The headMap() method of NavigableMap interface in Java 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 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 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)
