V tej vadnici bomo s pomočjo primerov spoznali razred Java HashMap in njegove različne operacije.
HashMap
Razred okviru Java zbirk zagotavlja funkcionalnost podatkovne strukture hash tabele.
Elemente shranjuje v parih ključ / vrednost . Tu so ključi edinstveni identifikatorji, ki se uporabljajo za povezovanje vsake vrednosti na zemljevidu.
HashMap
Razredu izvaja vmesnik Map.
![](https://cdn.wiki-base.com/7339188/java_hashmap_with_examples.png.webp)
Ustvari HashMap
Če želite ustvariti zemljevid zgoščevanja, moramo najprej uvoziti java.util.HashMap
paket. Ko uvozimo paket, tukaj je opisano, kako lahko v Javi ustvarimo hashmape.
// hashMap creation with 8 capacity and 0.6 load factor HashMap numbers = new HashMap();
V zgornji kodi smo ustvarili hashmap z imenom številke. Tu K predstavlja tip ključa, V pa tip vrednosti. Na primer
HashMap numbers = new HashMap();
Tu je vrsta ključev je String
in vrsta vrednot je Integer
.
Primer 1: Ustvari HashMap v 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); ) )
Izhod
HashMap: (Java = 8, JavaScript = 1, Python = 3)
V zgornjem primeru smo ustvarili HashMap
imenovane jezike.
Tu smo put()
metodo uporabili za dodajanje elementov v hashmap. Več o put()
metodi bomo izvedeli kasneje v tej vadnici.
Osnovne operacije na Java HashMap
HashMap
Razred določa različne metode za izvajanje različnih operacij na hashmaps. V tej vadnici si bomo ogledali nekatere pogosto uporabljene arraylist operacije:
- Dodaj elemente
- Dostopni elementi
- Spremeni elemente
- Odstranite elemente
1. V HashMap dodajte elemente
Za dodajanje enega elementa v hashmap uporabimo put()
metodo HashMap
razreda. Na primer
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); ) )
Izhod
Začetni HashMap: () HashMap po put (): (One = 1, Two = 2, Three = 3)
V zgornjem primeru smo ustvarili HashMap
imenovane številke. Tu smo put()
metodo uporabili za dodajanje elementov številkam.
Upoštevajte izjavo,
numbers.put("One", 1);
Tu posredujemo metodi String
vrednost One kot ključ in Integer
vrednost 1 kot vrednost put()
.
Priporočena branja
- Java HashMap put ()
- Java HashMap putAll ()
- Java HashMap putIfAbsent ()
2. Dostopite do elementov HashMap
Z get()
metodo lahko dostopamo do vrednosti iz hashmapa. Na primer
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); ) )
Izhod
HashMap: (1 = Java, 2 = Python, 3 = JavaScript) Vrednost v indeksu 1: Java
V zgornjem primeru opazite izraz,
languages.get(1);
Tu get()
metoda vzame ključ kot svoj argument in vrne ustrezno vrednost, povezano s ključem.
Mi lahko dostopate tudi tipke , vrednote in ključ / vrednost parov HashMap kot zastavljenih ogledov uporabo keySet()
, values()
in entrySet()
metode oz. Na primer
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()); ) )
Izhod
HashMap: (1 = Java, 2 = Python, 3 = JavaScript) Tipke: (1, 2, 3) Vrednosti: (Java, Python, JavaScript) Preslikave ključev / vrednosti: (1 = Java, 2 = Python, 3 = JavaScript )
V zgornjem primeru smo ustvarili hashmap z imenom jeziki. Tukaj dostopamo do ključev , vrednosti in preslikav ključev / vrednosti iz hashmapa.
Priporočena branja
- Java HashMap get ()
- Java Hashmap getOrDefault ()
- Java HashMap keySet ()
- Vrednosti Java HashMap ()
- Java HashMap entrySet ()
3. Spremenite vrednost HashMap
Z replace()
metodo lahko spremenimo vrednost, povezano s ključem v hashmapu. Na primer
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); ) )
Izhod
Izvirni HashMap: (1 = Java, 2 = Python, 3 = JavaScript) HashMap z uporabo replace (): (1 = Java, 2 = C ++, 3 = JavaScript)
V zgornjem primeru smo ustvarili hashmap z imenom jeziki. Upoštevajte izraz,
languages.replace(2, "C++");
Tu spremenimo vrednost, na katero se sklicuje tipka 2, z novo vrednostjo C ++.
HashMap
Razred določa tudi nekaj različic replace()
metode. Če želite izvedeti več, obiščite
- Zamenjava Java HashMap ()
- Java HashMap replaceAll ()
4. Odstranite elemente HashMap
Če želimo elemente odstraniti iz hashmapa, lahko uporabimo metodo remove (). Na primer
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); ) )
Izhod
HashMap: (1 = Java, 2 = Python, 3 = JavaScript) Odstranjena vrednost: Python Posodobljen HashMap: (1 = Java, 3 = JavaScript)
Tu remove()
metoda za svoj parameter vzame ključ . Nato vrne vrednost, povezano s ključem, in odstrani vnos .
Vnos lahko odstranimo tudi le pod določenimi pogoji. Na primer
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.
Če opcijski parametri ne uporablja, potem je privzeti zmogljivost bo 16 in privzeti faktor obremenitve bo 0,75 .