Java TreeMap (con esempi)

In questo tutorial, impareremo a conoscere la classe Java TreeMap e le sue operazioni con l'aiuto di esempi.

La TreeMapclasse del framework delle collezioni Java fornisce l'implementazione della struttura dei dati ad albero.

Implementa l'interfaccia NavigableMap.

Creazione di una TreeMap

Per creare un TreeMap, dobbiamo java.util.TreeMapprima importare il pacchetto. Una volta importato il pacchetto, ecco come possiamo creare un file TreeMapin Java.

 TreeMap numbers = new TreeMap();

Nel codice sopra, abbiamo creato un TreeMapnumero con nome senza argomenti. In questo caso, gli elementi in TreeMapvengono ordinati in modo naturale (ordine crescente).

Tuttavia, possiamo personalizzare l'ordinamento degli elementi utilizzando l' Comparatorinterfaccia. Lo impareremo più avanti in questo tutorial.

Qui,

  • Chiave: un identificatore univoco utilizzato per associare ogni elemento (valore) in una mappa
  • Valore: elementi associati da chiavi in ​​una mappa

Metodi di TreeMap

La TreeMapclasse fornisce vari metodi che ci consentono di eseguire operazioni sulla mappa.

Inserisci elementi in TreeMap

  • put() - inserisce la mappatura chiave / valore specificata (voce) nella mappa
  • putAll() - inserisce tutte le voci dalla mappa specificata a questa mappa
  • putIfAbsent() - inserisce la mappatura chiave / valore specificata nella mappa se la chiave specificata non è presente nella mappa

Per esempio,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( // Creating TreeMap of even numbers TreeMap evenNumbers = new TreeMap(); // Using put() evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); // Using putIfAbsent() evenNumbers.putIfAbsent("Six", 6); System.out.println("TreeMap of even numbers: " + evenNumbers); //Creating TreeMap of numbers TreeMap numbers = new TreeMap(); numbers.put("One", 1); // Using putAll() numbers.putAll(evenNumbers); System.out.println("TreeMap of numbers: " + numbers); ) )

Produzione

 Mappa ad albero dei numeri pari: (Quattro = 4, Sei = 6, Due = 2) Mappa ad albero dei numeri: (Quattro = 4, Uno = 1, Sei = 6, Due = 2)

Accedi agli elementi TreeMap

1. Utilizzo di entrySet (), keySet () e values ​​()

  • entrySet() - restituisce un insieme di tutte le mappature chiave / valori (voce) di una mappa ad albero
  • keySet() - restituisce un insieme di tutte le chiavi di una mappa ad albero
  • values() - restituisce un insieme di tutte le mappe di una mappa ad albero

Per esempio,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("TreeMap: " + numbers); // Using entrySet() System.out.println("Key/Value mappings: " + numbers.entrySet()); // Using keySet() System.out.println("Keys: " + numbers.keySet()); // Using values() System.out.println("Values: " + numbers.values()); ) )

Produzione

 TreeMap: (Uno = 1, Tre = 3, Due = 2) Mappature chiave / valore: (Uno = 1, Tre = 3, Due = 2) Chiavi: (Uno, tre, due) Valori: (1, 3, 2 )

2. Utilizzo di get () e getOrDefault ()

  • get()- Restituisce il valore associato alla chiave specificata. Restituisce null se la chiave non viene trovata.
  • getOrDefault()- Restituisce il valore associato alla chiave specificata. Restituisce il valore predefinito specificato se la chiave non viene trovata.

Per esempio,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("TreeMap: " + numbers); // Using get() int value1 = numbers.get("Three"); System.out.println("Using get(): " + value1); // Using getOrDefault() int value2 = numbers.getOrDefault("Five", 5); System.out.println("Using getOrDefault(): " + value2); ) )

Produzione

 TreeMap: (Uno = 1, Tre = 3, Due = 2) Utilizzo di get (): 3 Utilizzo di getOrDefault (): 5

Here, the getOrDefault() method does not find the key Five. Hence it returns the specified default value 5.

Remove TeeMap Elements

  • remove(key) - returns and removes the entry associated with the specified key from a TreeMap
  • remove(key, value) - removes the entry from the map only if the specified key is associated with the specified value and returns a boolean value

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("TreeMap: " + numbers); // remove method with single parameter int value = numbers.remove("Two"); System.out.println("Removed value: " + value); // remove method with two parameters boolean result = numbers.remove("Three", 3); System.out.println("Is the entry (Three=3) removed? " + result); System.out.println("Updated TreeMap: " + numbers); ) )

Output

 TreeMap: (One=1, Three=3, Two=2) Removed value = 2 Is the entry (Three=3) removed? True Updated TreeMap: (One=1)

Replace TreeMap Elements

  • replace(key, value) - replaces the value mapped by the specified key with the new value
  • replace(key, old, new) - replaces the old value with the new value only if the old value is already associated with the specified key
  • replaceAll(function) - replaces each value of the map with the result of the specified function

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); System.out.println("Original TreeMap: " + numbers); // Using replace() numbers.replace("Second", 22); numbers.replace("Third", 3, 33); System.out.println("TreeMap using replace: " + numbers); // Using replaceAll() numbers.replaceAll((key, oldValue) -> oldValue + 2); System.out.println("TreeMap using replaceAll: " + numbers); ) )

Output

 Original TreeMap: (First=1, Second=2, Third=3) TreeMap using replace(): (First=1, Second=22, Third=33) TreeMap using replaceAll(): (First=3, Second=24, Third=35)

In the above program notice the statement

 numbers.replaceAll((key, oldValue) -> oldValue + 2);

Here, we have passed a lambda expression as an argument.

The replaceAll() method accesses all the entries of the map. It then replaces all the elements with the new values (returned from the lambda expression).

Methods for Navigation

Since the TreeMap class implements NavigableMap, it provides various methods to navigate over the elements of the treemap.

1. First and Last Methods

  • firstKey() - returns the first key of the map
  • firstEntry() - returns the key/value mapping of the first key of the map
  • lastKey() - returns the last key of the map
  • lastEntry() - returns the key/value mapping of the last key of the map

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); System.out.println("TreeMap: " + numbers); // Using the firstKey() method String firstKey = numbers.firstKey(); System.out.println("First Key: " + firstKey); // Using the lastKey() method String lastKey = numbers.lastKey(); System.out.println("Last Key: " + lastKey); // Using firstEntry() method System.out.println("First Entry: " + numbers.firstEntry()); // Using the lastEntry() method System.out.println("Last Entry: " + numbers.lastEntry()); ) )

Output

 TreeMap: (First=1, Second=2, Third=3) First Key: First Last Key: Third First Entry: First=1 Last Entry: Third=3

2. Ceiling, Floor, Higher and Lower Methods

  • higherKey() - Returns the lowest key among those keys that are greater than the specified key.
  • higherEntry() - Returns an entry associated with a key that is lowest among all those keys greater than the specified key.
  • lowerKey() - Returns the greatest key among all those keys that are less than the specified key.
  • lowerEntry() - Returns an entry associated with a key that is greatest among all those keys that are less than the specified key.
  • ceilingKey() - Returns the lowest key among those keys that are greater than the specified key. If the key passed as an argument is present in the map, it returns that key.
  • ceilingEntry() - Returns an entry associated with a key that is lowest among those keys that are greater than the specified key. It an entry associated with the key passed an argument is present in the map, it returns the entry associated with that key.
  • floorKey() - Returns the greatest key among those keys that are less than the specified key. If the key passed as an argument is present, it returns that key.
  • floorEntry() - Returns an entry associated with a key that is greatest among those keys that are less than the specified key. If the key passed as argument is present, it returns that key.

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 5); numbers.put("Third", 4); numbers.put("Fourth", 6); System.out.println("TreeMap: " + numbers); // Using higher() System.out.println("Using higherKey(): " + numbers.higherKey("Fourth")); System.out.println("Using higherEntry(): " + numbers.higherEntry("Fourth")); // Using lower() System.out.println("Using lowerKey(): " + numbers.lowerKey("Fourth")); System.out.println("Using lowerEntry(): " + numbers.lowerEntry("Fourth")); // Using ceiling() System.out.println("Using ceilingKey(): " + numbers.ceilingKey("Fourth")); System.out.println("Using ceilingEntry(): " + numbers.ceilingEntry("Fourth")); // Using floor() System.out.println("Using floorKey(): " + numbers.floorKey("Fourth")); System.out.println("Using floorEntry(): " + numbers.floorEntry("Fourth")); ) )

Output

 TreeMap: (First=1, Fourth=6, Second=5, Third=4) Using higherKey(): Second Using higherEntry(): Second=5 Using lowerKey(): First Using lowerEntry(): First=1 Using ceilingKey(): Fourth Using ceilingEntry(): Fourth=6 Using floorkey(): Fourth Using floorEntry(): Fourth=6

3. pollFirstEntry() and pollLastEntry() Methods

  • pollFirstEntry() - returns and removes the entry associated with the first key of the map
  • pollLastEntry() - returns and removes the entry associated with the last key of the map

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); System.out.println("TreeMap: " + numbers); //Using the pollFirstEntry() method System.out.println("Using pollFirstEntry(): " + numbers.pollFirstEntry()); // Using the pollLastEntry() method System.out.println("Using pollLastEntry(): " + numbers.pollLastEntry()); System.out.println("Updated TreeMap: " + numbers); ) )

Output

 TreeMap: (First=1, Second=2, Third=3) Using pollFirstEntry(): First=1 Using pollLastEntry(): Third=3 Updated TreeMap: (Second=2)

4. headMap(), tailMap() and subMap() Methods

headMap(key, booleanValue)

The headMap() method returns all the key/value pairs of a treemap before the specified key (which is passed as an argument).

The booleanValue parameter is optional. Its default value is false.

If true is passed as a booleanValue, the method also includes the key/value pair of the key which is passed as an argument.

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); numbers.put("Fourth", 4); System.out.println("TreeMap: " + numbers); System.out.println("Using headMap() Method:"); // Using headMap() with default booleanValue System.out.println("Without boolean value: " + numbers.headMap("Fourth")); // Using headMap() with specified booleanValue System.out.println("With boolean value: " + numbers.headMap("Fourth", true)); ) )

Output

 TreeMap: (First=1, Fourth=4, Second=2, Third=3) Using headMap() Method: Without boolean value: (First=1) With boolean value: (First=1, Fourth=4)

tailMap(key, booleanValue)

The tailMap() method returns all the key/value pairs of a treemap starting from the specified key (which is passed as an argument).

The booleanValue is an optional parameter. Its default value is true.

If false is passed as a booleanValue, the method doesn't include the key/value pair of the specified key.

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); numbers.put("Fourth", 4); System.out.println("TreeMap: " + numbers); System.out.println("Using tailMap() Method:"); // Using tailMap() with default booleanValue System.out.println("Without boolean value: " + numbers.tailMap("Second")); // Using tailMap() with specified booleanValue System.out.println("With boolean value: " + numbers.tailMap("Second", false)); ) )

Output

 TreeMap: (First=1, Fourth=4, Second=2, Third=3) Using tailMap() Method: Without boolean value: (Second=2, Third=3) With boolean value: (Third=3)

subMap(k1, bV1, k2, bV2)

The subMap() method returns all the entries associated with keys between k1 and k2 including the entry of k1.

The bV1 and bV2 are optional boolean parameters. The default value of bV1 is true and the default value of bV2 is false.

If false is passed as bV1, the method returns all the entries associated with keys between k1 and k2 without including the entry of k1.

If true is passed as bV2, the method returns all the entries associated with keys between k1 and k2 including the entry of k2.

For example,

 import java.util.TreeMap; class Main ( public static void main(String() args) ( TreeMap numbers = new TreeMap(); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); numbers.put("Fourth", 4); System.out.println("TreeMap: " + numbers); System.out.println("Using subMap() Method:"); // Using subMap() with default booleanValue System.out.println("Without boolean value: " + numbers.subMap("Fourth", "Third")); // Using subMap() with specified booleanValue System.out.println("With boolean value: " + numbers.subMap("Fourth", false, "Third", true)); ) )

Output

 TreeMap: (First=1, Fourth=2, Second=2, Third=3) Using subMap() Method: Without boolean value: (Fourth=4, Second=2) With boolean value: (Second=2, Third=3)

Other Methods of TreeMap

Method Description
clone() Creates a copy of the TreeMap
containsKey() Searches the TreeMap for the specified key and returns a boolean result
containsValue() Searches the TreeMap for the specified value and returns a boolean result
size() Returns the size of the TreeMap
clear() Removes all the entries from the TreeMap

TreeMap Comparator

In all the examples above, treemap elements are sorted naturally (in ascending order). However, we can also customize the ordering of keys.

For this, we need to create our own comparator class based on which keys in a treemap are sorted. For example,

 import java.util.TreeMap; import java.util.Comparator; class Main ( public static void main(String() args) ( // Creating a treemap with a customized comparator TreeMap numbers = new TreeMap(new CustomComparator()); numbers.put("First", 1); numbers.put("Second", 2); numbers.put("Third", 3); numbers.put("Fourth", 4); System.out.println("TreeMap: " + numbers); ) // Creating a comparator class public static class CustomComparator implements Comparator ( @Override public int compare(String number1, String number2) ( int value = number1.compareTo(number2); // elements are sorted in reverse order if (value> 0) ( return -1; ) else if (value < 0) ( return 1; ) else ( return 0; ) ) ) )

Output

 TreeMap: (Third=3, Second=2, Fourth=4, First=1)

Nell'esempio precedente, abbiamo creato una mappa ad albero passando la classe CustomComparator come argomento.

La classe CustomComparator implementa l' Comparatorinterfaccia.

Quindi sovrascriviamo il compare()metodo per ordinare gli elementi in ordine inverso.

Per saperne di più, visita Java Comparator (documentazione ufficiale Java).

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