Why .compareTo() Is in an Interface While .equals() Is in a Class in Java

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I want to know why the .compareTo() is in the Comparable interface while a method like .equals is in the Object class. To me, it seems arbitrary why a method like .compareTo() is not in the Object class already.

To use .compareTo(), you implement the Comparable interface and implement the .compareTo() method for your purposes. For the .equals() method, you simply override the method in your class, since all classes inherit from the Object class.

My question is why is a method like .compareTo() in an interface that you implement rather than in a class like Object? Likewise, why is the .equals() method in the class Object and not in some interface to be implemented?

Best Answer

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.

Related Solutions

C# Interfaces – Abstract Class and Inheritance vs Interface

The difference between the abstract classes and interfaces in your case could allow the developer to well-define the pattern of your design with abstract class create "is a" relationship and interface create "can do" relationship.

Put it this way, if you have a Toy car and it would be not make sense to make them "is a" relationship with Minivan since you can use interface to exhibit toy car specific behaviour like water resistance tolerance.

So you can define like:

public Interface IToyCarResistance
{
  int ToleranceLevel { get; }
}

public class ToyCar : Car , IToyCarResistance
{
}

Java Comparison – Tricky compareTo, Inheritance, Easy to Extend

You could write a general compareTo method in class A, which uses a key computed by a method compareKey which is also in A, but can be overridden by each subclass to make as fine a distinction as it wants. If you make it return a float, you can keep subdividing the comparison criterion whenever a new level of inheriting classes is added (within reason, obviously).

Best Answer

Related Solutions

C# Interfaces – Abstract Class and Inheritance vs Interface

Java Comparison – Tricky compareTo, Inheritance, Easy to Extend

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