hashmap

Hashtable, HashMap, HashSet , hash table concept in Java collection framework

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Java’s Set and Map interfaces specify two very different collection types. A Set is just what it sounds like: a collection of distinct (non-equal) objects, with no other structure. A Map is, conceptually, also just what it sounds like: a mapping from a set of objects (the distinct keys) to a collection of objects (the values). Hashtable and HashMap both implement Map, HashSet implements Set, and they all use hash codes … Read more

Collision resolution in Java HashMap

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When you insert the pair (10, 17) and then (10, 20), there is technically no collision involved. You are just replacing the old value with the new value for a given key 10 (since in both cases, 10 is equal to 10 and also the hash code for 10 is always 10). Collision happens when multiple keys hash to the … Read more

C# Java HashMap equivalent

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Dictionary is probably the closest. System.Collections.Generic.Dictionary implements the System.Collections.Generic.IDictionary interface (which is similar to Java’s Map interface). Some notable differences that you should be aware of: Adding/Getting items Java’s HashMap has the put and get methods for setting/getting items myMap.put(key, value) MyObject value = myMap.get(key) C#’s Dictionary uses [] indexing for setting/getting items myDictionary[key] = value MyObject value = myDictionary[key] null keys Java’s HashMap allows null keys .NET’s Dictionary throws an ArgumentNullException if you try … Read more

C# Java HashMap equivalent

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Dictionary is probably the closest. System.Collections.Generic.Dictionary implements the System.Collections.Generic.IDictionary interface (which is similar to Java’s Map interface). Some notable differences that you should be aware of: Adding/Getting items Java’s HashMap has the put and get methods for setting/getting items myMap.put(key, value) MyObject value = myMap.get(key) C#’s Dictionary uses [] indexing for setting/getting items myDictionary[key] = value MyObject value = myDictionary[key] null keys Java’s HashMap allows null keys .NET’s Dictionary throws an ArgumentNullException if you try … Read more

Exception in thread “main” java.lang.StackOverflowError

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Your algorithm is fine. However int is too small for your computations, it fails for this input: At some point integer overflows to negative value and your implementation goes crazy, recursing infinitely. Change int num to long num and you’ll be fine – for some time. Later you’ll need BigInteger. Note that according to Wikipedia on Collatz conjecture (bold mine): The longest progression for … Read more