In java why do we use design patterns?

I would,Design patterns are solutions to general problems that software developers faced during software developments.These solutions were obtained by trail and error by numerous software developers over quite a substantial period of time.

why we use design patterns in java?Ans:-

1.They are reusable in multiple projects.

2.They provide the solution that help to define system architecture.

3.They capture the s/w engineering experiences.

4.They provide transparency  to the design of an application.

When should we use design patterns?

Ans:-We must use design patterns during the analysis and requirement phase(SDLC) .Design patterns ease the analysis and requirement phase SDLC by providing info base on prior hands-on experiences.