Comparable and overriding compare method for sorting an array lists of objects in java

Question

I wanted to sort an array list of objects based on one of the properties of object lets say here in my code 'name' , I searched and found out about using 'Comparator' for this option, the below class is my objects :

public class PhoneBook{

long number;
String name;
String family;
String address;

public long getNumber() {
    return number;
}

public void setNumber(long number) {
    this.number = number;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getFamily() {
    return family;
}

public void setFamily(String family) {
    this.family = family;
}

public String getAddress() {
    return address;
}

public void setAddress(String address) {
    this.address = address;
}

}

and I override the 'compare' method in my sort method, like this:

public void sort()
{
    Collections.sort(phoneBooks, new Comparator<PhoneBook>() {
        @Override
        public int compare(PhoneBook o1, PhoneBook o2) {
            return o1.getName().compareToIgnoreCase(o2.getName());
        }
    });
}

but my problem is that I get confused why without implementing 'Comparable' in the first code like this:

public class PhoneBook implements Comparable<PhoneBook>

the sort method work well, and actually I tried the above code and it gave me error so I remove the implement part but I saw it works.

Possible duplicate of [When should a class be Comparable and/or Comparator?](http://stackoverflow.com/questions/1440134/when-should-a-class-be-comparable-and-or-comparator) and (http://stackoverflow.com/questions/4108604/java-comparable-vs-comparator) — Rafik BELDI, Nov 29 '15 at 22:31

Yoda · Answer 1 · 2015-12-01T13:06:52.450

2

There are two Collections.sort() methods.

First receives only a Collection to sort and uses the compareTo method. In this case your class needs to implement Comparable.

The other one which you have used receives two arguments, first is Collection second one is the Comparator. This one doesn't care if your class implements Comparable<? super YourClass> as it uses provided Comparator.

Lambda come in handy when implementing functional interfaces:

Collections.sort(phoneBooks, (o1,  o2)-> o1.getName().compareToIgnoreCase(o2.getName()));

edited Dec 01 '15 at 13:06

answered Nov 29 '15 at 22:25

Yoda

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thanks, I'm trying to use Lambda that you mentioned above, but I have no idea how Lambda will work here, I tried the line that you wrote for Lambda but I'm confused about how it should e use and where, because it doesn't work. – Bita Mirshafiee Nov 30 '15 at 14:23
@BitaMirshafiee I added closing paranthesis. – Yoda Dec 01 '15 at 13:07

score 2 · Answer 2 · answered Nov 29 '15 at 22:27

There are two ways you can do what you wanted here - one is using a custom Comperator (as you did above), and the second is by implementing Comperable interface. You don't need to do both.

The first way (comparator) - you run the sort method with your comparator, and the sort method will use it in order to compare the objects. since it uses the comparator, it does not need you to implement compareTo and Comparable

The second way (comperable) - you run the sort method without any comperator. what happens here is that the sort method will compare your object using the compareTo method from Comperable interface that you implement in your object

Either way will work, but you don't need both

score 2 · Accepted Answer · answered Nov 29 '15 at 22:30

Comparable :

Comparable provides single sorting sequence. In other words, we can sort the collection on the basis of single element such as id or name or price etc.
Comparable affects the original class, actual class is modified.
Comparable provides compareTo() method to sort elements.
We can sort the list elements of Comparable type by Collections.sort(List) method.

Comparator :

Comparator provides multiple sorting sequence. In other words, we can sort the collection on the basis of multiple elements such as id, name and price etc.
Comparator doesn't affect the original class i.e. actual class is not modified.
Comparator provides compare() method to sort elements.
We can sort the list elements of Comparator type by Collections.sort(List,Comparator) method.

Comparable and overriding compare method for sorting an array lists of objects in java

3 Answers3