NavigableMap headMap() in Java

Last Updated : 02 Sep, 2024

The headMap() method of

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.

The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa.
The returned map supports all optional map operations that this map supports.
The returned map will throw an IllegalArgumentException on an attempt to insert a key outside its range.

Syntax

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

Where, K is the type of key maintained by this map and V is the value associated with the key in the map.

Parameters

: This function accepts two parameter:

toKey: This parameter refers to the key.
inclusive: This parameter decides whether key to be deleted should be compared with equality or not.

Return Value

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

Program 1

: When the key is integer and

second argument is missing

Java

// Java code to demonstrate the working of
// headMap() method

import java.io.*;
import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // Declaring the NavigableMap of Integer and String
        NavigableMap<Integer, String> nmmp = new TreeMap<>();

        // assigning the values in the NavigableMap
        // using put()
        nmmp.put(2, "two");
        nmmp.put(7, "seven");
        nmmp.put(3, "three");

        System.out.println("View of map with key less than"
                      + " or equal to 7 : " + nmmp.headMap(7));
    }
}

Output

View of map with key less than or equal to 7 : {2=two, 3=three}

Program 2

: With second argument.

Java

// Java code to demonstrate the working of
// headMap() method

import java.io.*;
import java.util.*;

public class GFG {

    public static void main(String[] args)
    {

        // Declaring the NavigableMap of Integer and String
        NavigableMap<Integer, String> nmmp = new TreeMap<>();

        // assigning the values in the NavigableMap
        // using put()
        nmmp.put(2, "two");
        nmmp.put(7, "seven");
        nmmp.put(3, "three");
        nmmp.put(9, "nine");

        // headMap with second argument as true
        System.out.println("View of map with key less than"
                     + " or equal to 7 : " + nmmp.headMap(7, true));
    }
}

Output

View of map with key less than or equal to 7 : {2=two, 3=three, 7=seven}

Reference

https://docs.oracle.com/javase/10/docs/api/java/util/NavigableMap.html#headMap(K, boolean)

NavigableSet in Java

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