When a key is added to or requested from a HashMap in OpenJDK, the flow of execution is the following: The key is transformed into a 32-bit value using the developer-defined hashCode() method. The 32-bit value is then transformed by a second hash function (of which Andrew’s answer contains the source code) into an offset … Read more
A dictionary is a general concept that maps keys to values. There are many ways to implement such a mapping. A hashtable is a specific way to implement a dictionary. Besides hashtables, another common way to implement dictionaries is red-black trees. Each method has its own pros and cons. A red-black tree can always perform … Read more
It would tell you the same thing if you had a Dictionary<SomeType, string>, SomeType being a reference type, and you tried to pass null as the key, it is not something affecting only nullable type like bool?. You can use any type as the key, nullable or not. It all comes down to the fact … Read more
Well, you could use Dictionary<string, dynamic> in C# 4 / .NET 4 – but other than that, you can’t do it with exactly the code shown because there’s no type which is implicitly convertible to int, string and double. (You could write your own one, but you’d have to list each type separately.) You could … Read more
Sleep(1) is a documented way in Windows to yield the processor and allow other threads to run. You can find this code in the Reference Source with comments: // Our memory model guarantee if we pick up the change in bucket from another processor, // we will see the ‘isWriterProgress’ flag to be true or … Read more
You call nextElement() twice in your loop. This call moves the enumeration pointer forward. You should modify your code like the following: while (e.hasMoreElements()) { String param = e.nextElement(); System.out.println(param); }
I had the same need and did some research and ended up using libcfu It’s simple and readable so if I have a need to modify, I can do it without spending too much time to understand. It’s also of BSD license. No need to change my structs (to embed say a next pointer) I … Read more
MSDN: A hash function must have the following properties: If two objects compare as equal, the GetHashCode method for each object must return the same value. However, if two objects do not compare as equal, the GetHashCode methods for the two object do not have to return different values. The GetHashCode method for an object … Read more
If I know the number of keys and exactly what are those keys, is there any way in python to make a dict (or a hashtable) work more efficiently? I vaguely remember that if you know the keys, you can design the hash function smartly (perfect hash?) and allocate the space beforehand. Python doesn’t expose … Read more
The standard trade offs between these data structures apply. Binary Trees medium complexity to implement (assuming you can’t get them from a library) inserts are O(logN) lookups are O(logN) Linked lists (unsorted) low complexity to implement inserts are O(1) lookups are O(N) Hash tables high complexity to implement inserts are O(1) on average lookups are … Read more