Why is .compareTo() in an interface while .equals() is in a

The early [JLS 1.0 §4.3.2](http://titanium.cs.berkeley.edu/doc/java-langspec-1.0.pdf) defines the class `Object` in the following way:

> 4.3.2. The Class `Object`

>The standard class `Object` is a superclass (§8.1) of all other classes. (...) All class and array types inherit the methods of class `Object`,
which are summarized here (...)

> The members of `Object` are as follows:

> The method `getClass` returns the `Class` (§20.3) object (...)

> The method `toString` returns a `String` representation of the object.

> The methods `equals` and `hashCode` are declared for the benefit of hashtables
such as `java.util.Hashtable` (§21.7). The method equals defines a notion
of object equality, which is based on value, not reference, comparison.

> The method `clone` is used to make a duplicate of an object.

> The methods `wait`, `notify`, and `notifyAll` are used in concurrent programming
using threads, as described in §17.

> The method `finalize` is run just before an object is destroyed and is
described in §12.6.

So, that's why `equals` is in `Object` but `compareTo` is in a separate interface. Java devs included only the methods immediately needed for all  uses intended by then. To be hashable, every `Object` would need `equals` and `hashCode` - but not all objects would need to have a concept of **ordering**, which is what `compareTo` is used for.