In object-oriented computer programming, SOLID is a mnemonic acronym for five design principles intended to make software designs more understandable, flexible and maintainable. It is not related to the GRASP software design principles. The principles are a subset of many principles promoted by Robert C. Martin. Though they apply to any object-oriented design, the SOLID principles can also form a core philosophy for methodologies such as agile development or adaptive software development. The theory of SOLID principles was introduced by Martin in his 2000 paper Design Principles and Design Patterns, although the SOLID acronym itself was introduced later by Michael Feathers.
a class should have only a single responsibility (i.e. only changes to one part of the software's specification should be able to affect the specification of the class).
"software entities … should be open for extension, but closed for modification."
"objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program." See also design by contract.
"many client-specific interfaces are better than one general-purpose interface."[4]
one should "depend upon abstractions, [not] concretions."