Sort Java Collection

Question

I have a Java collection:

Collection<CustomObject> list = new ArrayList<CustomObject>(); 

CustomObject has an id field now before display list I want to sort this collection by that id.

Is there any way I could that do that?

Answer 1

Use a Comparator:

List<CustomObject> list = new ArrayList<CustomObject>(); Comparator<CustomObject> comparator = new Comparator<CustomObject>() {     @Override     public int compare(CustomObject left, CustomObject right) {         return left.getId() - right.getId(); // use your logic     } };  Collections.sort(list, comparator); // use the comparator as much as u want System.out.println(list); 

Additionally, if CustomObjectimplements Comparable, then just use Collections.sort(list)

With JDK 8 the syntax is much simpler.

List<CustomObject> list = getCustomObjectList(); Collections.sort(list, (left, right) -> left.getId() - right.getId()); System.out.println(list); 

Much simplier

List<CustomObject> list = getCustomObjectList(); list.sort((left, right) -> left.getId() - right.getId()); System.out.println(list); 

Simplest

List<CustomObject> list = getCustomObjectList(); list.sort(Comparator.comparing(CustomObject::getId)); System.out.println(list); 

Obviously the initial code can be used for JDK 8 too.

Answer 2

The question is: "Sort Collection". So you can't use Collections.sort(List<T> l, Comparator<? super T> comparator).

Some tips:

For Collection type:

Comparator<String> defaultComparator = new Comparator<String>() {    @Override    public int compare(String o1, String o2) {        return o1.compareTo(o2);    } };  Collection<String> collection = getSomeStringCollection(); String[] strings = collection.toArray(new String[collection.size()]); Arrays.sort(strings, defaultComparator); List<String> sortedStrings = Arrays.asList(strings);  Collection<String> collection = getSomeStringCollection(); List<String> list = new ArrayList(collection); Collections.sort(list, defaultComparator); collection = list; // if you wish 

For List type:

List<String> list = getSomeStringList(); Collections.sort(list, defaultComparator); 

For Set type:

Set<String> set = getSomeStringSet(); // Than steps like in 'For Collection type' section or use java.util.TreeSet // TreeSet sample: // Sorted using java.lang.Comparable. Set<String> naturalSorted = new TreeSet(set);  Set<String> set = getSomeStringSet(); Set<String> sortedSet = new TreeSet(defaultComparator); sortedSet.addAll(set); 

Java 8 version. There is java.util.List#sort(Comparator<? super E> c) method

List<String> list = getSomeStringList(); list.sort(defaultComparator); 

or

List<String> list = getSomeStringList(); list.sort((String o1, String o2) -> o1.compareTo(o2)); 

or for types that implements Comparable:

List<String> list = getSomeStringList(); list.sort(String::compareTo);