Sort Java Collection

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.

A slightly different example say if you have a class that doesn't implement Comparable but you still want to sort it on a field or method.

Collections.sort(allMatching, new Comparator<ClassOne>() {
  @Override public int compare(final ClassOne o1, final ClassOne o2) {
    if (o1.getMethodToSort() > o2.getMethodToSort()) {
      return 1;
    } else if (o1.getMethodToSort() < o2.getMethodToSort()) {
      return -1;
    }  
    return 0;
  }
});

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);