Design pattern:

https://www.tutorialspoint.com/design_pattern/design_pattern_overview.htm

But the most common one I have seen are in the projects I worked so far are:

• Dependency Injection Pattern: http://www.journaldev.com/2394/java-dependency-injection-design-pattern-example-tutorial
• DTO Pattern: https://stackoverflow.com/questions/1051182/what-is-data-transfer-object
• Builder Pattern: https://jlordiales.me/2012/12/13/the-builder-pattern-in-practice/
• Simple Factory Pattern: https://alvinalexander.com/java/java-factory-pattern-example
• Strategy Pattern: https://dzone.com/articles/design-patterns-strategy
• Singleton Pattern: http://www.oodesign.com/singleton-pattern.html (Bad Practice to create your own, usually you will depend on a framework or library to create a singleton - https://stackoverflow.com/questions/137975/what-is-so-bad-about-singletons\

Why design Patterns?

Someone already solved your problems

• Creational Design Patterns
  • Simple Factory
  • Builder
  • Factory Method
  • ...etc
• Behavior Design Patterns
  • Stragegy
  • Template
  • ..etc
• Structural Design Patterns
  • Adapter
  • Bridge
  • Composite
  • ..etc