Not all objects can be compared, but all objects can be checked for equality. If nothing else, one can see if two objects exist at the same location in memory (reference equality).

What does it mean to compareTo() on two Thread objects? How is one thread “greater than” another? How do you compare two ArrayList<T>s?

The Object contract applies to all Java classes. If even one class cannot be compared to other instances of its own class, then Object cannot require it to be part of the interface.

Joshua Bloch uses the key words “natural ordering” when explaining why a class might want to implement Comparable. Not every class has a natural ordering as I mentioned in my examples above, so not every class should implement Comparable nor should Object have the compareTo method.

…the compareTo method is not declared in Object. … It is
similar in character to Object‘s equals method, except that it
permits order comparisons in addition to simple equality comparisons,
and it is generic. By implementing Comparable, a class indicates
that its instances have a natural ordering.

_{^{Effective Java, Second Edition: Joshua Bloch. Item 12, Page 62. Ellipses remove references to other chapters and code examples.}}

For cases where you do want to impose an ordering on a non-Comparable class that does not have a natural ordering, you can always supply a Comparator instance to help sort it.

The JLS §4.3.2 defines the class object in the following way:

4.3.2. The Class Object

The class Object is a superclass (§8.1.4) of all other classes.

All class and array types inherit (§8.4.8) the methods of class Object, which are summarized as follows:

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

• The method equals defines a notion of object equality, which is based on value, not reference, comparison.

• The method finalize is run just before an object is destroyed (§12.6).

• The method getClass returns the Class object that represents the class of the object.

• A Class object exists for each reference type. It can be used, for example, to discover the fully qualified name of a class, its members, its immediate superclass, and any interfaces that it implements.

  The type of a method invocation expression of getClass is Class<? extends |T|> where T is the class or interface searched (§15.12.1) for getClass.

  A class method that is declared synchronized (§8.4.3.6) synchronizes on the monitor associated with the Class object of the class.

• The method hashCode is very useful, together with the method equals, in hashtables such as java.util.Hashmap.

• The methods wait, notify, and notifyAll are used in concurrent programming using threads (§17.2).

• The method toString returns a String representation of the object.

So, that’s why equals is in Object but compareTo is in a separate interface. I would speculate that they wanted to keep Object as minimal as possible. They probably figured that nearly all Objects would need equals and hashCode (which is really just a form of equality testing) but not all objects would need to have a concept of ordering, which is what compareTo is used for.

In addition to Snowman’s excellent answer, remember that Comparable has been a generic interface for a long time. A type doesn’t implement compareTo(object), it implements compareTo(T) where T is its own type. This cannot be implemented on object, since object does not know the class that will be derived from it.

object could have defined an compareTo(object) method, but this would have allowed not just what Snowman points out, a comparison between two ArrayList<T>s or between two Threads, but even a comparison between an ArrayList<T> and a Thread. That’s even more nonsensical.

Suppose I have two object references: X identifies an instance of String holding the content “George”; Y identifies instance of Point holding the coordinates [12,34]. Consider the following two questions:

• Do X and Y identify equivalent objects?

• Should X sort before, after, or equivalent to Y?

The fact that X and Y identify instances of unrelated types does not pose any problem when considering the first question. Objects can only be considered equivalent if their types share a common base that defines them as being equivalent; since String and Point have no such base (their only common base type regards all distinct objects as non-equivalent) the answer is simply “no”.

The fact that the types are unrelated, however, poses a huge problem with regard to the second question. Some types define ordering relationships among their instances, and some ordering relationships may even extend over multiple types [e.g. it would be possible for BigInteger and BigDecimal to define comparison methods that would allow instances of either type to be ranked relative to instances of the other], but it is not possible in general to take two arbitrary instances and ask “Should X sort before, after, or equivalent to Y” and derive a total ordering. It would be possible to ask “Should X sort before, after, equivalent, or unranked with respect to Y” if objects were required to report a consistent ordering though not a total one, but most sorting algorithms require total orderings. Thus, even if all objects could implement a compareTo method if “unranked” was a valid return, such a method wouldn’t be useful enough to justify its existence.