Java TreeMap headMap() Method

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Description

TheJava TreeMap headMap(K toKey) method is used to return a view of the portion of this map whose keys are strictly less than toKey.

Declaration

Following is the declaration for java.util.TreeMap.headMap() method.

public SortedMap<K,V> headMap(K toKey)

Parameters

toKey − This is the high endpoint (exclusive) of the keys in the returned map.

Return Value

The method call returns a view of the portion of this map whose keys are strictly less than toKey.

Exception

• ClassCastException − This exception is thrown if toKey is not compatible with this map's comparator (or, if the map has no comparator, if toKey does not implement Comparable). Implementations may, but are not required to, throw this exception if toKey cannot be compared to keys currently in the map.

• NullPointerException − This exception is thrown if toKey is null and this map uses natural ordering, or its comparator does not permit null keys.

• IllegalArgumentException − This exception is thrown if this map itself has a restricted range, and toKey lies outside the bounds of the range.

Java TreeMap headMap(K toKey,boolean inclusive) Method

Description

TheheadMap(K toKey,boolean inclusive) method is used to return a view of the portion of this map whose keys are less than (or equal to, if inclusive is true)toKey.

Declaration

Following is the declaration for java.util.TreeMap.headMap() method.

public NavigableMap<K,V> headMap(K toKey,boolean inclusive)

Parameters

• toKey − This is the high endpoint of the keys in the returned map.

• inclusive − This is true if the high endpoint is to be included in the returned view.

Return Value

The method call returns a view of the portion of this map whose keys are less than (or equal to, if inclusive is true) toKey.

Exception

• ClassCastException − This exception is thrown if toKey is not compatible with this map's comparator.

• NullPointerException − This exception is thrown if toKey is null and this map uses natural ordering, or its comparator does not permit null keys.

• IllegalArgumentException − This exception is thrown if this map itself has a restricted range, and toKey lies outside the bounds of the range.

Getting Head Map based on given Key from a TreeMap of Integer, Integer Pairs Example

The following example shows the usage of Java TreeMap headMap(K key) method to get a view of the portion of this map whose keys are strictly less than given key. We've created a TreeMap and SortedMap objects of Integer,Integer pairs. Then few entries are added to the treemap object, and using headMap() we're getting a view for the given key and printed it.

package com.tutorialspoint;

import java.util.SortedMap;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating maps 
      TreeMap<Integer, Integer> treemap = new TreeMap<>();
      SortedMap<Integer, Integer> treemaphead = new TreeMap<>();

      // populating tree map
      treemap.put(2, 2);
      treemap.put(1, 1);
      treemap.put(3, 3);
      treemap.put(6, 6);
      treemap.put(5, 5);

      // getting head map
      treemaphead = treemap.headMap(3);

      System.out.println("Checking values of the sorted map");
      System.out.println("Value is: "+ treemaphead);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the sorted map
Value is: {1=1, 2=2}

Getting Head Map based on given Key from a TreeMap of Integer, String Pairs Example

The following example shows the usage of Java TreeMap headMap(K key) method to get a view of the portion of this map whose keys are strictly less than given key. We've created a TreeMap and SortedMap objects of Integer,String pairs. Then few entries are added to the treemap object, and using headMap() we're getting a view for the given key and printed it.

package com.tutorialspoint;

import java.util.SortedMap;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating maps 
      TreeMap<Integer, String> treemap = new TreeMap<>();
      SortedMap<Integer, String> treemaphead = new TreeMap<>();

      // populating tree map
      treemap.put(2, "two");
      treemap.put(1, "one");
      treemap.put(3, "three");
      treemap.put(6, "six");
      treemap.put(5, "five");

      // getting head map
      treemaphead = treemap.headMap(3);

      System.out.println("Checking values of the sorted map");
      System.out.println("Value is: "+ treemaphead);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the sorted map
Value is: {1=one, 2=two}

Getting Head Map based on given Key from a TreeMap of Integer, Object Pairs Example

The following example shows the usage of Java TreeMap headMap(K key, boolean inclusive) method to get a view of the portion of this map whose keys are strictly less than given key while including the high end point. We've created a TreeMap and SortedMap objects of Integer,Student pairs. Then few entries are added to the treemap object, and using headMap() we're getting a view for the given key and printed it.

package com.tutorialspoint;

import java.util.NavigableMap;
import java.util.TreeMap;

public class TreeMapDemo {
   public static void main(String[] args) {

      // creating maps 
      TreeMap<Integer, String> treemap = new TreeMap<>();
      NavigableMap<Integer, String> treemapheadincl = new TreeMap<>();

      // populating tree map
      treemap.put(2, "two");
      treemap.put(1, "one");
      treemap.put(3, "three");
      treemap.put(6, "six");
      treemap.put(5, "five");

      // getting head map inclusive 3
      treemapheadincl = treemap.headMap(3,true);

      System.out.println("Checking values of the map");
      System.out.println("Value is: "+ treemapheadincl);
   }    
}

Output

Let us compile and run the above program, this will produce the following result.

Checking values of the map
Value is: {1=one, 2=two, 3=three}