Java HashMap Under the Hood

1. Ikhtisar

Pada artikel ini, kita akan menjelajahi implementasi paling populer dari antarmuka Peta dari Java Collections Framework secara lebih rinci, melanjutkan di mana artikel intro kami tinggalkan.

Sebelum kita memulai penerapan, penting untuk menunjukkan bahwa antarmuka koleksi List dan Set utama memperluas Collection tetapi Map tidak.

Sederhananya, HashMap menyimpan nilai berdasarkan kunci dan menyediakan API untuk menambahkan, mengambil, dan memanipulasi data yang disimpan dengan berbagai cara. Implementasinya didasarkan pada prinsip-prinsip hashtable , yang pada awalnya terdengar agak rumit tetapi sebenarnya sangat mudah dipahami.

Pasangan nilai-kunci disimpan dalam apa yang dikenal sebagai keranjang yang bersama-sama membentuk apa yang disebut tabel, yang sebenarnya adalah larik internal.

Setelah kita mengetahui kunci di mana suatu objek disimpan atau akan disimpan, operasi penyimpanan dan pengambilan terjadi dalam waktu yang konstan , O (1) dalam peta hash berdimensi baik.

Untuk memahami bagaimana peta hash bekerja di bawah tenda, seseorang perlu memahami mekanisme penyimpanan dan pengambilan yang digunakan oleh HashMap. Kami akan banyak fokus pada ini.

Terakhir, pertanyaan terkait HashMap cukup umum dalam wawancara , jadi ini adalah cara yang solid untuk mempersiapkan wawancara atau mempersiapkannya.

2. API put ()

Untuk menyimpan nilai dalam peta hash, kita memanggil API put yang mengambil dua parameter; kunci dan nilai yang sesuai:

V put(K key, V value);

Saat sebuah nilai ditambahkan ke peta di bawah sebuah kunci, API hashCode () dari objek kunci dipanggil untuk mengambil apa yang disebut sebagai nilai hash awal.

Untuk melihat ini beraksi, mari kita buat sebuah objek yang akan bertindak sebagai kunci. Kami hanya akan membuat satu atribut untuk digunakan sebagai kode hash untuk mensimulasikan tahap pertama hashing:

public class MyKey { private int id; @Override public int hashCode() { System.out.println("Calling hashCode()"); return id; } // constructor, setters and getters }

Sekarang kita dapat menggunakan objek ini untuk memetakan nilai di peta hash:

@Test public void whenHashCodeIsCalledOnPut_thenCorrect() { MyKey key = new MyKey(1); Map map = new HashMap(); map.put(key, "val"); }

Tidak banyak yang terjadi pada kode di atas, tetapi perhatikan output konsol. Memang metode hashCode dipanggil:

Calling hashCode()

Selanjutnya, hash () API dari peta hash dipanggil secara internal untuk menghitung nilai hash akhir menggunakan nilai hash awal.

Nilai hash terakhir ini pada akhirnya bermuara pada indeks dalam larik internal atau yang kita sebut lokasi keranjang.

Fungsi hash dari HashMap terlihat seperti ini:

static final int hash(Object key) { int h; return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16); }

Yang harus kita catat di sini hanyalah penggunaan kode hash dari objek kunci untuk menghitung nilai hash akhir.

Sementara di dalam fungsi put , nilai hash akhir digunakan seperti ini:

public V put(K key, V value) { return putVal(hash(key), key, value, false, true); }

Perhatikan bahwa fungsi putVal internal dipanggil dan diberi nilai hash akhir sebagai parameter pertama.

Orang mungkin bertanya-tanya mengapa kunci digunakan lagi di dalam fungsi ini karena kita telah menggunakannya untuk menghitung nilai hash.

Alasannya adalah peta hash menyimpan kunci dan nilai di lokasi bucket sebagai objek Map.Entry .

Seperti dibahas sebelumnya, semua antarmuka kerangka kerja koleksi Java memperluas antarmuka Koleksi tetapi Peta tidak. Bandingkan deklarasi antarmuka Map yang kita lihat sebelumnya dengan antarmuka Set :

public interface Set extends Collection

Alasannya adalah bahwa peta tidak secara tepat menyimpan elemen tunggal seperti halnya koleksi lain, melainkan kumpulan pasangan kunci-nilai.

Jadi metode generik dari antarmuka Collection seperti add , toArray tidak masuk akal ketika berhubungan dengan Map .

Konsep yang telah kita bahas dalam tiga paragraf terakhir menjadi salah satu pertanyaan wawancara Java Collections Framework yang paling populer . Jadi, perlu dipahami.

Satu atribut khusus dengan peta hash adalah bahwa ia menerima nol nilai-nilai dan kunci nol:

@Test public void givenNullKeyAndVal_whenAccepts_thenCorrect(){ Map map = new HashMap(); map.put(null, null); }

Ketika kunci null ditemukan selama operasi put , itu secara otomatis diberi nilai hash akhir 0 , yang berarti itu menjadi elemen pertama dari array yang mendasarinya.

Ini juga berarti bahwa ketika kuncinya adalah null, tidak ada operasi hashing dan oleh karena itu, API hashCode dari kunci tersebut tidak dipanggil, yang pada akhirnya menghindari pengecualian pointer null.

Selama operasi put , saat kita menggunakan kunci yang sudah digunakan sebelumnya untuk menyimpan nilai, ia mengembalikan nilai sebelumnya yang terkait dengan kunci:

@Test public void givenExistingKey_whenPutReturnsPrevValue_thenCorrect() { Map map = new HashMap(); map.put("key1", "val1"); String rtnVal = map.put("key1", "val2"); assertEquals("val1", rtnVal); }

jika tidak, ia mengembalikan null:

@Test public void givenNewKey_whenPutReturnsNull_thenCorrect() { Map map = new HashMap(); String rtnVal = map.put("key1", "val1"); assertNull(rtnVal); }

Ketika put mengembalikan null, itu juga bisa berarti bahwa nilai sebelumnya yang terkait dengan kunci adalah null, belum tentu itu adalah pemetaan nilai kunci baru:

@Test public void givenNullVal_whenPutReturnsNull_thenCorrect() { Map map = new HashMap(); String rtnVal = map.put("key1", null); assertNull(rtnVal); }

The containsKey API dapat digunakan untuk membedakan antara skenario seperti yang akan kita lihat dalam ayat berikutnya.

3. Get API

Untuk mengambil objek yang sudah disimpan di peta hash, kita harus mengetahui kunci yang menjadi tempat penyimpanannya. Kami memanggil get API dan meneruskannya ke objek utama:

@Test public void whenGetWorks_thenCorrect() { Map map = new HashMap(); map.put("key", "val"); String val = map.get("key"); assertEquals("val", val); }

Secara internal, prinsip hashing yang sama digunakan. API hashCode () dari objek kunci dipanggil untuk mendapatkan nilai hash awal:

@Test public void whenHashCodeIsCalledOnGet_thenCorrect() { MyKey key = new MyKey(1); Map map = new HashMap(); map.put(key, "val"); map.get(key); }

Kali ini, API hashCode MyKey dipanggil dua kali; sekali untuk put dan sekali untuk mendapatkan :

Calling hashCode() Calling hashCode()

Nilai ini kemudian diulang dengan memanggil hash () API internal untuk mendapatkan nilai hash akhir.

As we saw in the previous section, this final hash value ultimately boils down to a bucket location or an index of the internal array.

The value object stored in that location is then retrieved and returned to the calling function.

When the returned value is null, it could mean that the key object is not associated with any value in the hash map:

@Test public void givenUnmappedKey_whenGetReturnsNull_thenCorrect() { Map map = new HashMap(); String rtnVal = map.get("key1"); assertNull(rtnVal); }

Or it could simply mean that the key was explicitly mapped to a null instance:

@Test public void givenNullVal_whenRetrieves_thenCorrect() { Map map = new HashMap(); map.put("key", null); String val=map.get("key"); assertNull(val); }

To distinguish between the two scenarios, we can use the containsKey API, to which we pass the key and it returns true if and only if a mapping was created for the specified key in the hash map:

@Test public void whenContainsDistinguishesNullValues_thenCorrect() { Map map = new HashMap(); String val1 = map.get("key"); boolean valPresent = map.containsKey("key"); assertNull(val1); assertFalse(valPresent); map.put("key", null); String val = map.get("key"); valPresent = map.containsKey("key"); assertNull(val); assertTrue(valPresent); }

For both cases in the above test, the return value of the get API call is null but we are able to distinguish which one is which.

4. Collection Views in HashMap

HashMap offers three views that enable us to treat its keys and values as another collection. We can get a set of all keys of the map:

@Test public void givenHashMap_whenRetrievesKeyset_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); Set keys = map.keySet(); assertEquals(2, keys.size()); assertTrue(keys.contains("name")); assertTrue(keys.contains("type")); }

The set is backed by the map itself. So any change made to the set is reflected in the map:

@Test public void givenKeySet_whenChangeReflectsInMap_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); assertEquals(2, map.size()); Set keys = map.keySet(); keys.remove("name"); assertEquals(1, map.size()); }

We can also obtain a collection view of the values:

@Test public void givenHashMap_whenRetrievesValues_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); Collection values = map.values(); assertEquals(2, values.size()); assertTrue(values.contains("baeldung")); assertTrue(values.contains("blog")); }

Just like the key set, any changes made in this collection will be reflected in the underlying map.

Finally, we can obtain a set view of all entries in the map:

@Test public void givenHashMap_whenRetrievesEntries_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); Set
    
      entries = map.entrySet(); assertEquals(2, entries.size()); for (Entry e : entries) }
    

Remember that a hash map specifically contains unordered elements, therefore we assume any order when testing the keys and values of entries in the for each loop.

Many times, you will use the collection views in a loop as in the last example, and more specifically using their iterators.

Just remember that the iterators for all the above views are fail-fast.

If any structural modification is made on the map, after the iterator has been created, a concurrent modification exception will be thrown:

@Test(expected = ConcurrentModificationException.class) public void givenIterator_whenFailsFastOnModification_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); Set keys = map.keySet(); Iterator it = keys.iterator(); map.remove("type"); while (it.hasNext()) { String key = it.next(); } }

The only allowed structural modification is a remove operation performed through the iterator itself:

public void givenIterator_whenRemoveWorks_thenCorrect() { Map map = new HashMap(); map.put("name", "baeldung"); map.put("type", "blog"); Set keys = map.keySet(); Iterator it = keys.iterator(); while (it.hasNext()) { it.next(); it.remove(); } assertEquals(0, map.size()); }

The final thing to remember about these collection views is the performance of iterations. This is where a hash map performs quite poorly compared with its counterparts linked hash map and tree map.

Iteration over a hash map happens in worst case O(n) where n is the sum of its capacity and the number of entries.

5. HashMap Performance

The performance of a hash map is affected by two parameters: Initial Capacity and Load Factor. The capacity is the number of buckets or the underlying array length and the initial capacity is simply the capacity during creation.

The load factor or LF, in short, is a measure of how full the hash map should be after adding some values before it is resized.

The default initial capacity is 16 and default load factor is 0.75. We can create a hash map with custom values for initial capacity and LF:

Map hashMapWithCapacity=new HashMap(32); Map hashMapWithCapacityAndLF=new HashMap(32, 0.5f);

The default values set by the Java team are well optimized for most cases. However, if you need to use your own values, which is very okay, you need to understand the performance implications so that you know what you are doing.

When the number of hash map entries exceeds the product of LF and capacity, then rehashing occurs i.e. another internal array is created with twice the size of the initial one and all entries are moved over to new bucket locations in the new array.

A low initial capacity reduces space cost but increases the frequency of rehashing. Rehashing is obviously a very expensive process. So as a rule, if you anticipate many entries, you should set a considerably high initial capacity.

On the flip side, if you set the initial capacity too high, you will pay the cost in iteration time. As we saw in the previous section.

So a high initial capacity is good for a large number of entries coupled with little to no iteration.

A low initial capacity is good for few entries with a lot of iteration.

6. Collisions in the HashMap

A collision, or more specifically, a hash code collision in a HashMap, is a situation where two or more key objects produce the same final hash value and hence point to the same bucket location or array index.

This scenario can occur because according to the equals and hashCode contract, two unequal objects in Java can have the same hash code.

It can also happen because of the finite size of the underlying array, that is, before resizing. The smaller this array, the higher the chances of collision.

That said, it's worth mentioning that Java implements a hash code collision resolution technique which we will see using an example.

Keep in mind that it's the hash value of the key that determines the bucket the object will be stored in. And so, if the hash codes of any two keys collide, their entries will still be stored in the same bucket.

And by default, the implementation uses a linked list as the bucket implementation.

The initially constant time O(1)put and get operations will occur in linear time O(n) in the case of a collision. This is because after finding the bucket location with the final hash value, each of the keys at this location will be compared with the provided key object using the equals API.

To simulate this collision resolution technique, let's modify our earlier key object a little:

public class MyKey { private String name; private int id; public MyKey(int id, String name) { this.id = id; this.name = name; } // standard getters and setters @Override public int hashCode() { System.out.println("Calling hashCode()"); return id; } // toString override for pretty logging @Override public boolean equals(Object obj) { System.out.println("Calling equals() for key: " + obj); // generated implementation } }

Notice how we're simply returning the id attribute as the hash code – and thus force a collision to occur.

Also, note that we've added log statements in our equals and hashCode implementations – so that we know exactly when the logic is called.

Let's now go ahead to store and retrieve some objects that collide at some point:

@Test public void whenCallsEqualsOnCollision_thenCorrect() { HashMap map = new HashMap(); MyKey k1 = new MyKey(1, "firstKey"); MyKey k2 = new MyKey(2, "secondKey"); MyKey k3 = new MyKey(2, "thirdKey"); System.out.println("storing value for k1"); map.put(k1, "firstValue"); System.out.println("storing value for k2"); map.put(k2, "secondValue"); System.out.println("storing value for k3"); map.put(k3, "thirdValue"); System.out.println("retrieving value for k1"); String v1 = map.get(k1); System.out.println("retrieving value for k2"); String v2 = map.get(k2); System.out.println("retrieving value for k3"); String v3 = map.get(k3); assertEquals("firstValue", v1); assertEquals("secondValue", v2); assertEquals("thirdValue", v3); }

In the above test, we create three different keys – one has a unique id and the other two have the same id. Since we use id as the initial hash value, there will definitely be a collision during both storage and retrieval of data with these keys.

In addition to that, thanks to the collision resolution technique we saw earlier, we expect each of our stored values to be retrieved correctly, hence the assertions in the last three lines.

When we run the test, it should pass, indicating that collisions were resolved and we will use the logging produced to confirm that the collisions indeed occurred:

storing value for k1 Calling hashCode() storing value for k2 Calling hashCode() storing value for k3 Calling hashCode() Calling equals() for key: MyKey [name=secondKey, id=2] retrieving value for k1 Calling hashCode() retrieving value for k2 Calling hashCode() retrieving value for k3 Calling hashCode() Calling equals() for key: MyKey [name=secondKey, id=2]

Notice that during storage operations, k1 and k2 were successfully mapped to their values using only the hash code.

However, storage of k3 was not so simple, the system detected that its bucket location already contained a mapping for k2. Therefore, equals comparison was used to distinguish them and a linked list was created to contain both mappings.

Any other subsequent mapping whose key hashes to the same bucket location will follow the same route and end up replacing one of the nodes in the linked list or be added to the head of the list if equals comparison returns false for all existing nodes.

Likewise, during retrieval, k3 and k2 were equals-compared to identify the correct key whose value should be retrieved.

On a final note, from Java 8, the linked lists are dynamically replaced with balanced binary search trees in collision resolution after the number of collisions in a given bucket location exceed a certain threshold.

This change offers a performance boost, since, in the case of a collision, storage and retrieval happen in O(log n).

This section is very common in technical interviews, especially after the basic storage and retrieval questions.

7. Conclusion

In this article, we have explored HashMap implementation of Java Map interface.

Kode sumber lengkap untuk semua contoh yang digunakan dalam artikel ini dapat ditemukan di proyek GitHub.