Java TreeMap class

Java TreeMap class is a red-black tree based implementation. It provides an efficient means of storing key-value pairs in sorted order.

The important points about Java TreeMap class are:

Java TreeMap contains values based on the key. It implements the NavigableMap interface and extends AbstractMap class.
Java TreeMap contains only unique elements.
Java TreeMap cannot have a null key but can have multiple null values.
Java TreeMap is non synchronized.
Java TreeMap maintains ascending order.

TreeMap class declaration

Let's see the declaration for java.util.TreeMap class.

public class TreeMap<K,V> extends AbstractMap<K,V> implements NavigableMap<K,V>, Cloneable, Serializable

TreeMap class Parameters

Let's see the Parameters for java.util.TreeMap class.

K: It is the type of keys maintained by this map.
V: It is the type of mapped values.

Constructors of Java TreeMap class

Constructor	Description
TreeMap()	It is used to construct an empty tree map that will be sorted using the natural order of its key.
TreeMap(Comparator<? super K> comparator)	It is used to construct an empty tree-based map that will be sorted using the comparator comp.
TreeMap(Map<? extends K,? extends V> m)	It is used to initialize a treemap with the entries from m, which will be sorted using the natural order of the keys.
TreeMap(SortedMap<K,? extends V> m)	It is used to initialize a treemap with the entries from the SortedMap sm, which will be sorted in the same order as sm.

Methods of Java TreeMap class

Method	Description
Map.Entry<K,V> ceilingEntry(K key)	It returns the key-value pair having the least key, greater than or equal to the specified key, or null if there is no such key.
K ceilingKey(K key)	It returns the least key, greater than the specified key or null if there is no such key.
void clear()	It removes all the key-value pairs from a map.
Object clone()	It returns a shallow copy of TreeMap instance.
Comparator<? super K> comparator()	It returns the comparator that arranges the key in order, or null if the map uses the natural ordering.
NavigableSet<K> descendingKeySet()	It returns a reverse order NavigableSet view of the keys contained in the map.
NavigableMap<K,V> descendingMap()	It returns the specified key-value pairs in descending order.
Map.Entry firstEntry()	It returns the key-value pair having the least key.
Map.Entry<K,V> floorEntry(K key)	It returns the greatest key, less than or equal to the specified key, or null if there is no such key.
void forEach(BiConsumer<? super K,? super V> action)	It performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
SortedMap<K,V> headMap(K toKey)	It returns the key-value pairs whose keys are strictly less than toKey.
NavigableMap<K,V> headMap(K toKey, boolean inclusive)	It returns the key-value pairs whose keys are less than (or equal to if inclusive is true) toKey.
Map.Entry<K,V> higherEntry(K key)	It returns the least key strictly greater than the given key, or null if there is no such key.
K higherKey(K key)	It is used to return true if this map contains a mapping for the specified key.
Set keySet()	It returns the collection of keys exist in the map.
Map.Entry<K,V> lastEntry()	It returns the key-value pair having the greatest key, or null if there is no such key.
Map.Entry<K,V> lowerEntry(K key)	It returns a key-value mapping associated with the greatest key strictly less than the given key, or null if there is no such key.
K lowerKey(K key)	It returns the greatest key strictly less than the given key, or null if there is no such key.
NavigableSet<K> navigableKeySet()	It returns a NavigableSet view of the keys contained in this map.
Map.Entry<K,V> pollFirstEntry()	It removes and returns a key-value mapping associated with the least key in this map, or null if the map is empty.
Map.Entry<K,V> pollLastEntry()	It removes and returns a key-value mapping associated with the greatest key in this map, or null if the map is empty.
V put(K key, V value)	It inserts the specified value with the specified key in the map.
void putAll(Map<? extends K,? extends V> map)	It is used to copy all the key-value pair from one map to another map.
V replace(K key, V value)	It replaces the specified value for a specified key.
boolean replace(K key, V oldValue, V newValue)	It replaces the old value with the new value for a specified key.
void replaceAll(BiFunction<? super K,? super V,? extends V> function)	It replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
NavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)	It returns key-value pairs whose keys range from fromKey to toKey.
SortedMap<K,V> subMap(K fromKey, K toKey)	It returns key-value pairs whose keys range from fromKey, inclusive, to toKey, exclusive.
SortedMap<K,V> tailMap(K fromKey)	It returns key-value pairs whose keys are greater than or equal to fromKey.
NavigableMap<K,V> tailMap(K fromKey, boolean inclusive)	It returns key-value pairs whose keys are greater than (or equal to, if inclusive is true) fromKey.
boolean containsKey(Object key)	It returns true if the map contains a mapping for the specified key.
boolean containsValue(Object value)	It returns true if the map maps one or more keys to the specified value.
K firstKey()	It is used to return the first (lowest) key currently in this sorted map.
V get(Object key)	It is used to return the value to which the map maps the specified key.
K lastKey()	It is used to return the last (highest) key currently in the sorted map.
V remove(Object key)	It removes the key-value pair of the specified key from the map.
Set<Map.Entry<K,V>> entrySet()	It returns a set view of the mappings contained in the map.
int size()	It returns the number of key-value pairs exists in the hashtable.
Collection values()	It returns a collection view of the values contained in the map.

Java TreeMap Example

import java.util.*;
class TreeMap1{
 public static void main(String args[]){
   TreeMap<Integer,String> map=new TreeMap<Integer,String>();  
	  map.put(100,"Amit");  
	  map.put(102,"Ravi");  
	  map.put(101,"Vijay");  
	  map.put(103,"Rahul");  
	  
	  for(Map.Entry m:map.entrySet()){  
	   System.out.println(m.getKey()+" "+m.getValue());  
	  }  
 }
}

Output:100 Amit
       101 Vijay
       102 Ravi
       103 Rahul

Java TreeMap Example: remove()

import java.util.*;
public class TreeMap2 {
   public static void main(String args[]) {
	TreeMap<Integer,String> map=new TreeMap<Integer,String>();  
	  map.put(100,"Amit");  
	  map.put(102,"Ravi");  
	  map.put(101,"Vijay");  
	  map.put(103,"Rahul");  
	  System.out.println("Before invoking remove() method");
	  for(Map.Entry m:map.entrySet())
	  {
		  System.out.println(m.getKey()+" "+m.getValue());	  
	  }
	  map.remove(102);	  
	  System.out.println("After invoking remove() method");
	  for(Map.Entry m:map.entrySet())
	  {
		  System.out.println(m.getKey()+" "+m.getValue());	  
	  }
	  }
}

Output:

Before invoking remove() method
100 Amit
101 Vijay
102 Ravi
103 Rahul
After invoking remove() method
100 Amit
101 Vijay
103 Rahul

Java TreeMap Example: NavigableMap

import java.util.*;
class TreeMap3{
 public static void main(String args[]){
   NavigableMap<Integer,String> map=new TreeMap<Integer,String>();  
	  map.put(100,"Amit");  
	  map.put(102,"Ravi");  
	  map.put(101,"Vijay");  
	  map.put(103,"Rahul");  
	  //Maintains descending order
	  System.out.println("descendingMap: "+map.descendingMap());
	  //Returns key-value pairs whose keys are less than or equal to the specified key.
	  System.out.println("headMap: "+map.headMap(102,true));
	  //Returns key-value pairs whose keys are greater than or equal to the specified key.
	  System.out.println("tailMap: "+map.tailMap(102,true));
	  //Returns key-value pairs exists in between the specified key.
	  System.out.println("subMap: "+map.subMap(100, false, 102, true)); 
 }
}

descendingMap: {103=Rahul, 102=Ravi, 101=Vijay, 100=Amit}
headMap: {100=Amit, 101=Vijay, 102=Ravi}
tailMap: {102=Ravi, 103=Rahul}
subMap: {101=Vijay, 102=Ravi}

Java TreeMap Example: SortedMap

import java.util.*;
class TreeMap4{
 public static void main(String args[]){
   SortedMap<Integer,String> map=new TreeMap<Integer,String>();  
	  map.put(100,"Amit");  
	  map.put(102,"Ravi");  
	  map.put(101,"Vijay");  
	  map.put(103,"Rahul");  
	  //Returns key-value pairs whose keys are less than the specified key.
	  System.out.println("headMap: "+map.headMap(102));
	  //Returns key-value pairs whose keys are greater than or equal to the specified key.
	  System.out.println("tailMap: "+map.tailMap(102));
	  //Returns key-value pairs exists in between the specified key.
	  System.out.println("subMap: "+map.subMap(100, 102));  
 }
}

headMap: {100=Amit, 101=Vijay}
tailMap: {102=Ravi, 103=Rahul}
subMap: {100=Amit, 101=Vijay}

What is difference between HashMap and TreeMap?

HashMap	TreeMap
1) HashMap can contain one null key.	TreeMap cannot contain any null key.
2) HashMap maintains no order.	TreeMap maintains ascending order.

Java TreeMap Example: Book

import java.util.*;  
class Book {  
int id;  
String name,author,publisher;  
int quantity;  
public Book(int id, String name, String author, String publisher, int quantity) {  
    this.id = id;  
    this.name = name;  
    this.author = author;  
    this.publisher = publisher;  
    this.quantity = quantity;  
}  
}  
public class MapExample {  
public static void main(String[] args) {  
    //Creating map of Books  
    Map<Integer,Book> map=new TreeMap<Integer,Book>();  
    //Creating Books  
    Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);  
    Book b2=new Book(102,"Data Communications & Networking","Forouzan","Mc Graw Hill",4);  
    Book b3=new Book(103,"Operating System","Galvin","Wiley",6);  
    //Adding Books to map 
    map.put(2,b2);
    map.put(1,b1);
    map.put(3,b3);
    
    //Traversing map
    for(Map.Entry<Integer, Book> entry:map.entrySet()){  
    	int key=entry.getKey();
    	Book b=entry.getValue();
        System.out.println(key+" Details:");
        System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity); 
    }  
}  
}  

Output:

1 Details:
101 Let us C Yashwant Kanetkar BPB 8
2 Details:
102 Data Communications & Networking Forouzan Mc Graw Hill 4
3 Details:
103 Operating System Galvin Wiley 6

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