Wow, I happened to visit this thread exactly one year after I posted my last answer here. 🙂 Scala 2.9.1. mutable.Map comes with a withDefaultValue method. REPL session: scala> import collection.mutable import collection.mutable scala> mutable.Map[Int, String]().withDefaultValue(“”) res18: scala.collection.mutable.Map[Int,String] = Map() scala> res18(3) res19: String = “”
Your uniqueRecs has String type of the key. You have to do: List<String> keys = new ArrayList<>(uniqueRecs.keySet()); or List<ARecord> values = new ArrayList<>(uniqueRecs.values());
If you need to add all the values in a Map, try this: float sum = 0.0f; for (float f : map.values()) { sum += f; } At the end, the sum variable will contain the answer. So yes, for traversing a Map‘s values it’s best to use a for loop.
The following article addresses this question in some detail: HashMap requires a better hashCode() – JDK 1.4 Part II. According to that article, the main reason to move to power-of-two sizes was that bit masking is faster than integer division. This is not without adverse consequences, which are explained by one of the original authors: … Read more
You can see the implementation of hashCode function in java.util.ArrayList as public int hashCode() { int hashCode = 1; Iterator<E> i = iterator(); while (i.hasNext()) { E obj = i.next(); hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode()); } return hashCode; } It is one such example and your Eclipse generated code follows a … Read more
Most compilers should define std::hash_map for you; in the coming C++0x standard, it will be part of the standard library as std::unordered_map. The STL Page on it is fairly standard. If you use Visual Studio, Microsoft has a page on it. If you want to use your class as the value, not as the key, … Read more
the reference to sharedData which is final is thread safe since it can never be changed. The contents of the Map is NOT thread safe because it needs to be either wrapped with preferably a Guava ImmutableMap implementation or java.util.Collections.unmodifiableMap() or use one of the Map implementations in the java.util.concurrent package. Only if you do … Read more
The template arguments for an unordered_map looks like this: template< class Key, class T, class Hash = std::hash<Key>, class KeyEqual = std::equal_to<Key>, class Allocator = std::allocator< std::pair<const Key, T> > > class unordered_map; std::hash is not specialized for tuples (scroll down to Standard specializations for library types). Therefore you need to provide your own, something … Read more
1. Why is this so? HashMap is newer than Hashtable and fixes some of its limitations. I can only guess what the designers were thinking, but here are my guesses: Hashtable calculates a hash for each key by calling hashCode on each key. This would fail if the key were null, so this could be … Read more
If you wish to avoid rehashing the HashMap, and you know that no other elements will be placed into the HashMap, then you must take into account the load factor as well as the initial capacity. The load factor for a HashMap defaults to 0.75. The calculation to determine whether rehashing is necessary occurs whenever … Read more