Java TreeMap (med exempel)

I denna handledning lär vi oss om klassen Java TreeMap och dess funktioner med hjälp av exempel.

Den TreeMapklass av samlingarna ram Java ger genomförandet trädet datastruktur.

Den implementerar NavigableMap-gränssnittet.

Skapa en TreeMap

För att skapa ett TreeMapmåste vi först importera java.util.TreeMappaketet. När vi väl har importerat paketet, här är hur vi kan skapa en TreeMapi Java.

 TreeMap numbers = new TreeMap();

I koden ovan har vi skapat ett TreeMapnamn utan några argument. I det här fallet TreeMapsorteras elementen i naturligt (stigande ordning).

Vi kan dock anpassa sorteringen av element med hjälp av Comparatorgränssnittet. Vi lär oss om det senare i den här handledningen.

Här,

  • Nyckel - en unik identifierare som används för att associera varje element (värde) på en karta
  • Värde - element som är associerade med nycklar på en karta

Metoder för TreeMap

I TreeMapklassen tillhandahåller olika metoder som gör att vi kan utföra operationer på kartan.

Infoga element i TreeMap

  • put() - infogar den angivna nyckel- / värdekartläggningen (post) på kartan
  • putAll() - infogar alla poster från specificerad karta till denna karta
  • putIfAbsent() - infogar den angivna nyckel- / värdekartläggningen på kartan om den angivna nyckeln inte finns på kartan

Till exempel,

 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); ) )

Produktion

 TreeMap av jämna siffror: (Fyra = 4, Sex = 6, Två = 2) TreeMap av siffror: (Fyra = 4, En = 1, Sex = 6, Två = 2)

Åtkomst till TreeMap-element

1. Använda entrySet (), keySet () och värden ()

  • entrySet() - returnerar en uppsättning av alla nyckel / värden mappning (inmatning) för en treemap
  • keySet() - returnerar en uppsättning av alla tangenter på en trädkarta
  • values() - returnerar en uppsättning av alla kartor på en trädkarta

Till exempel,

 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()); ) )

Produktion

 TreeMap: (One = 1, Three = 3, Two = 2) Key / Value mappings: (One = 1, Three = 3, Two = 2) Keys: (One, Three, Two) Värden: (1, 3, 2 )

2. Använda get () och getOrDefault ()

  • get()- Returnerar värdet som är associerat med den angivna nyckeln. Returnerar null om nyckeln inte hittas.
  • getOrDefault()- Returnerar värdet som är associerat med den angivna nyckeln. Returnerar det angivna standardvärdet om nyckeln inte hittas.

Till exempel,

 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); ) )

Produktion

 TreeMap: (En = 1, Tre = 3, Två = 2) Använda get (): 3 Använda getOrDefault (): 5

Här getOrDefault()hittar metoden inte nyckeln Fem. Därför returnerar det angivna standardvärdet 5.

Ta bort TeeMap Elements

  • remove(key) - returnerar och tar bort posten som är associerad med den angivna nyckeln från en TreeMap
  • remove(key, value) - tar bara bort posten från kartan om den angivna nyckeln är associerad med det angivna värdet och returnerar ett booleskt värde

Till exempel,

 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); ) )

Produktion

TreeMap: (One = 1, Three = 3, Two = 2) Borttaget värde = 2 Tar posten (Three = 3) bort? Sann uppdaterad TreeMap: (One = 1)

Byt ut TreeMap-element

  • replace(key, value) - ersätter värdet som mappats av den angivna nyckeln med det nya värdet
  • replace(key, old, new) - ersätter det gamla värdet med det nya värdet endast om det gamla värdet redan är associerat med den angivna nyckeln
  • replaceAll(function) - ersätter varje värde på kartan med resultatet av den angivna funktionen

Till exempel,

 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); ) )

Produktion

 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)

I ovanstående exempel har vi skapat en trekarta som passerar CustomComparator-klassen som ett argument.

CustomComparator-klassen implementerar Comparatorgränssnittet.

Vi åsidosätter sedan compare()metoden för att sortera element i omvänd ordning.

För att lära dig mer, besök Java Comparator (officiell Java-dokumentation).

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