Many of the collection types existed before Iterable<T> (which was only introduced in 1.5) – there was little reason to add a constructor to accept Iterable<T> as well as Collection<T> but changing the existing constructor would have been a breaking change. Personally I would use Iterable<T> if that allows you to do everything you want … Read more
You can do an enhanced for loop (for java 5 and higher) for iteration on array’s elements: String[] elements = {“a”, “a”, “a”, “a”}; for (String s: elements) { //Do your stuff here System.out.println(s); }
TreeSet has a log(n) time complexity guarantuee for add()/remove()/contains() methods. Sorting an ArrayList takes n*log(n) operations, but add()/get() takes only 1 operation. So if you’re mainly retrieving, and don’t sort often, ArrayList is the better choice. If you sort often but dont retrieve that much TreeSet would be a better choice.
Since Java 8 you can make use of Map#computeIfAbsent(). Map<String, List<User>> usersByCountry = new HashMap<>(); for (User user : listOfUsers) { usersByCountry.computeIfAbsent(user.getCountry(), k -> new ArrayList<>()).add(user); } Or, make use of Stream API’s Collectors#groupingBy() to go from List to Map directly: Map<String, List<User>> usersByCountry = listOfUsers.stream().collect(Collectors.groupingBy(User::getCountry)); In Java 7 or below, best what you can … Read more
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 … Read more
As others have noted, there is no in-built way. But you can always subclass that TreeSet, with your constructor(s) of choice, and add in the required functionality: public class UpdateableTreeSet<T extends Updateable> extends TreeSet<T> { // definition of updateable interface Updateable{ void update(Object value); } // constructors here … // ‘update’ method; returns false if … Read more
If you have another list that contains all the items you would like to add you can do arList.addAll(otherList). Alternatively, if you will always add the same elements to the list you could create a new list that is initialized to contain all your values and use the addAll() method, with something like Integer[] otherList … Read more
What you want is a very expensive operation in an ArrayList. It requires shifting every element between the beginning of the list and the location of C down by one. However, if you really want to do it: int index = url.indexOf(itemToMove); url.remove(index); url.add(0, itemToMove); If this is a frequent operation for you, and random … Read more
All standard implementations of java.util.List already implement java.io.Serializable. So even though java.util.List itself is not a subtype of java.io.Serializable, it should be safe to cast the list to Serializable, as long as you know it’s one of the standard implementations like ArrayList or LinkedList. If you’re not sure, then copy the list first (using something … Read more
You can use a lambda: Collectors.toMap(p -> p.getLast(), Function.identity()) or, more concisely, you can use a method reference using ::: Collectors.toMap(Person::getLast, Function.identity()) and instead of Function.identity, you can simply use the equivalent lambda: Collectors.toMap(Person::getLast, p -> p) If you use Netbeans you should get hints whenever an anonymous class can be replaced by a lambda.