SOLID is an acronym for five design principles that aim to make software designs more understandable, flexible, and maintainable. These principles were introduced by Robert C. Martin, also known as Uncle Bob. Here is an overview of each principle as applied to Java development.

A class should have only one reason to change, meaning that a class should only have one job or responsibility. This principle helps to achieve high cohesion and low coupling, making the system easier to understand and modify.

Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. This means that the behavior of a module can be extended without modifying its source code, typically achieved through polymorphism and abstraction.

Objects of a superclass should be replaceable with objects of a subclass without affecting the correctness of the program. This principle ensures that a subclass can stand in for its superclass without altering the desirable properties of the program (correctness, task performed, etc.).

Clients should not be forced to depend on interfaces they do not use. This principle advocates for creating specific interfaces rather than a single general-purpose interface, thus ensuring that classes depend only on the methods they need.

High-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces). Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions. This principle decouples software modules, leading to a more flexible and reusable codebase.

By adhering to the SOLID principles, Java developers can create more maintainable, understandable, and flexible software systems. These principles guide the design and implementation process, ensuring that the system is well-structured and robust.