hashmap - w3toppers.com

what is the best way to get a sub HashMap based on a list of Keys?

With Java8 streams, there is a functional (elegant) solution. If keys is the list of keys to keep and map is the source Map. keys.stream() .filter(map::containsKey) .collect(Collectors.toMap(Function.identity(), map::get)); Complete example: List<Integer> keys = new ArrayList<>(); keys.add(2); keys.add(3); keys.add(42); // this key is not in the map Map<Integer, String> map = new HashMap<>(); map.put(1, “foo”); map.put(2, … Read more

How to convert a string to a HashMap? [duplicate]

You can do that with Guava’s Splitter.MapSplitter: Map<String, String> properties = Splitter.on(“,”) .withKeyValueSeparator(“:”) .split(inputString);

Hashmap vs Array performance

HashMap uses an array underneath so it can never be faster than using an array correctly. Random.nextInt() is many times slower than what you are testing, even using array to test an array is going to bias your results. The reason your array benchmark is so slow is due to the equals comparisons, not the … Read more

Sorting the Map in descending order based on the value [duplicate]

Since you can have duplicate values you shouldn’t be using a Set at all. Change to a List and sort it instead. Your entriesSortedByValues would look something like this: static <K,V extends Comparable<? super V>> List<Entry<K, V>> entriesSortedByValues(Map<K,V> map) { List<Entry<K,V>> sortedEntries = new ArrayList<Entry<K,V>>(map.entrySet()); Collections.sort(sortedEntries, new Comparator<Entry<K,V>>() { @Override public int compare(Entry<K,V> e1, Entry<K,V> … Read more

Why are immutable objects in hashmaps so effective?

String#hashCode: private int hash; … public int hashCode() { int h = hash; if (h == 0 && count > 0) { int off = offset; char val[] = value; int len = count; for (int i = 0; i < len; i++) { h = 31*h + val[off++]; } hash = h; } return … Read more

Theoretical limit for number of keys (objects) that can be stored in a HashMap?

Is there a theoretical limit for the number of key entries that can be stored in a HashMap or does it purely depend on the heapmemory available ? Looking at the documentation of that class, I would say that the theoretical limit is Integer.MAX_VALUE (231-1 = 2147483647) elements. This is because to properly implement this … Read more

Comparing two hashmaps for equal values and same key sets?

Simply use : mapA.equals(mapB); Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent the same mappings

How does ConcurrentHashMap work internally?

I would read the source of ConcurrentHashMap as it is rather complicated in the detail. In short it has Multiple partitions which can be locked independently. (16 by default) Using concurrent Locks operations for thread safety instead of synchronized. Has thread safe Iterators. synchronizedCollection’s iterators are not thread safe. Does not expose the internal locks. … Read more

How to iterate through a Hashmap, print the key/value and remove the value in Rust?

There are at least two reasons why this is disallowed: You would need to have two concurrent mutable references to map — one held by the iterator used in the for loop and one in the variable map to call map.remove. You have references to the key and the value within the map when trying … Read more

Which is faster, Hash lookup or Binary search?

For very small collections the difference is going to be negligible. At the low end of your range (500k items) you will start to see a difference if you’re doing lots of lookups. A binary search is going to be O(log n), whereas a hash lookup will be O(1), amortized. That’s not the same as … Read more