Java HashMap (z primeri)

V tej vadnici bomo s pomočjo primerov spoznali razred Java HashMap in njegove različne operacije.

HashMapRazred 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.

HashMapRazredu izvaja vmesnik Map.

Izvajanje Java HashMap

Ustvari HashMap

Če želite ustvariti zemljevid zgoščevanja, moramo najprej uvoziti java.util.HashMappaket. 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 Stringin 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 HashMapimenovane 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

HashMapRazred 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 HashMaprazreda. 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 HashMapimenovane številke. Tu smo put()metodo uporabili za dodajanje elementov številkam.

Upoštevajte izjavo,

 numbers.put("One", 1);

Tu posredujemo metodi Stringvrednost One kot ključ in Integervrednost 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 ++.

HashMapRazred 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 .

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