I denne vejledning lærer vi om Java TreeMap-klassen og dens operationer ved hjælp af eksempler.
Den TreeMap
klasse af Java kollektioner rammer giver gennemførelsen træ datastruktur.
Den implementerer NavigableMap-grænsefladen.
Oprettelse af et TreeMap
For at oprette en TreeMap
skal vi først importere java.util.TreeMap
pakken. Når vi først har importeret pakken, kan du oprette en TreeMap
i Java her.
TreeMap numbers = new TreeMap();
I ovenstående kode har vi oprettet et TreeMap
navngivet nummer uden nogen argumenter. I dette tilfælde TreeMap
sorteres elementerne i naturligt (stigende rækkefølge).
Vi kan dog tilpasse sorteringen af elementer ved hjælp af Comparator
grænsefladen. Vi lærer om det senere i denne vejledning.
Her,
- Nøgle - en unik identifikator, der bruges til at knytte hvert element (værdi) til et kort
- Værdi - elementer tilknyttet nøgler på et kort
Metoder til TreeMap
Den TreeMap
klasse giver forskellige metoder, der tillader os at udføre operationer på kortet.
Indsæt elementer til TreeMap
put()
- indsætter den angivne nøgle- / værdikortlægning (post) på kortetputAll()
- indsætter alle poster fra det angivne kort til dette kortputIfAbsent()
- indsætter den angivne nøgle- / værdikortlægning på kortet, hvis den angivne nøgle ikke er til stede på kortet
For eksempel,
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 af lige tal: (Fire = 4, Six = 6, Two = 2) TreeMap of numbers: (Four = 4, One = 1, Six = 6, Two = 2)
Adgang til TreeMap-elementer
1. Brug af entrySet (), keySet () og værdier ()
entrySet()
- returnerer et sæt af alle nøgle / værdikortlægning (indtastning) af et treemapkeySet()
- returnerer et sæt af alle tasterne på et trækortvalues()
- returnerer et sæt af alle kortene på et trækort
For eksempel,
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) Værdier: (1, 3, 2 )
2. Brug af get () og getOrDefault ()
get()
- Returnerer den værdi, der er knyttet til den angivne nøgle. Returnerer nul, hvis nøglen ikke findes.getOrDefault()
- Returnerer den værdi, der er knyttet til den angivne nøgle. Returnerer den angivne standardværdi, hvis nøglen ikke findes.
For eksempel,
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: (One = 1, Three = 3, Two = 2) Brug af get (): 3 Brug af getOrDefault (): 5
Her finder getOrDefault()
metoden ikke nøglen Five. Derfor returnerer den den angivne standardværdi 5.
Fjern TeeMap Elements
remove(key)
- returnerer og fjerner den post, der er knyttet til den angivne nøgle fra et TreeMapremove(key, value)
- fjerner kun posten fra kortet, hvis den angivne nøgle er knyttet til den angivne værdi og returnerer en boolsk værdi
For eksempel,
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) Fjernet værdi = 2 Er posten (Three = 3) fjernet? Ægte opdateret TreeMap: (One = 1)
Udskift TreeMap-elementer
replace(key, value)
- erstatter den værdi, der er kortlagt af den angivne nøgle, med den nye værdireplace(key, old, new)
- erstatter kun den gamle værdi med den nye værdi, hvis den gamle værdi allerede er knyttet til den angivne nøglereplaceAll(function)
- erstatter hver værdi på kortet med resultatet af den angivne funktion
For eksempel,
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 mapfirstEntry()
- returns the key/value mapping of the first key of the maplastKey()
- returns the last key of the maplastEntry()
- 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 mappollLastEntry()
- 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 ovenstående eksempel har vi oprettet et treemap, der passerer CustomComparator-klassen som et argument.
CustomComparator-klassen implementerer Comparator
grænsefladen.
Vi tilsidesætter derefter compare()
metoden til at sortere elementer i omvendt rækkefølge.
For at lære mere, besøg Java Comparator (officiel Java-dokumentation).