TreeSet : headSet

SortedSet<E> headSet(E toElement)

Returns a view of the portion of this set whose elements are strictly less than toElement.

import java.util.*;

class TreeSetHeadSet{
 public static void main(String args[]){
  TreeSet<Integer> mySet = new TreeSet<> ();
  
  /* Add Elements too mySet */
  for(int i=20; i>0; i-=2){
   mySet.add(i);
  }
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  for(int i=1; i<22; i+=2){
   System.out.print("HeadSet for value " + i +" is ");
   System.out.println(mySet.headSet(i));
  }
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

HeadSet for value 1 is []
HeadSet for value 3 is [2]
HeadSet for value 5 is [2, 4]
HeadSet for value 7 is [2, 4, 6]
HeadSet for value 9 is [2, 4, 6, 8]
HeadSet for value 11 is [2, 4, 6, 8, 10]
HeadSet for value 13 is [2, 4, 6, 8, 10, 12]
HeadSet for value 15 is [2, 4, 6, 8, 10, 12, 14]
HeadSet for value 17 is [2, 4, 6, 8, 10, 12, 14, 16]
HeadSet for value 19 is [2, 4, 6, 8, 10, 12, 14, 16, 18]
HeadSet for value 21 is [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

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

2. 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). 

3.throws NullPointerException if toElement is null and this set uses natural ordering, or its comparator does not permit null elements

import java.util.*;

class TreeSetHeadSetNullPointer{
 public static void main(String args[]){
  TreeSet<Integer> mySet = new TreeSet<> ();
  
  /* Add Elements too mySet */
  for(int i=20; i>0; i-=2){
   mySet.add(i);
  }
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  System.out.println("Getting the headSet of null");
  mySet.headSet(null);
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

Getting the headSet of 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 java.util.TreeSet.headSet(TreeSet.java:372)
        at TreeSetHeadSetNullPointer.main(TreeSetHeadSetNullPointer.java:16)

4. Throws IllegalArgumentException, if this set itself has a restricted range, and toElement lies outside the bounds of the range

import java.util.*;

class TreeSetHeadSetIllegalArg{
 public static void main(String args[]){
  TreeSet<Integer> mySet = new TreeSet<> ();
  NavigableSet<Integer> subSet;
  
  /* Add Elements too mySet */
  for(int i=20; i>0; i-=2){
   mySet.add(i);
  }
  
  System.out.println("Elements in mySet are");
  System.out.println(mySet+"\n");
  
  subSet = mySet.subSet(5, true, 15, true);
  
  System.out.println("\nElements in subSet are");
  System.out.println(subSet+"\n");
  
  System.out.println("Getting the headSet of 4");
  subSet.headSet(4);
 }
}

Output

Elements in mySet are
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]


Elements in subSet are
[6, 8, 10, 12, 14]

Getting the headSet of 4
Exception in thread "main" java.lang.IllegalArgumentException: toKey out of rang
e
        at java.util.TreeMap$AscendingSubMap.headMap(TreeMap.java:1719)
        at java.util.TreeSet.headSet(TreeSet.java:338)
        at java.util.TreeSet.headSet(TreeSet.java:372)
        at TreeSetHeadSetIllegalArg.main(TreeSetHeadSetIllegalArg.java:22)

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