In .NET 3.5 you can use a Lookup instead of a Dictionary. var items = new List<KeyValuePair<int, String>>(); items.Add(new KeyValuePair<int, String>(1, “first”)); items.Add(new KeyValuePair<int, String>(1, “second”)); var lookup = items.ToLookup(kvp => kvp.Key, kvp => kvp.Value); foreach (string x in lookup[1]) { Console.WriteLine(x); } The Lookup class is immutable. If you want a mutable version you … Read more
A synchronized list only synchronizes methods of this list. It means a thread won’t be able to modify the list while another thread is currently running a method from this list. The object is locked while processing method. As an example, Let’s say two threads run addAllon your list, with 2 different lists (A=A1,A2,A3 and … Read more
Java implements erasure, so there’s no way to tell on runtime if genericObject is an instance of Set<String> or not. The only way to guarantee this is to use bounds on your generics, or check all elements in the set. Compile-time Generic Bounds Using bounds checking, which will be checked at compile-time: public <T extends … Read more
The compiler doesn’t like it, because byte[] isn’t Byte[]. What you can do is use commons-lang’s ArrayUtils.toPrimitive(wrapperCollection): Byte[] bytes = pdu.toArray(new Byte[pdu.size()]); return ArrayUtils.toPrimitive(bytes); If you can’t use commons-lang, simply loop through the array and fill another array of type byte[] with the values (they will be automatically unboxed) If you can live with Byte[] … Read more
I agree with Stuart Marks that list == null ? Stream.empty() : list.stream() is the right way to do this (see his answer), or at least the right way to do this pre-Java 9 (see edit below), but I’ll leave this answer up to demonstrate usage of the Optional API. <T> Stream<T> getStream(List<T> list) { … Read more
No, annotation elements can only be primitive types, Strings, enum types, Class, other annotations, or arrays of any of these. The typical way to represent these kinds of structures would be to declare another annotation type public @interface TableMapping { public String dbName(); public String tableName(); } then say @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface column { … Read more
If I got your question right: List <Object> getList(int numOfElements){ return IntStream.range(0, numOfElements) .mapToObj(Object::new) // or x -> new Object(x).. or any other constructor .collect(Collectors.toList()); } If you want the same object n times: Collections.nCopies(n, T)
The “pair” generic class for .NET is Tuple. You would use it like: var strings=new List<Tuple<string, string>>(); strings.Add(Tuple.Create(“REFERENCE”, “Ref”)); A dictionary is a perfectly acceptable substitute if the left-most string is unique (ie a key). You’ll get errors otherwise. As to whether it’s better to use the built-in collections or create an actual object, depends … Read more
What is logic behind ordering object elements? how “(this.grade – s.grade)” if positive 1 moves “this.grade” front and puts “s.grade” next in order, why not other way around? Using negative numbers to say “this is less than that”, positive numbers to say “this is more than that” and 0 to say “these 2 things are … Read more
The BiMap from Google Guava looks like it will suit you. A bimap (or “bidirectional map”) is a map that preserves the uniqueness of its values as well as that of its keys. This constraint enables bimaps to support an “inverse view”, which is another bimap containing the same entries as this bimap but with … Read more