Difference between Comparable and Comparator interfaces

[ashni Prakash]

Can someone tell me the difference between Comparable and Comparator interface and which to use when with examples.

Thanks

[Sergey Petunin]

Comparable interface is defined in the java.lang package and is implemented by a class if the objects of this class can be compared to each other. The comparation can be done using the compareTo() method of this interface. This method should return 0 if the objects are equal, 1 if the argument is smaller and -1 if the argument is bigger than the object on which we're calling this method.

Here's an example of a comparable class. We define the rule for comparation: the RichMan object is bigger if it's int field "money" is greater than one of another RichMan .

class RichMan implements Comparable<RichMan> {
 
    int money;
 
    String name;
 
    public int compareTo(RichMan anotherRichMan) {
    if (money > anotherRichMan.money)
    return 1;
    else if (money < anotherRichMan.money)
    return -1;
    else return 0;
    }
}

Comparator interface is defined in the java.util package and is used for sorting collections, e.g., with Collections.sort method. It defines a method compare(T o1, T o2) which is used for defining a custom sort order. For example, you want to sort a list of RichMan objects not by their money amount, but using their name order. Here's how this can be done:

class NameComparator implements Comparator<RichMan> {
    public int compare(RichMan man1, RichMan man2) {
        return man1.name.compareTo(man2.name);
    }
}

So to sort a list of RichMan objects by their names, you can say:

Collections.sort(richMenList, new NameComparator());

[ December 18, 2007: Message edited by: Serge Petunin ]

[Vishwanath Krishnamurthi]

Hi,
And also, the custom sort order can be specified for a sorted collection like TreeMap with a comparator as a constructor argument;

TreeMap()
Constructs a new, empty map, sorted according to the keys' natural order.

TreeMap(Comparator c)
Constructs a new, empty map, sorted according to the given comparator