Java TreeMap (z primeri)

V tej vadnici bomo s pomočjo primerov spoznali razred Java TreeMap in njegovo delovanje.

TreeMapRazred okviru Java zbirk zagotavlja izvajanje na drevo podatki strukturo.

Izvaja vmesnik NavigableMap.

Ustvarjanje TreeMap

Če želite ustvariti a TreeMap, moramo najprej uvoziti java.util.TreeMappaket. Ko uvozimo paket, tukaj je opisano, kako lahko TreeMapv Javi ustvarimo a .

 TreeMap numbers = new TreeMap();

V zgornji kodi smo TreeMapbrez argumentov ustvarili imenovane številke. V tem primeru so elementi v TreeMaprazvrščeni naravno (naraščajoče).

Razvrščanje elementov pa lahko prilagodimo z uporabo Comparatorvmesnika. O tem bomo izvedeli kasneje v tej vadnici.

Tukaj,

  • Ključ - enolični identifikator, ki se uporablja za povezovanje vsakega elementa (vrednosti) na zemljevidu
  • Vrednost - elementi, povezani s ključi na zemljevidu

Metode TreeMap

TreeMapRazred določa različne metode, ki nam omogočajo, da izvajanje operacij na zemljevidu.

Vstavite elemente v TreeMap

  • put() - na zemljevid vstavi določeno preslikavo ključ / vrednost (vnos)
  • putAll() - na ta zemljevid vstavi vse vnose z določenega zemljevida
  • putIfAbsent() - na zemljevid vstavi določeno preslikavo ključa / vrednosti, če določeni ključ ni prisoten na zemljevidu

Na primer

 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); ) )

Izhod

 TreeMap parnih števil: (Four = 4, Six = 6, Two = 2) TreeMap števil:: Four = 4, One = 1, Six = 6, Two = 2)

Dostopite do elementov TreeMap

1. Uporaba entrySet (), keySet () in vrednosti ()

  • entrySet() - vrne nabor vseh preslikav ključev / vrednosti (vnos) drevesne karte
  • keySet() - vrne nabor vseh tipk drevesnega zemljevida
  • values() - vrne niz vseh zemljevidov drevesnega zemljevida

Na primer

 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()); ) )

Izhod

 TreeMap: (One = 1, Three = 3, Two = 2) Preslikave ključa / vrednosti: (One = 1, Three = 3, Two = 2) Tipke: (One, Three, Two) Vrednosti: (1, 3, 2 )

2. Uporaba get () in getOrDefault ()

  • get()- Vrne vrednost, povezano z navedenim ključem. Vrne nič, če ključa ni mogoče najti.
  • getOrDefault()- Vrne vrednost, povezano z navedenim ključem. Vrne določeno privzeto vrednost, če ključa ni mogoče najti.

Na primer

 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); ) )

Izhod

 TreeMap: (One = 1, Three = 3, Two = 2) Uporaba get (): 3 Uporaba getOrDefault (): 5

Tu getOrDefault()metoda ne najde ključnega peterca. Zato vrne določeno privzeto vrednost 5.

Odstranite elemente TeeMap

  • remove(key) - vrne in odstrani vnos, povezan z določenim ključem, iz TreeMap
  • remove(key, value) - odstrani vnos z zemljevida samo, če je navedeni ključ povezan z navedeno vrednostjo in vrne logično vrednost

Na primer

 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); ) )

Izhod

TreeMap: (One = 1, Three = 3, Two = 2) Odstranjena vrednost = 2 Ali je vnos (Three = 3) odstranjen? Resnično posodobljen drevesni zemljevid: (ena = 1)

Zamenjajte elemente TreeMap

  • replace(key, value) - nadomesti vrednost, ki jo preslika določeni ključ, z novo vrednostjo
  • replace(key, old, new) - staro vrednost nadomesti z novo vrednostjo le, če je stara vrednost že povezana z določenim ključem
  • replaceAll(function) - zamenja vsako vrednost zemljevida z rezultatom določene funkcije

Na primer

 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); ) )

Izhod

 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)

V zgornjem primeru smo ustvarili drevesni zemljevid, ki kot argument posreduje razred CustomComparator.

Razred CustomComparator izvaja Comparatorvmesnik.

Nato compare()metodo preglasimo, da razvrstimo elemente v obratnem vrstnem redu.

Če želite izvedeti več, obiščite Java Comparator (uradna dokumentacija za Java).

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