Java List interface contains(Object o) method signature

Question

Possible Duplicate:
Why aren’t Java Collections remove methods generic?

The Java Collection<E> interface has a contains method with the following signature:

boolean contains(Object o)

Since the interface is generic with type E shouldn't the signature be
boolean contains(E o)

to reflect that and only allow arguments of type E.

The same question can be asked regarding the remove(Object o) method

Looking at the [Collection JavaDoc](http://docs.oracle.com/javase/7/docs/api/java/util/Collection.html#contains(java.lang.Object)), it looks like the method is there from Java 1.2, were generics didn't exist yet in Java language. — Luiggi Mendoza, Oct 25 '12 at 00:07
This question has been asked and answered many times before: [Why does Java Map take an untyped parameter for the get and remove methods?](http://stackoverflow.com/questions/4269147/why-does-java-mapk-v-take-an-untyped-parameter-for-the-get-and-remove-methods) and [Why aren't Java Collections remove methods generic?](http://stackoverflow.com/questions/104799/why-arent-java-collections-remove-methods-generic) — Daniel Pryden, Oct 25 '12 at 00:08
Thanks, Daniel, for the reference. This is exactly what I needed. — Jonathan, Oct 25 '12 at 00:31

score 4 · Accepted Answer · edited May 23 '17 at 11:55

The contains and remove methods accept any object because the they accept (and can succeed with) objects that might not be instances of E. The contract for contains is:

returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e))

Note that o need not actually be an object in the collection; it must merely pass the equals test.

The same idea goes for remove.

See also this thread, where it is pointed out that making contains and remove generic would have broken a lot of existing, perfectly valid code.

Java List interface contains(Object o) method signature

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