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Java Java Data Structures - Retired Organizing Data Interfaces

Aditya Puri
Aditya Puri
1,080 Points
Posted by Aditya Puri
Aditya Puri
1,080 Points

Question about Comparable interface

So what I can understand is that the Comparable interface contains a method compareTo that compares 2 objects. Many classes have implemented this interface including String and Date. For String, it compares them according to the letters that they contain..for Date it compares them according to which date comes first. Am I right?

So then when we write int dateCmp = this.mCreationDate.compareTo(other.mCreationDate) then is the compareTo method of the class date is being called here?

Also why do we need to implement the Comparable interface to use the compareTo object? We are making the compareTo method from scratch anyway, then why do we need the interface? Can't we make the compareTo method without implementing the interface?

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

I assume you're right about how dates and strings are compared. The online documentation for a standard class should provide an explanation. Yes, that is using the compareTo method of the date class.

1 Answer

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

Interfaces require you provide implementations, but the advantage is you can use the object in new ways. interfaces define types in addition to those of normal inheritance hierarchy. If implementing a interface, the object can be plugged into places (either methods or language features) that expect/rely on a Comparable type. According to the docs for Comparable: "Lists (and arrays) of objects that implement this interface can be sorted automatically by Collections.sort (and Arrays.sort). Objects that implement this interface can be used as keys in a sorted map or as elements in a sorted set, without the need to specify a comparator."

Aditya Puri
Aditya Puri
1,080 Points
Aditya Puri
Aditya Puri
1,080 Points

i don't understand what you said......

Simon Coates
Simon Coates
28,694 Points
Simon Coates
Simon Coates
28,694 Points

Objects have an inheritance hierarchy with single inheritance. A String object is thus, also an Object, and can be used where anywhere an Object or String is expected. Classes that implement interfaces get additional types - A string for example is of Object type, of String type, and by virture of interfaces of Comparable, Serilializable and CharSequence types. As such, a String can be used (predictably) in places that expect any of these types (as defined by inheritance or interface implementation). Being Comparable specifically means that an object can be used for sorted collections. (treemap for instance).