It's a mistake in Java too. It's often convenient to simply skip implementing equals() and hashCode() because you don't plan to use an object as the key to a map. In that case, attempting to use it as a key should fail to compile, so you know you need to go back and add it.

The Comparable interface shows how it should be done.

(But on the other hand, if you were starting over it would be better to learn from Go's interfaces.)