V tomto tutoriále sa pomocou príkladov dozvieme o triede Java TreeMap a jej operáciách.
TreeMapTrieda zbierok rámca Java poskytuje implementáciu stromu štruktúry dát.
Implementuje rozhranie NavigableMap.
Vytvorenie TreeMap
Aby sme vytvorili TreeMap, musíme najskôr importovať java.util.TreeMapbalík. Po importovaní balíka môžeme vytvoriť TreeMapv jazyku Java.
TreeMap numbers = new TreeMap();
Vo vyššie uvedenom kóde sme vytvorili TreeMappomenované čísla bez akýchkoľvek argumentov. V tomto prípade sú prvky v TreeMapzoradené prirodzene (vzostupne).
Triedenie prvkov však môžeme prispôsobiť pomocou Comparatorrozhrania. Dozvieme sa o tom neskôr v tomto návode.
Tu,
- Kľúč - jedinečný identifikátor, ktorý sa používa na priradenie každého prvku (hodnoty) k mape
- Hodnota - prvky spojené s klávesmi na mape
Metódy TreeMap
TreeMapTrieda ponúka celý rad možností, ktoré nám umožňujú vykonávať operácie na mape.
Vložte prvky do TreeMap
put()- vloží zadané mapovanie kľúča / hodnoty (záznamu) do mapyputAll()- vloží všetky záznamy z určenej mapy na túto mapuputIfAbsent()- vloží zadané mapovanie kľúča / hodnoty do mapy, ak sa uvedený kľúč na mape nenachádza
Napríklad,
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); ) )
Výkon
TreeMap párnych čísel: (štyri = 4, šesť = 6, dva = 2) TreeMap čísel: (štyri = 4, jeden = 1, šesť = 6, dva = 2)
Prístup k prvkom TreeMap
1. Pomocou položiek entrySet (), keySet () a values ()
entrySet()- vráti množinu všetkých mapovaní (vstupov) kľúčov / hodnôt TreemapkeySet()- vráti sadu všetkých kľúčov stromovej mapyvalues()- vráti súbor všetkých máp stromovej mapy
Napríklad,
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()); ) )
Výkon
TreeMap: (Jeden = 1, Tri = 3, Dva = 2) Mapovanie kľúčov / hodnôt: (Jeden = 1, Tri = 3, Dva = 2) Klávesy: (Jeden, Tri, Dva) Hodnoty: (1, 3, 2 )
2. Používanie get () a getOrDefault ()
get()- Vráti hodnotu spojenú so zadaným kľúčom. Ak sa kľúč nenájde, vráti hodnotu null.getOrDefault()- Vráti hodnotu spojenú so zadaným kľúčom. Vráti zadanú predvolenú hodnotu, ak sa kľúč nenájde.
Napríklad,
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); ) )
Výkon
TreeMap: (jeden = 1, tri = 3, dva = 2) pomocou get (): 3 pomocou getOrDefault (): 5
Tu getOrDefault()metóda nenájde kľúčovú päťku. Preto vráti zadanú predvolenú hodnotu 5.
Odstráňte prvky TeeMap
remove(key)- vráti a odstráni položku spojenú so zadaným kľúčom z TreeMapremove(key, value)- odstráni položku z mapy, iba ak je zadaný kľúč spojený so zadanou hodnotou a vráti boolovskú hodnotu
Napríklad,
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); ) )
Výkon
TreeMap: (Jeden = 1, Tri = 3, Dva = 2) Odstránená hodnota = 2 Je položka (Tri = 3) odstránená? Skutočne aktualizovaná mapa stromu: (jeden = 1)
Vymeňte prvky TreeMap
replace(key, value)- nahradí hodnotu mapovanú zadaným kľúčom novou hodnotoureplace(key, old, new)- nahradí starú hodnotu novou hodnotou iba v prípade, že je stará hodnota už spojená so zadaným kľúčomreplaceAll(function)- nahradí každú hodnotu mapy výsledkom zadanej funkcie
Napríklad,
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); ) )
Výkon
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)
Vo vyššie uvedenom príklade sme vytvorili stromovú mapu prechádzajúcu triedou CustomComparator ako argument.
Trieda CustomComparator implementuje Comparatorrozhranie.
Potom compare()metódu prepíšeme na zoradenie prvkov v opačnom poradí.
Ak sa chcete dozvedieť viac, navštívte Java Comparator (oficiálna dokumentácia Java).
