C# Java HashMap equivalent

java

hashmap

exception-handling

custom-dictionary

byAlex Kataev·Aug 20, 2024

Comparing Java and C#, you'll find a twin to Java's HashMap<K, V> in C#: Dictionary<K, V>. These map a key to a value and in Java, requires importing java.util.HashMap.

Java:

HashMap<String, Integer> fruitsInBasket = new HashMap<>();
fruitsInBasket.put("banana", 99); // And a banana ain't one
int bananaCount = fruitsInBasket.get("banana");

C# equips you with a similar tool, but with a different command structure:

Dictionary<string, int> fruitsInBasket = new Dictionary<string, int>();
fruitsInBasket["banana"] = 99; // Jay Z's troubles don't extend to grocery shopping
int bananaCount = fruitsInBasket["banana"];

Swiftly and cleanly, a mapping of String keys to Integer values is set.

Interface associations and exception handling

C#'s Dictionary implements IDictionary, giving you the foundation for mapping keys to their associated values. Draw parallels here to Java's Map interface; both facilitate potent collection behaviors.

Using TryGetValue and ContainsKey

C#'s TryGetValue is your friend against the dreaded KeyNotFoundException. Comparatively safer than direct [key] access:

if (fruitsInBasket.TryGetValue("apple", out int fruitCount)) {
    // Found apples! fruitCount is their quantity
} else {
    // No apples. Hard times.
}

If you purely want to check for a key's existence, reach for ContainsKey:

if (fruitsInBasket.ContainsKey("apple")) {
    // The apple of my eye exists
}

Duplicate keys and Null Key Handling

C#'s Dictionary<K,V> throws an ArgumentException when a duplicate key attempts squatting, unlike HashMap which blindly overwrites the old tenant. Duplicate detection or collision dodging logic is key for a good user experience.

Null keys aren't a go in C#. ArgumentNullException is the bouncer against keys of null, so you might need to bake a custom Dictionary wrapper to handle these scenarios.

The thread-safe option and Hashtable

Multi-threaded applications present their own challenges, particularly given Dictionary isn't natively thread-safe. ConcurrentDictionary provides a better option here in C#, standing parallel to Collections.synchronizedMap in Java.

For a non-generic collection akin to HashMap, C# presents the Hashtable. Though be aware, typeless means not exactly safe for type.

Example codes of practical usage

Crafting with Dictionary<K, V> needs care. Neglecting to consider exception handling, duplicate keys and null key handling can generate bugs. Here's how you might employ it in algorithm challenges such as counting frequencies:

Dictionary<char, int> frequency = new Dictionary<char, int>();
foreach (char letter in "pineapple") {
    if (frequency.ContainsKey(letter)) {
        frequency[letter]++; // +1 pineapple whoah!
    } else {
        frequency[letter] = 1; // First pineapple sighting
    }
}

Dictionary customization

Forge your own custom Dictionary to architect the features you need, such as handling null-keys gracefully or overwriting values with abandon. Subclassing a Dictionary opens up the gate to override and enhance methods.

Diving into the nitty-gritty

Hashing and the innards

The humble workings of C#'s Dictionary and Java's HashMap rest on the shoulders of hashing. Both employ arrays of buckets and hash codes to determine a cosy home for our (key, value) pairs.

Performance considerations

Given perfect conditions, HashMap and Dictionary trot along in constant time (O(1)) for adding and pulling elements. Overcrowded buckets bringing performance down to O(n) - where 'n' represents the bucket's item count - can dampen the flutter of joy in your heart.

Serialization and deserialization

Converting these data structures to JSON, binary or another storage/transfer-friendly format can be critical in certain applications. So embrace the System.Runtime.Serialization namespace in C# for all your serialization needs.

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