, focused methods: By writing small, focused methods that do one thing and do it well, you can make it easier to test your code. Small methods are easier to understand, easier to test, and less likely to have bugs.

4. Use SOLID design principles: The SOLID principles of object-oriented design provide guidelines for creating flexible, maintainable, and testable code. By following these principles, you can create an application that is easy to test and that can be easily modified to meet changing requirements.

5. Avoid static methods and global state: Static methods and global state can make it difficult to test your code, since they introduce hidden dependencies and make it hard to isolate the code under test. By avoiding these constructs, you can make it easier to test your code and ensure that it is flexible and maintainable.

6. Write automated tests: Writing automated tests is an essential part of designing for testability. By writing tests that cover all aspects of your application, you can ensure that your code is reliable and free of bugs. Automated tests also provide a safety net for refactoring, since they help to catch regressions and ensure that changes to the code do not introduce new bugs.

By following these design principles and practices, you can create a Java application that is easy to test and maintain. This can save time and effort in the long run, since bugs are caught early and changes can be made more easily.