I am looking to implement a sort feature for my address book application.
I want to sort an ArrayList<Contact> contactArray. Contact is a class which contains four fields: name, home number, mobile number and address. I want to sort on name.
How can I write a custom sort function to do this?
This question is related to
java
sorting
collections
arraylist
This page tells you all you need to know about sorting collections, such as ArrayList.
Basically you need to
Contact class implement the Comparable interface by
public int compareTo(Contact anotherContact) within it.Collections.sort(myContactList);,
myContactList is ArrayList<Contact> (or any other collection of Contact).There's another way as well, involving creating a Comparator class, and you can read about that from the linked page as well.
Example:
public class Contact implements Comparable<Contact> {
....
//return -1 for less than, 0 for equals, and 1 for more than
public compareTo(Contact anotherContact) {
int result = 0;
result = getName().compareTo(anotherContact.getName());
if (result != 0)
{
return result;
}
result = getNunmber().compareTo(anotherContact.getNumber());
if (result != 0)
{
return result;
}
...
}
}
BalusC and bguiz have already given very complete answers on how to use Java's built-in Comparators.
I just want to add that google-collections has an Ordering class which is more "powerful" than the standard Comparators. It might be worth checking out. You can do cool things such as compounding Orderings, reversing them, ordering depending on a function's result for your objects...
Here is a blog post that mentions some of its benefits.
The Collections.sort is a good sort implementation. If you don't have The comparable implemented for Contact, you will need to pass in a Comparator implementation
Of note:
The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log(n) performance. The specified list must be modifiable, but need not be resizable. This implementation dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. This avoids the n2 log(n) performance that would result from attempting to sort a linked list in place.
The merge sort is probably better than most search algorithm you can do.
You need make your Contact classes implement Comparable, and then implement the compareTo(Contact) method. That way, the Collections.sort will be able to sort them for you. Per the page I linked to, compareTo 'returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object.'
For example, if you wanted to sort by name (A to Z), your class would look like this:
public class Contact implements Comparable<Contact> {
private String name;
// all the other attributes and methods
public compareTo(Contact other) {
return this.name.compareTo(other.name);
}
}
In addition to what was already posted you should know that since Java 8 we can shorten our code and write it like:
Collection.sort(yourList, Comparator.comparing(YourClass::getFieldToSortOn));
or since List now have sort method
yourList.sort(Comparator.comparing(YourClass::getFieldToSortOn));
Since Java 8, functional interfaces (interfaces with only one abstract method - they can have more default or static methods) can be easily implemented using:
arguments -> body source::method.Since Comparator<T> has only one abstract method int compare(T o1, T o2) it is functional interface.
So instead of (example from @BalusC answer)
Collections.sort(contacts, new Comparator<Contact>() {
public int compare(Contact one, Contact other) {
return one.getAddress().compareTo(other.getAddress());
}
});
we can reduce this code to:
Collections.sort(contacts, (Contact one, Contact other) -> {
return one.getAddress().compareTo(other.getAddress());
});
We can simplify this (or any) lambda by skipping
{return ... }So instead of
(Contact one, Contact other) -> {
return one.getAddress().compareTo(other.getAddress();
}
we can write
(one, other) -> one.getAddress().compareTo(other.getAddress())
Also now Comparator has static methods like comparing(FunctionToComparableValue) or comparing(FunctionToValue, ValueComparator) which we could use to easily create Comparators which should compare some specific values from objects.
In other words we can rewrite above code as
Collections.sort(contacts, Comparator.comparing(Contact::getAddress));
//assuming that Address implements Comparable (provides default order).
By using lambdaj you can sort a collection of your contacts (for example by their name) as it follows
sort(contacts, on(Contact.class).getName());
or by their address:
sort(contacts, on(Contacts.class).getAddress());
and so on. More in general, it offers a DSL to access and manipulate your collections in many ways, like filtering or grouping your contacts based on some conditions, aggregate some of their property values, etc.
Ok, I know this was answered a long time ago... but, here's some new info:
Say the Contact class in question already has a defined natural ordering via implementing Comparable, but you want to override that ordering, say by name. Here's the modern way to do it:
List<Contact> contacts = ...;
contacts.sort(Comparator.comparing(Contact::getName).reversed().thenComparing(Comparator.naturalOrder());
This way it will sort by name first (in reverse order), and then for name collisions it will fall back to the 'natural' ordering implemented by the Contact class itself.
I did it by the following way. number and name are two arraylist. I have to sort name .If any change happen to name arralist order then the number arraylist also change its order.
public void sortval(){
String tempname="",tempnum="";
if (name.size()>1) // check if the number of orders is larger than 1
{
for (int x=0; x<name.size(); x++) // bubble sort outer loop
{
for (int i=0; i < name.size()-x-1; i++) {
if (name.get(i).compareTo(name.get(i+1)) > 0)
{
tempname = name.get(i);
tempnum=number.get(i);
name.set(i,name.get(i+1) );
name.set(i+1, tempname);
number.set(i,number.get(i+1) );
number.set(i+1, tempnum);
}
}
}
}
}
You shoud use the Arrays.sort function. The containing classes should implement Comparable.
use this method:
private ArrayList<myClass> sortList(ArrayList<myClass> list) {
if (list != null && list.size() > 1) {
Collections.sort(list, new Comparator<myClass>() {
public int compare(myClass o1, myClass o2) {
if (o1.getsortnumber() == o2.getsortnumber()) return 0;
return o1.getsortnumber() < o2.getsortnumber() ? 1 : -1;
}
});
}
return list;
}
`
and use: mySortedlist = sortList(myList);
No need to implement comparator in your class.
If you want inverse order swap 1 and -1
Source: Stackoverflow.com