java.util.TreeSet.headSet() Method

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Description

The headSet(E toElement,boolean inclusive) method is used to return a view of the portion of this set whose elements are less than (or equal to, if inclusive is true) toElement. The returned set is backed by this set, so changes in the returned set are reflected in this set, and vice-versa.

Declaration

Following is the declaration for java.util.TreeSet.headSet() method.

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

Parameters

• toElement − This is the high endpoint of the returned set.

• 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 set whose elements are less than (or equal to, if inclusive is true) toElement.

Exception

• ClassCastException − This is thrown if toElement is not compatible with this set's comparator.

• NullPointerException − This is thrown if toElement is null and this set uses natural ordering, or its comparator does not permit null elements.

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

Example

The following example shows the usage of java.util.TreeSet.headSet() method.

package com.tutorialspoint;

import java.util.Iterator;
import java.util.TreeSet;

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

      // creating TreeSet 
      TreeSet <Integer>tree = new TreeSet<Integer>();
      TreeSet <Integer>treeheadsetincl = new TreeSet<Integer>();

      // adding in the tree
      tree.add(12);
      tree.add(13);
      tree.add(14);
      tree.add(15);
      tree.add(16);
      tree.add(17);

      // getting values for 15 inclusive true
      treeheadsetincl = (TreeSet)tree.headSet(15, true);  
    
      // creating iterator
      Iterator iterator;
      iterator = treeheadsetincl.iterator();

      //Displaying the tree set data
      System.out.println("Tree set data for '15' inclusive TRUE: ");     
      
      while (iterator.hasNext()) {
         System.out.println(iterator.next() + " ");
      }

      // getting values for 15 inclusive false
      treeheadsetincl = (TreeSet)tree.headSet(15, false);  
    
      // creating iterator     
      iterator = treeheadsetincl.iterator();

      //Displaying the tree set data
      System.out.println("Tree set data for '15' inclusive FALSE: ");     
      
      while (iterator.hasNext()) {
         System.out.println(iterator.next() + " ");
      }
   }    
}

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

Tree set data for '15' inclusive TRUE: 
12 
13 
14 
15 
Tree set data for '15' inclusive FALSE: 
12 
13 
14