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Don't extend classes in Java, only implement interfaces. You can go so far as to prohibited extending classes using static analysis tooling.

• chopin 4y ago
  You can make them final. This is especially useful when writing libraries.
  • plmpsu 4y ago
    Even better. Or sealed if the usecase applies.
    • LecroJS 4y ago
      Newbie OO q: What’s the benefit/use case of sealing or making an object final? My experience is in javascript so I wonder why I’d ever need something like Object.seal or Object.freeze. Is it really that difficult to maintain object instances that do not change after some point?
      • PawBer 4y ago
        It's just a tool to enforce the hierarchy in code. Once the codebase gets big it's hard to remember all the details about your API.