Difference between Comparator and Comparable in java

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One of the common interview question is “What are differences between Comparator and Comparable”. or “How will you sort collection of employee objects by its id or name”.For that we can use two interfaces.i.e. Comparator and Comparable.Before we actually see differences,let me give you brief introduction of both.

Comparable interface:

Class whose objects to be sorted must implement this interface.In this,we have to implement compareTo(Object) method.
For example:

If any class implements comparable inteface then collection of that object can be sorted automatically using Collection.sort() or Arrays.sort().Object will be sort on the basis of compareTo method in that class.
Objects which implement Comparable in java can be used as keys in a SortedMap like TreeMap or SortedSet like TreeSet without implementing any other interface.

Comparator interface:

Class whose objects to be sorted do not need to implement this interface.Some third class can implement this interface to sort.e.g.CountrySortByIdComparator class can implement Comparator interface to sort collection of country object by id. For example:

Using Comparator interface,we can write different sorting based on different attributes of objects to be sorted.You can use anonymous comparator to compare at particular line of code. For example:

Comparator vs Comparable

Parameter
Comparable
Comparator
Sorting logic
Sorting logic must be in same class whose objects are being sorted. Hence this is called natural ordering of objects
Sorting logic is in separate class. Hence we can write different sorting based on different attributes of objects to be sorted. E.g. Sorting using id,name etc.
Implementation

Class whose objects to be sorted must implement this interface.e.g Country class needs to implement comparable to collection of country object by id

Class whose objects to be sorted do not need to implement this interface.Some other class can implement this interface. E.g.-CountrySortByIdComparator class can implement Comparator interface to sort collection of country object by id
Sorting method
int compareTo(Object o1)
This method compares this object with o1 object and returns  a integer.Its value has following meaning
1. positive – this object is greater than o1
2. zero – this object equals to o1
3. negative – this object is less than o1
int compare(Object o1,Object o2)
This method compares o1 and o2 objects. and returns  a integer.Its value has following meaning.
1. positive – o1 is greater than o2
2. zero – o1 equals to o2
3. negative – o1 is less than o1
Calling method
Collections.sort(List)
Here objects will be sorted on the basis of CompareTo method
Collections.sort(List, Comparator)
Here objects will be sorted on the basis of Compare method in Comparator
Package
Java.lang.Comparable
Java.util.Comparator

Java code:

For Comparable:

We will create class country having attribute id and name.This class will implement Comparable interface and implement CompareTo method to sort collection of country object by id.

1. Country.java

2.ComparableMain.java

Output:

For Comparator:

We will create class country having attribute id and name and will create another class CountrySortByIdComparator which will implement Comparator interface and implement compare method to sort collection of country object by id and we will also see how to use anonymous comparator.

1.Country.java

2.CountrySortbyIdComparator.java

3.ComparatorMain.java

Output:

Please go through  top 50 core java interview questions for more interview questions.

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