U ovom uputstvu uz primjere ćemo naučiti o klasi Java HashMap i njezinim raznim operacijama.
HashMapKlasa Java zbirki okvira pruža funkcionalnost strukture hash tablice podataka.
Pohranjuje elemente u parove ključ / vrijednost . Ovdje su ključevi jedinstveni identifikatori koji se koriste za pridruživanje svake vrijednosti na karti.
HashMapKlasa implementira Karta sučelje.
Implementacija Java HashMap-a
Stvorite HashMap
Da bismo stvorili hash kartu, prvo moramo uvesti java.util.HashMappaket. Nakon što uvozimo paket, evo kako možemo stvoriti hashmape u Javi.
// hashMap creation with 8 capacity and 0.6 load factor HashMap numbers = new HashMap();
U gornjem kodu stvorili smo hashmapu s brojevima. Ovdje K predstavlja tip ključa, a V predstavlja vrstu vrijednosti. Na primjer,
HashMap numbers = new HashMap();
Ovdje je vrsta ključeva je Stringi vrsta vrijednosti je Integer.
Primjer 1: Stvorite HashMap u Javi
import java.util.HashMap; class Main ( public static void main(String() args) ( // create a hashmap HashMap languages = new HashMap(); // add elements to hashmap languages.put("Java", 8); languages.put("JavaScript", 1); languages.put("Python", 3); System.out.println("HashMap: " + languages); ) )
Izlaz
HashMap: (Java = 8, JavaScript = 1, Python = 3)
U gornjem primjeru stvorili smo HashMapimenovane jezike.
Ovdje smo koristili put()metodu za dodavanje elemenata u hashmapu. Više o put()metodi naučit ćemo kasnije u ovom vodiču.
Osnovne operacije na Java HashMap-u
HashMapKlasa pruža različite metode za obavljanje različitih operacija na hashmaps. U ovom ćemo uputstvu pogledati neke često korištene arraylist operacije:
- Dodaj elemente
- Pristupni elementi
- Promijenite elemente
- Uklonite elemente
1. Dodajte elemente u HashMap
Da bismo dodali jedan element u hashmapu, koristimo put()metodu HashMapklase. Na primjer,
import java.util.HashMap; class Main ( public static void main(String() args) ( // create a hashmap HashMap numbers = new HashMap(); System.out.println("Initial HashMap: " + numbers); // put() method to add elements numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("HashMap after put(): " + numbers); ) )
Izlaz
Početna HashMap: () HashMap nakon put (): (One = 1, Two = 2, Three = 3)
U gornjem primjeru stvorili smo HashMapimenovane brojeve. Ovdje smo put()metodu koristili za dodavanje elemenata brojevima.
Primijetite izjavu,
numbers.put("One", 1);
Ovdje metodi prenosimo Stringvrijednost One kao ključ i Integervrijednost 1 kao vrijednost put().
Preporučena čitanja
- Java HashMap put ()
- Java HashMap putAll ()
- Java HashMap putIfAbsent ()
2. Pristupite elementima HashMap
get()Metodu možemo koristiti za pristup vrijednosti iz hashmape. Na primjer,
import java.util.HashMap; class Main ( public static void main(String() args) ( HashMap languages = new HashMap(); languages.put(1, "Java"); languages.put(2, "Python"); languages.put(3, "JavaScript"); System.out.println("HashMap: " + languages); // get() method to get value String value = languages.get(1); System.out.println("Value at index 1: " + value); ) )
Izlaz
HashMap: (1 = Java, 2 = Python, 3 = JavaScript) Vrijednost u indeksu 1: Java
U gornjem primjeru primijetite izraz,
languages.get(1);
Ovdje get()metoda uzima ključ kao svoj argument i vraća odgovarajuću vrijednost povezanu s ključem.
Mi također mogu pristupiti tipke , vrijednosti i ključ / vrijednost parova hashmap kao set pogleda korištenjem keySet(), values()te entrySet()metode respektivno. Na primjer,
import java.util.HashMap; class Main ( public static void main(String() args) ( HashMap languages = new HashMap(); languages.put(1, "Java"); languages.put(2, "Python"); languages.put(3, "JavaScript"); System.out.println("HashMap: " + languages); // return set view of keys // using keySet() System.out.println("Keys: " + languages.keySet()); // return set view of values // using values() System.out.println("Values: " + languages.values()); // return set view of key/value pairs // using entrySet() System.out.println("Key/Value mappings: " + languages.entrySet()); ) )
Izlaz
HashMap: (1 = Java, 2 = Python, 3 = JavaScript) Tipke: (1, 2, 3) Vrijednosti: (Java, Python, JavaScript) preslikavanja ključa / vrijednosti: (1 = Java, 2 = Python, 3 = JavaScript )
U gornjem primjeru stvorili smo hashmapu nazvanu jezici. Ovdje pristupamo ključevima , vrijednostima i preslikavanjima ključeva / vrijednosti s hashmape.
Preporučena čitanja
- Java HashMap get ()
- Java Hashmap getOrDefault ()
- Java HashMap keySet ()
- Vrijednosti Java HashMap ()
- Java HashMap entrySet ()
3. Promijenite vrijednost HashMap-a
replace()Metodu možemo koristiti za promjenu vrijednosti povezane s ključem u hashmapi. Na primjer,
import java.util.HashMap; class Main ( public static void main(String() args) ( HashMap languages = new HashMap(); languages.put(1, "Java"); languages.put(2, "Python"); languages.put(3, "JavaScript"); System.out.println("Original HashMap: " + languages); // change element with key 2 languages.replace(2, "C++"); System.out.println("HashMap using replace(): " + languages); ) )
Izlaz
Izvorna HashMap: (1 = Java, 2 = Python, 3 = JavaScript) HashMap pomoću replace (): (1 = Java, 2 = C ++, 3 = JavaScript)
U gornjem primjeru stvorili smo hashmapu nazvanu jezici. Primijetite izraz,
languages.replace(2, "C++");
Ovdje mijenjamo vrijednost na koju upućuje ključ 2 novom vrijednošću C ++.
The HashMap class also provides some variations of the replace() method. To learn more, visit
- Java HashMap replace()
- Java HashMap replaceAll()
4. Remove HashMap Elements
To remove elements from a hashmap, we can use the remove() method. For example,
import java.util.HashMap; class Main ( public static void main(String() args) ( HashMap languages = new HashMap(); languages.put(1, "Java"); languages.put(2, "Python"); languages.put(3, "JavaScript"); System.out.println("HashMap: " + languages); // remove element associated with key 2 String value = languages.remove(2); System.out.println("Removed value: " + value); System.out.println("Updated HashMap: " + languages); ) )
Output
HashMap: (1=Java, 2=Python, 3=JavaScript) Removed value: Python Updated HashMap: (1=Java, 3=JavaScript)
Here, the remove() method takes the key as its parameter. It then returns the value associated with the key and removes the entry.
We can also remove the entry only under certain conditions. For example,
remove(2, "C++");
Here, the remove() method only removes the entry if the key 2 is associated with the value C++. Since 2 is not associated with C++, it doesn't remove the entry.
To learn more, visit Java HashMap remove().
Other Methods of HashMap
| Method | Description |
|---|---|
| clear() | removes all mappings from the HashMap |
| compute() | computes a new value for the specified key |
| computeIfAbsent() | computes value if a mapping for the key is not present |
| computeIfPresent() | computes a value for mapping if the key is present |
| merge() | merges the specified mapping to the HashMap |
| clone() | makes the copy of the HashMap |
| containsKey() | checks if the specified key is present in Hashmap |
| containsValue() | checks if Hashmap contains the specified value |
| size() | returns the number of items in HashMap |
| isEmpty() | checks if the Hashmap is empty |
Iterate through a HashMap
To iterate through each entry of the hashmap, we can use Java for-each loop. We can iterate through keys only, vales only, and key/value mapping. For example,
import java.util.HashMap; import java.util.Map.Entry; class Main ( public static void main(String() args) ( // create a HashMap HashMap languages = new HashMap(); languages.put(1, "Java"); languages.put(2, "Python"); languages.put(3, "JavaScript"); System.out.println("HashMap: " + languages); // iterate through keys only System.out.print("Keys: "); for (Integer key : languages.keySet()) ( System.out.print(key); System.out.print(", "); ) // iterate through values only System.out.print("Values: "); for (String value : languages.values()) ( System.out.print(value); System.out.print(", "); ) // iterate through key/value entries System.out.print("Entries: "); for (Entry entry : languages.entrySet()) ( System.out.print(entry); System.out.print(", "); ) ) )
Output
HashMap: (1=Java, 2=Python, 3=JavaScript) Keys: 1, 2, 3, Values: Java, Python, JavaScript, Entries: 1=Java, 2=Python, 3=JavaScript,
Note that we have used the Map.Entry in the above example. It is the nested class of the Map interface that returns a view (elements) of the map.
We first need to import the java.util.Map.Entry package in order to use this class.
This nested class returns a view (elements) of the map.
Creating HashMap from Other Maps
In Java, we can also create a hashmap from other maps. For example,
import java.util.HashMap; import java.util.TreeMap; class Main ( public static void main(String() args) ( // create a treemap TreeMap evenNumbers = new TreeMap(); evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); System.out.println("TreeMap: " + evenNumbers); // create hashmap from the treemap HashMap numbers = new HashMap(evenNumbers); numbers.put("Three", 3); System.out.println("HashMap: " + numbers); ) )
Output
TreeMap: (Four=4, Two=2) HashMap: (Two=2, Three=3, Four=4)
In the above example, we have created a TreeMap named evenNumbers. Notice the expression,
numbers = new HashMap(evenNumbers)
Here, we are creating a HashMap named numbers using the TreeMap. To learn more about treemap, visit Java TreeMap.
Note: While creating a hashmap, we can include optional parameters: capacity and load factor. For example,
HashMap numbers = new HashMap(8, 0.6f);
Here,
- 8 (capacity is 8) - This means it can store 8 entries.
- 0.6f (load factor is 0.6) - This means whenever our hash table is filled by 60%, the entries are moved to a new hash table double the size of the original hash table.
Ako se ne koristi proizvoljni parametri, zadani kapacitet će biti 16 , a zadana faktor opterećenja će biti 0,75 .
