collections

问题 I have a Map that uses a Set for the key type, like this: Map<Set<Thing>, Val> map; When I query map.containsKey(myBunchOfThings), it returns false, and I don't understand why. I can iterate through each key in the keyset and verify there is a key that (1) has the same hashCode, and (2) is equals() to myBunchOfThings. System.out.println(map.containsKey(myBunchOfThings)); // false. for (Set<Thing> k : map.keySet()) { if (k.hashCode() == myBunchOfThings.hashCode() && k.equals(myBunchOfThings) {

问题 I have a Map that uses a Set for the key type, like this: Map<Set<Thing>, Val> map; When I query map.containsKey(myBunchOfThings), it returns false, and I don't understand why. I can iterate through each key in the keyset and verify there is a key that (1) has the same hashCode, and (2) is equals() to myBunchOfThings. System.out.println(map.containsKey(myBunchOfThings)); // false. for (Set<Thing> k : map.keySet()) { if (k.hashCode() == myBunchOfThings.hashCode() && k.equals(myBunchOfThings) {

问题 I understand that a dictionary is not an ordered collection and one should not depend on the order of insertion and retrieval in a dictionary. However, this is what I noticed: Added 20 key value pairs to a Dictionary Retrieved them by doing a foreach(KeyValuePair...) The order of retrieval was same as the order in which they were added. Tested for around 16 key value pairs. Is this by design? 回答1: It's by coincidence, although predictably so. You absolutely shouldn't rely on it. Usually it

问题 When I want to make a value type read-only outside of my class I do this: public class myClassInt { private int m_i; public int i { get { return m_i; } } public myClassInt(int i) { m_i = i; } } What can I do to make a List<T> type readonly (so they can't add/remove elements to/from it) outside of my class? Now I just declare it public: public class myClassList { public List<int> li; public myClassList() { li = new List<int>(); li.Add(1); li.Add(2); li.Add(3); } } 回答1: You can expose it

问题 I'm trying to find an implementation of java.util.List and java.util.Set at the same time in Java. I want this class to allow only unique elements (as Set ) and preserve their order (like List ). Does it exist in JDK 6? It's important to have List<T>#add(int, T) so I can insert into a specific position. 回答1: TreeSet is sorted by element order; LinkedHashSet retains insertion order. Hopefully one of those is what you were after. You've specified that you want to be able to insert at an

问题 I'm trying to find an implementation of java.util.List and java.util.Set at the same time in Java. I want this class to allow only unique elements (as Set ) and preserve their order (like List ). Does it exist in JDK 6? It's important to have List<T>#add(int, T) so I can insert into a specific position. 回答1: TreeSet is sorted by element order; LinkedHashSet retains insertion order. Hopefully one of those is what you were after. You've specified that you want to be able to insert at an

问题 For trait Item case class TypeA(i: Int) extends Item case class TypeB(i: Int) extends Item consider a Scala list of items such as val myList = List(TypeA(1), TypeB(11), TypeB(12), TypeA(2), TypeB(21), TypeA(3), TypeB(31)) The goal is to define a new slice method that can be applied onto myList and which takes a predicate or condition as argument; for instance myList.slice { x => x.isInstanceOf[TypeA] } would deliver List(List(TypeA(1), TypeB(11), TypeB(12)), List(TypeA(2), TypeB(21)), List

问题 I have a set of objects in a Vector from which I'd like to select a random subset (e.g. 100 items coming back; pick 5 randomly). In my first (very hasty) pass I did an extremely simple and perhaps overly clever solution: Vector itemsVector = getItems(); Collections.shuffle(itemsVector); itemsVector.setSize(5); While this has the advantage of being nice and simple, I suspect it's not going to scale very well, i.e. Collections.shuffle() must be O(n) at least. My less clever alternative is

问题 This question already has answers here : How to remove elements from a generic list while iterating over it? (26 answers) Closed 6 years ago . I'm trying to remove an item from an ArrayList and I get this Exception: Collection was modified; enumeration operation may not execute. Any ideas? 回答1: You are removing the item during a foreach , yes? Simply, you can't. There are a few common options here: use List<T> and RemoveAll with a predicate iterate backwards by index, removing matching items

问题 This question already has answers here : DataTable iteration gives unwanted data (2 answers) Anonymous type result from sql query execution entity framework (5 answers) Closed 5 days ago . My previous question was closed due to typo. But my issue is unresolved. So rephrasing the question. Hi, I have a query which returns data dynamically. Number of columns, their name and datatypes are not known. It's like a Select * from table_name I need to get the result in a list. I tried with IEnumerable