collections – Page 17

Java Stream: divide into two lists by boolean predicate

August 15, 2023 by Tarik

Collectors.partitioningBy: Map<Boolean, List<Employee>> partitioned = listOfEmployees.stream().collect( Collectors.partitioningBy(Employee::isActive)); The resulting map contains two lists, corresponding to whether or not the predicate was matched: List<Employee> activeEmployees = partitioned.get(true); List<Employee> formerEmployees = partitioned.get(false); There are a couple of reasons to use partitioningBy over groupingBy (as suggested by Juan Carlos Mendoza): Firstly, the parameter of groupingBy is a Function<Employee, … Read more

Java: cast collection type to subtype [duplicate]

August 15, 2023 by Tarik

You can cast through the untyped List interface: List<A> a = new ArrayList<A>(); List<B> b = (List)a;

IndexOf predicate?

August 14, 2023 by Tarik

Are you looking for List<T>.FindIndex(predicate)?

jQuery add elements to empty selection?

August 13, 2023 by Tarik

Quoting from the jQuery website: Given a jQuery object that represents a set of DOM elements, the .add() method constructs a new jQuery object from the union of those elements and the ones passed into the method. Hence, when you do .add() it will not save the added elements, you would have to explicitly assign … Read more

C# / .NET equivalent for Java Collections.emptyList()?

August 13, 2023 by Tarik

Personally, I think this is better than any of the other answers: static readonly IList<T> EmptyList = new T[0]; Arrays implement IList<T>. You cannot add to an array. You cannot assign to an element in an empty array (because there is none). This is, in my opinion, a lot simpler than new List<T>().AsReadOnly(). You still … Read more

HashMap with Null Key and Null Value

August 13, 2023 by Tarik

HashMap does not call hashcode when null is passed as key and null Key is handled as special case. Put Method HashMap puts null key in bucket 0 and maps null as key to passed value. HashMap does it by linked list data structure. HashMap uses linked list data structure internally. Linked list data structure … Read more

How to check in Groovy that object is a list or collection or array?

August 12, 2023 by Tarik

A List is a Collection, so the checks aren’t mutually exclusive: def foo = … boolean isCollection = foo instanceof Collection boolean isList = foo instanceof List boolean isSet = foo instanceof Set boolean isArray = foo != null && foo.getClass().isArray()

When is the unmodifiablemap (really) necessary?

August 12, 2023 by Tarik

Collections.unmodifiableMap guarantees that the map will not be modified. It’s mostly useful if you want to return a read-only view of an internal map from a method call, e.g: class A { private Map importantData; public Map getImportantData() { return Collections.unmodifiableMap(importantData); } } This gives you a fast method that does not risk the client … Read more

How to sort an arraylist of objects by a property?

August 11, 2023 by Tarik

You can use Collections.sort with a custom Comparator<HockeyPlayer>. class HockeyPlayer { public final int goalsScored; // … }; List<HockeyPlayer> players = // … Collections.sort(players, new Comparator<HockeyPlayer>() { @Override public int compare(HockeyPlayer p1, HockeyPlayer p2) { return p1.goalsScored – p2.goalsScored; // Ascending } }); The comparision part can also be written this way : players.sort(Comparator.comparingInt(HockeyPLayer::goalsScored)); Alternatively, … Read more