Why do we have contains(Object o) instead of contains(E e)?

Question

Is it to maintain backwards compatibility with older (un-genericized) versions of Collection? Or is there a more subtle detail that I am missing? I see this pat

Answer 1

Because otherwise it could have only be compared to the exact match of parameter type, specifically wildcarded collections would have stopped working, e.g.

class Base
{
}

class Derived
  extends Base
{
}

Collection< ? extends Base > c = ...;

Derived d = ...;

Base base_ref = d;

c.contains( d ); // Would have produced compile error

c.contains( base_ref ); // Would have produced compile error

EDIT
For doubters who think that's not one of the reasons, here is a modified array list with a would be generified contains method

class MyCollection< E > extends ArrayList< E >
{
    public boolean myContains( E e )
    {
        return false;
    }
}

MyCollecttion< ? extends Base > c2 = ...;

c2.myContains( d ); // does not compile
c2.myContains( base_ref ); // does not compile

Basically contains( Object o ) is a hack to make this very common use case to work with Java Generics.