Panduan untuk Java HashMap

1. Ikhtisar

Di artikel ini, kita akan melihat cara menggunakan HashMap di Java, dan kita akan melihat cara kerjanya secara internal.

Kelas yang sangat mirip dengan HashMap adalah Hashtable . Silakan merujuk ke beberapa artikel kami yang lain untuk mempelajari lebih lanjut tentang kelas java.util.Hashtable itu sendiri dan perbedaan antara HashMap dan Hashtable .

2. Penggunaan Dasar

Pertama-tama, mari kita lihat apa artinya HashMap adalah peta. Peta adalah pemetaan nilai kunci, yang berarti bahwa setiap kunci dipetakan tepat ke satu nilai dan kita dapat menggunakan kunci tersebut untuk mengambil nilai yang sesuai dari peta.

Orang mungkin bertanya mengapa tidak menambahkan nilai ke daftar. Mengapa kita membutuhkan HashMap ? Alasan sederhananya adalah kinerja. Jika kita ingin mencari elemen tertentu dalam daftar, kompleksitas waktunya adalah O (n) dan jika daftar tersebut diurutkan, maka akan menjadi O (log n) menggunakan, misalnya, pencarian biner.

Keuntungan dari HashMap adalah kompleksitas waktu untuk memasukkan dan mengambil nilai rata-rata adalah O (1) . Kami akan melihat bagaimana itu bisa dicapai nanti. Pertama mari kita lihat bagaimana menggunakan HashMap .

2.1. Mendirikan

Mari buat kelas sederhana yang akan kita gunakan di seluruh artikel:

public class Product { private String name; private String description; private List tags; // standard getters/setters/constructors public Product addTagsOfOtherProdcut(Product product) { this.tags.addAll(product.getTags()); return this; } }

2.2. Taruh

Sekarang kita dapat membuat HashMap dengan kunci bertipe String dan elemen bertipe Product :

Map productsByName = new HashMap(); 

Dan tambahkan produk ke HashMap kami :

Product eBike = new Product("E-Bike", "A bike with a battery"); Product roadBike = new Product("Road bike", "A bike for competition"); productsByName.put(eBike.getName(), eBike); productsByName.put(roadBike.getName(), roadBike); 

2.3. Dapatkan

Kita bisa mengambil nilai dari peta dengan kuncinya:

Product nextPurchase = productsByName.get("E-Bike"); assertEquals("A bike with a battery", nextPurchase.getDescription());

Jika kami mencoba menemukan nilai untuk kunci yang tidak ada di peta, kami akan mendapatkan nilai null :

Product nextPurchase = productsByName.get("Car"); assertNull(nextPurchase);

Dan jika kita memasukkan nilai kedua dengan kunci yang sama, kita hanya akan mendapatkan nilai terakhir yang dimasukkan untuk kunci itu:

Product newEBike = new Product("E-Bike", "A bike with a better battery"); productsByName.put(newEBike.getName(), newEBike); assertEquals("A bike with a better battery", productsByName.get("E-Bike").getDescription());

2.4. Null sebagai Kuncinya

HashMap juga memungkinkan kita memiliki null sebagai kunci:

Product defaultProduct = new Product("Chocolate", "At least buy chocolate"); productsByName.put(null, defaultProduct); Product nextPurchase = productsByName.get(null); assertEquals("At least buy chocolate", nextPurchase.getDescription());

2.5. Nilai dengan Kunci yang Sama

Selanjutnya, kita dapat memasukkan objek yang sama dua kali dengan kunci yang berbeda:

productsByName.put(defaultProduct.getName(), defaultProduct); assertSame(productsByName.get(null), productsByName.get("Chocolate"));

2.6. Hapus Nilai

Kami dapat menghapus pemetaan nilai kunci dari HashMap :

productsByName.remove("E-Bike"); assertNull(productsByName.get("E-Bike"));

2.7. Periksa Apakah Kunci atau Nilai Ada di Peta

Untuk memeriksa apakah kunci ada di peta, kita bisa menggunakan metode containsKey () :

productsByName.containsKey("E-Bike");

Atau, untuk memeriksa apakah ada nilai di peta, kita bisa menggunakan metode containsValue () :

productsByName.containsValue(eBike);

Kedua pemanggilan metode akan mengembalikan nilai true dalam contoh kita. Meskipun terlihat sangat mirip, ada perbedaan penting dalam performa antara kedua pemanggilan metode ini. Kompleksitas untuk memeriksa apakah sebuah kunci ada adalah O (1) , sedangkan kompleksitas untuk memeriksa sebuah elemen adalah O (n), karena itu perlu untuk mengulang semua elemen di peta.

2.8. Iterasi Melalui HashMap

Ada tiga cara dasar untuk mengulangi semua pasangan nilai kunci di HashMap .

Kita dapat melakukan iterasi atas set semua kunci:

for(String key : productsByName.keySet()) { Product product = productsByName.get(key); }

Atau kita dapat mengulang himpunan semua entri:

for(Map.Entry entry : productsByName.entrySet()) { Product product = entry.getValue(); String key = entry.getKey(); //do something with the key and value }

Terakhir, kita dapat mengulang semua nilai:

List products = new ArrayList(productsByName.values());

3. Kunci

Kami dapat menggunakan kelas apa pun sebagai kunci di HashMap kami . Namun, agar peta berfungsi dengan baik, kita perlu menyediakan implementasi untuk equals () dan hashCode (). Katakanlah kita ingin memiliki peta dengan produk sebagai kunci dan harga sebagai nilainya:

HashMap priceByProduct = new HashMap(); priceByProduct.put(eBike, 900);

Mari terapkan metode equals () dan hashCode () :

@Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } Product product = (Product) o; return Objects.equals(name, product.name) && Objects.equals(description, product.description); } @Override public int hashCode() { return Objects.hash(name, description); }

Perhatikan bahwa hashCode () dan equals () perlu diganti hanya untuk kelas yang ingin kita gunakan sebagai kunci peta, bukan untuk kelas yang hanya digunakan sebagai nilai di peta. Kami akan melihat mengapa ini diperlukan di bagian 5 artikel ini.

4. Additional Methods as of Java 8

Java 8 added several functional-style methods to HashMap. In this section, we'll look at some of these methods.

For each method, we'll look at two examples. The first example shows how to use the new method, and the second example shows how to achieve the same in earlier versions of Java.

As these methods are quite straightforward, we won't look at more detailed examples.

4.1. forEach()

The forEach method is the functional-style way to iterate over all elements in the map:

productsByName.forEach( (key, product) -> { System.out.println("Key: " + key + " Product:" + product.getDescription()); //do something with the key and value }); 

Prior to Java 8:

for(Map.Entry entry : productsByName.entrySet()) { Product product = entry.getValue(); String key = entry.getKey(); //do something with the key and value }

Our article Guide to the Java 8 forEach covers the forEach loop in greater detail.

4.2. getOrDefault()

Using the getOrDefault() method, we can get a value from the map or return a default element in case there is no mapping for the given key:

Product chocolate = new Product("chocolate", "something sweet"); Product defaultProduct = productsByName.getOrDefault("horse carriage", chocolate); Product bike = productsByName.getOrDefault("E-Bike", chocolate);

Prior to Java 8:

Product bike2 = productsByName.containsKey("E-Bike") ? productsByName.get("E-Bike") : chocolate; Product defaultProduct2 = productsByName.containsKey("horse carriage") ? productsByName.get("horse carriage") : chocolate; 

4.3. putIfAbsent()

With this method, we can add a new mapping, but only if there is not yet a mapping for the given key:

productsByName.putIfAbsent("E-Bike", chocolate); 

Prior to Java 8:

if(productsByName.containsKey("E-Bike")) { productsByName.put("E-Bike", chocolate); }

Our article Merging Two Maps with Java 8 takes a closer look at this method.

4.4. merge()

And with merge(), we can modify the value for a given key if a mapping exists, or add a new value otherwise:

Product eBike2 = new Product("E-Bike", "A bike with a battery"); eBike2.getTags().add("sport"); productsByName.merge("E-Bike", eBike2, Product::addTagsOfOtherProdcut);

Prior to Java 8:

if(productsByName.containsKey("E-Bike")) { productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2); } else { productsByName.put("E-Bike", eBike2); } 

4.5. compute()

With the compute() method, we can compute the value for a given key:

productsByName.compute("E-Bike", (k,v) -> { if(v != null) { return v.addTagsOfOtherProdcut(eBike2); } else { return eBike2; } });

Prior to Java 8:

if(productsByName.containsKey("E-Bike")) { productsByName.get("E-Bike").addTagsOfOtherProdcut(eBike2); } else { productsByName.put("E-Bike", eBike2); } 

It's worth noting that the methods merge() and compute() are quite similar. The compute() method accepts two arguments: the key and a BiFunction for the remapping. And merge() accepts three parameters: the key, a default value to add to the map if the key doesn't exist yet, and a BiFunction for the remapping.

5. HashMap Internals

In this section, we'll look at how HashMap works internally and what are the benefits of using HashMap instead of a simple list, for example.

As we've seen, we can retrieve an element from a HashMap by its key. One approach would be to use a list, iterate over all elements, and return when we find an element for which the key matches. Both the time and space complexity of this approach would be O(n).

With HashMap, we can achieve an average time complexity of O(1) for the put and get operations and space complexity of O(n). Let's see how that works.

5.1. The Hash Code and Equals

Instead of iterating over all its elements, HashMap attempts to calculate the position of a value based on its key.

The naive approach would be to have a list that can contain as many elements as there are keys possible. As an example, let's say our key is a lower-case character. Then it's sufficient to have a list of size 26, and if we want to access the element with key ‘c', we'd know that it's the one at position 3, and we can retrieve it directly.

However, this approach would not be very effective if we have a much bigger keyspace. For example, let's say our key was an integer. In this case, the size of the list would have to be 2,147,483,647. In most cases, we would also have far fewer elements, so a big part of the allocated memory would remain unused.

HashMap stores elements in so-called buckets and the number of buckets is called capacity.

When we put a value in the map, the key's hashCode() method is used to determine the bucket in which the value will be stored.

To retrieve the value, HashMap calculates the bucket in the same way – using hashCode(). Then it iterates through the objects found in that bucket and use key's equals() method to find the exact match.

5.2. Keys' Immutability

In most cases, we should use immutable keys. Or at least, we must be aware of the consequences of using mutable keys.

Let's see what happens when our key changes after we used it to store a value in a map.

For this example, we'll create the MutableKey:

public class MutableKey { private String name; // standard constructor, getter and setter @Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } MutableKey that = (MutableKey) o; return Objects.equals(name, that.name); } @Override public int hashCode() { return Objects.hash(name); } }

And here goes the test:

MutableKey key = new MutableKey("initial"); Map items = new HashMap(); items.put(key, "success"); key.setName("changed"); assertNull(items.get(key));

As we can see, we're no longer able to get the corresponding value once the key has changed, instead, null is returned. This is because HashMap is searching in the wrong bucket.

The above test case may be surprising if we don't have a good understanding of how HashMap works internally.

5.3. Collisions

For this to work correctly, equal keys must have the same hash, however, different keys can have the same hash. If two different keys have the same hash, the two values belonging to them will be stored in the same bucket. Inside a bucket, values are stored in a list and retrieved by looping over all elements. The cost of this is O(n).

As of Java 8 (see JEP 180), the data structure in which the values inside one bucket are stored is changed from a list to a balanced tree if a bucket contains 8 or more values, and it's changed back to a list if, at some point, only 6 values are left in the bucket. This improves the performance to be O(log n).

5.4. Capacity and Load Factor

To avoid having many buckets with multiple values, the capacity is doubled if 75% (the load factor) of the buckets become non-empty. The default value for the load factor is 75%, and the default initial capacity is 16. Both can be set in the constructor.

5.5. Summary of put and get Operations

Let's summarize how the put and get operations work.

When we add an element to the map,HashMap calculates the bucket. If the bucket already contains a value, the value is added to the list (or tree) belonging to that bucket. If the load factor becomes bigger than the maximum load factor of the map, the capacity is doubled.

When we want to get a value from the map,HashMap calculates the bucket and gets the value with the same key from the list (or tree).

6. Conclusion

Di artikel ini, kami melihat cara menggunakan HashMap dan cara kerjanya secara internal. Bersama dengan ArrayList , HashMap adalah salah satu struktur data yang paling sering digunakan di Java, jadi sangat berguna untuk memiliki pengetahuan yang baik tentang cara menggunakannya dan cara kerjanya. Artikel kami The Java HashMap Under the Hood mencakup internal HashMap secara lebih rinci.

Seperti biasa, kode sumber lengkap tersedia di Github.