NavigableSet : headSet (E toElement, boolean inclusive)

NavigableSet<E> headSet(E toElement, boolean inclusive)

Returns a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement.

import java.util.*;

class NavigableSetHeadSetFlag{
 public static void main(String args[]){
  NavigableSet<Integer> mySet = new TreeSet<> ();
  
  /* Add Elements too mySet */
  for(int i=10; i>0; i-=2){
   mySet.add(i);
  }
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  for(int i=0; i<12; i+=2){
   System.out.println("\nHeadSet for value " + i +" is ");
   System.out.print("inclusive is false ");
   System.out.println(mySet.headSet(i, false));
   System.out.print("inclusive is true ");
   System.out.println(mySet.headSet(i, true));
  }
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10]

HeadSet for value 0 is
inclusive is false []
inclusive is true []

HeadSet for value 2 is
inclusive is false []
inclusive is true [2]

HeadSet for value 4 is
inclusive is false [2]
inclusive is true [2, 4]

HeadSet for value 6 is
inclusive is false [2, 4]
inclusive is true [2, 4, 6]

HeadSet for value 8 is
inclusive is false [2, 4, 6]
inclusive is true [2, 4, 6, 8]

HeadSet for value 10 is
inclusive is false [2, 4, 6, 8]
inclusive is true [2, 4, 6, 8, 10]

1. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa.

import java.util.*;

class NavigableSetHeadSetFlag1{
 public static void main(String args[]){
  NavigableSet<Integer> mySet = new TreeSet<> ();
  NavigableSet<Integer> headSet;
  
  /* Add Elements too mySet */
  for(int i=10; i>0; i-=2){
   mySet.add(i);
  }
  
  headSet = mySet.headSet(10, true);
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  System.out.println("Elements in headSet are");
  System.out.println(headSet+"\n");
  
  System.out.println("\nAdding 3 to headSet");
  headSet.add(3);
  
  System.out.println("Remving 4 from mySet\n");
  mySet.remove(4);
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  System.out.println("Elements in headSet are");
  System.out.println(headSet+"\n");

 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10]

Elements in headSet are
[2, 4, 6, 8, 10]


Adding 3 to headSet
Remving 4 from mySet

Elements in mySet are
[2, 3, 6, 8, 10]

Elements in headSet are
[2, 3, 6, 8, 10]

2. The returned set will throw an IllegalArgumentException on an attempt to insert an element outside its range.

import java.util.*;

class NavigableSetHeadSetFlagIllegal{
 public static void main(String args[]){
  NavigableSet<Integer> mySet = new TreeSet<> ();
  NavigableSet<Integer> headSet;
  
  /* Add Elements too mySet */
  for(int i=10; i>0; i-=2){
   mySet.add(i);
  }
  
  headSet = mySet.headSet(10, true);
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  System.out.println("Elements in headSet are");
  System.out.println(headSet+"\n");
  
  System.out.println("\nAdding 11 to headSet");
  headSet.add(11);
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10]

Elements in headSet are
[2, 4, 6, 8, 10]


Adding 11 to headSet
Exception in thread "main" java.lang.IllegalArgumentException: key out of range
        at java.util.TreeMap$NavigableSubMap.put(TreeMap.java:1407)
        at java.util.TreeSet.add(TreeSet.java:255)
        at NavigableSetHeadSetFlagIllegal.main(NavigableSetHeadSetFlagIllegal.java:22)

3. throws ClassCastException, if toElement is not compatible with this set's comparator (or, if the set has no comparator, if toElement does not implement Comparable).

4. throws NullPointerException if toElement is null and this set does not permit null elements

import java.util.*;

class NavigableSetHeadSet1NullPointer{
 public static void main(String args[]){
  NavigableSet<Integer> mySet = new TreeSet<> ();
  NavigableSet<Integer> headSet;
  
  /* Add Elements too mySet */
  for(int i=10; i>0; i-=2){
   mySet.add(i);
  }
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  System.out.println("Getting the Head Set upto null");
  headSet = mySet.headSet(null, true);
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10]

Getting the Head Set upto null
Exception in thread "main" java.lang.NullPointerException
        at java.util.TreeMap.compare(TreeMap.java:1188)
        at java.util.TreeMap$NavigableSubMap.<init>(TreeMap.java:1266)
        at java.util.TreeMap$AscendingSubMap.<init>(TreeMap.java:1699)
        at java.util.TreeMap.headMap(TreeMap.java:891)
        at java.util.TreeSet.headSet(TreeSet.java:338)
        at NavigableSetHeadSet1NullPointer.main(NavigableSetHeadSet1NullPointer.Java:17)

 

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