Java TreeMap (met voorbeelden)

In deze tutorial leren we met behulp van voorbeelden over de Java TreeMap-klasse en de bewerkingen ervan.

De TreeMapklasse van het Java-verzamelingsraamwerk biedt de implementatie van de boomgegevensstructuur.

Het implementeert de NavigableMap-interface.

Een TreeMap maken

Om een ​​te maken TreeMap, moeten we eerst het java.util.TreeMappakket importeren . Nadat we het pakket hebben geïmporteerd, kunnen we als volgt een TreeMapin Java maken.

 TreeMap numbers = new TreeMap();

In de bovenstaande code hebben we een TreeMapbenoemd nummer gemaakt zonder argumenten. In dit geval worden de elementen in TreeMapnatuurlijk gesorteerd (oplopende volgorde).

We kunnen het sorteren van elementen echter aanpassen door de Comparatorinterface te gebruiken. We zullen er later in deze tutorial over leren.

Hier,

  • Sleutel - een unieke identificatie die wordt gebruikt om elk element (waarde) in een kaart te associëren
  • Waarde - elementen die zijn gekoppeld door sleutels in een kaart

Methoden van TreeMap

De TreeMapklasse biedt verschillende methoden waarmee we bewerkingen op de kaart kunnen uitvoeren.

Voeg elementen in TreeMap in

  • put() - voegt de opgegeven sleutel / waarde-toewijzing (invoer) in op de kaart
  • putAll() - voegt alle gegevens van de opgegeven kaart in op deze kaart
  • putIfAbsent() - voegt de opgegeven sleutel / waarde-toewijzing in op de kaart als de opgegeven sleutel niet aanwezig is op de kaart

Bijvoorbeeld,

 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); ) )

Uitvoer

 TreeMap van even getallen: (Four = 4, Six = 6, Two = 2) TreeMap met getallen: (Four = 4, One = 1, Six = 6, Two = 2)

Toegang tot TreeMap Elements

1. entrySet (), keySet () en values ​​() gebruiken

  • entrySet() - retourneert een set van alle sleutel / waarden mapping (invoer) van een treemap
  • keySet() - geeft een set van alle sleutels van een boomstructuur terug
  • values() - geeft een set van alle kaarten van een boomstructuur terug

Bijvoorbeeld,

 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()); ) )

Uitvoer

 TreeMap: (One = 1, Three = 3, Two = 2) Sleutel- / waardetoewijzingen: (One = 1, Three = 3, Two = 2) Keys: (One, Three, Two) Waarden: (1, 3, 2 )

2. Get () en getOrDefault () gebruiken

  • get()- Retourneert de waarde die is gekoppeld aan de opgegeven sleutel. Retourneert null als de sleutel niet wordt gevonden.
  • getOrDefault()- Retourneert de waarde die is gekoppeld aan de opgegeven sleutel. Retourneert de opgegeven standaardwaarde als de sleutel niet wordt gevonden.

Bijvoorbeeld,

 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); ) )

Uitvoer

 TreeMap: (One = 1, Three = 3, Two = 2) Get () gebruiken: 3 getOrDefault () gebruiken: 5

Hier vindt de getOrDefault()methode de sleutel Vijf niet. Daarom retourneert het de opgegeven standaardwaarde 5.

Verwijder TeeMap Elements

  • remove(key) - retourneert en verwijdert het item dat is gekoppeld aan de opgegeven sleutel uit een TreeMap
  • remove(key, value) - verwijdert het item alleen van de kaart als de opgegeven sleutel is gekoppeld aan de opgegeven waarde en retourneert een booleaanse waarde

Bijvoorbeeld,

 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); ) )

Uitvoer

TreeMap: (One = 1, Three = 3, Two = 2) Verwijderde waarde = 2 Is het item (Three = 3) verwijderd? Echte bijgewerkte TreeMap: (One = 1)

Vervang TreeMap-elementen

  • replace(key, value) - vervangt de waarde die is toegewezen door de opgegeven sleutel door de nieuwe waarde
  • replace(key, old, new) - vervangt de oude waarde alleen door de nieuwe waarde als de oude waarde al is gekoppeld aan de opgegeven sleutel
  • replaceAll(function) - vervangt elke waarde van de kaart door het resultaat van de opgegeven functie

Bijvoorbeeld,

 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); ) )

Uitvoer

 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)

In het bovenstaande voorbeeld hebben we een treemap gemaakt die de klasse CustomComparator als een argument doorgeeft.

De CustomComparator-klasse implementeert de Comparatorinterface.

We overschrijven vervolgens de compare()methode om elementen in omgekeerde volgorde te sorteren.

Bezoek voor meer informatie Java Comparator (officiële Java-documentatie).

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