Do you ever store a null value? If not, you can just do: Foo value = map.get(key); if (value != null) { … } else { // No such key } Otherwise, you could just check for existence if you get a null value returned: Foo value = map.get(key); if (value != null) { … … Read more
One of the numerous niceties of Ruby is the possibility to extend existing classes with your own methods. That’s called “class reopening” or monkey-patching (the meaning of the latter can vary, though). So, take a look here: car = {:make => “bmw”, :year => “2003”} # => {:make=>”bmw”, :year=>”2003″} car.to_json # NoMethodError: undefined method `to_json’ … Read more
Hash your objects yourself manually, and use the resulting strings as keys for a regular JavaScript dictionary. After all, you are in the best position to know what makes your objects unique. That’s what I do. Example: var key = function(obj){ // Some unique object-dependent key return obj.totallyUniqueEmployeeIdKey; // Just an example }; var dict … Read more
There is no difference between the objects; you have a HashMap<String, Object> in both cases. There is a difference in the interface you have to the object. In the first case, the interface is HashMap<String, Object>, whereas in the second it’s Map<String, Object>. But the underlying object is the same. The advantage to using Map<String, … Read more
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] = … Read more
Short answer Use a TreeMap. This is precisely what it’s for. If this map is passed to you and you cannot determine the type, then you can do the following: SortedSet<String> keys = new TreeSet<>(map.keySet()); for (String key : keys) { String value = map.get(key); // do something } This will iterate across the map … Read more
This is fairly trivial to do with Array.prototype.reduce: var arr = [ { key: ‘foo’, val: ‘bar’ }, { key: ‘hello’, val: ‘world’ } ]; var result = arr.reduce(function(map, obj) { map[obj.key] = obj.val; return map; }, {}); console.log(result); // { foo:’bar’, hello:’world’ } Note: Array.prototype.reduce() is IE9+, so if you need to support older … Read more
If your data structure has many-to-one mapping between keys and values you should iterate over entries and pick all suitable keys: public static <T, E> Set<T> getKeysByValue(Map<T, E> map, E value) { Set<T> keys = new HashSet<T>(); for (Entry<T, E> entry : map.entrySet()) { if (Objects.equals(value, entry.getValue())) { keys.add(entry.getKey()); } } return keys; } In … Read more
LinkedHashMap is precisely what you’re looking for. It is exactly like HashMap, except that when you iterate over it, it presents the items in the insertion order.
Rails has an except/except! method that returns the hash with those keys removed. If you’re already using Rails, there’s no sense in creating your own version of this. class Hash # Returns a hash that includes everything but the given keys. # hash = { a: true, b: false, c: nil} # hash.except(:c) # => … Read more