java - Is instanceof considered bad practice? If so, under what circumstances is instanceof still preferable?

Question

Over the years, I've tried to avoid instanceof whenever possible. Using polymorphism or the visitor pattern where applicable. I suppose it simply eases maintenance in some situations... Are there any other drawbacks that one should be aware of?

I do however see it here and there in the Java libraries so I suppose it has its place? Under what circumstances is it preferable? Is it ever unavoidable?

Answer 1

It's definitely has its place in a stock implementation of equals. E.g.

public boolean equals ( Object o )
{
  if ( this == o )
  {
     return true;
  }

  if ( ! (o instanceof MyClass) )
  {
    return false;
  }

  // Compare fields
  ...
}

One neat thing to know about instanceof is that its LHS can be null and in that case the expression evaluates to false.