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Java Set

  • Set models the mathematical set abstraction.  

  • Contains no methods other than those inherited from Collection. 

  • Two Set objects are equal if they contain the same elements.   

  • Lists are usually used to keep things in some kind of order, Lists allow you to manually override the ordering of  elements by adding or removing elements via the element's index. 

Set has 3 different implementations

  1. TreeSet 

  2. HashSet 

  3. LinkedHashSet 

TreeSet

  • TreeSet implements SortedSet which is sub interface of Set. 

  • Elements in TreeSet are sorted by natural ascending order or custom comparision rules. 

  • Null value is not allowed. 

  • Duplicates and dissimilar objects are not allowed. 

  • Elements cannot be access randomly. 

  • TreeSet uses tree data structure to store values. 

java-set-0
 
java-set-1
 

TreeSet Example

TreeSetDemo.java

import java.util.ArrayList;

import java.util.NavigableSet;

import java.util.SortedSet;

import java.util.TreeSet;

 

public class TreeSetDemo {

 

        public static void main(String[] args) {

 

                // create a TreeSet

                TreeSet set1 = new TreeSet();

                TreeSet set2 = new TreeSet();

                TreeSet<Integer> set3 = new TreeSet();

 

                ArrayList list = new ArrayList();

                list.add(10);

                list.add(20);

                list.add(30);

                list.add(40);

               

                // Adding Elements to TreeSet

                set1.add(30);

                set1.add(40);

                set1.add(10);

                set1.add(20);

 

                set2.addAll(list);

                set3.addAll(list);

                // cannot add duplicates, dissimilar and null values in TreeSet

 

                // set.add("A");

                // set.add(new Integer(10));

                // set.add(null);

                System.out.println("Contents of TreeSet set1: " + set1);

                System.out.println("Contents of TreeSet set2: " + set2);

                System.out.println("Contents of TreeSet set3: " + set3);

               

                // Getting the elements

                Object o1[] = set1.toArray();

                System.out.println(o1[0]);

                Integer i1 = (Integer) o1[2];

                System.out.println(i1);

 

                Integer i2[] = set3.toArray(new Integer[set3.size()]);// getting exactly same size as set3

                Integer i3[] = set3.toArray(new Integer[5]);// specifying size explicitly

 

                Object o2 = set1.first();

                System.out.println(o2);

                Object o3 = set1.last();

                System.out.println(o3);

               

                // Searching

                boolean b1 = set1.contains(10);

                System.out.println(b1);

                boolean b2 = set1.containsAll(set2);

                System.out.println(b1);

                Object o4 = set1.ceiling(25);

                System.out.println(o4);

                Object o5 = set1.floor(25);

                System.out.println(o5);

                Object o6 = set1.higher(25);

                System.out.println(o6);

                Object o7 = set1.lower(25);

                System.out.println(o7);

               

                // Getting collection

                SortedSet ref1 = set1.headSet(35);

                System.out.println(ref1);

                SortedSet ref2 = set1.tailSet(35);

                System.out.println(ref2);

                SortedSet ref3 = set1.subSet(15, 35);

                System.out.println(ref3);

                NavigableSet ref4 = set1.descendingSet();

                NavigableSet ref5 = set1.headSet(35, true);

                System.out.println(ref5);

                NavigableSet ref6 = set1.tailSet(35, false);

                System.out.println(ref6);

                NavigableSet ref7 = set1.subSet(15, true, 35, false);

                System.out.println(ref7);

               

                // Removing elements

                boolean b3 = set1.remove(30);

                System.out.println(b3);

                boolean b4 = set2.removeAll(set1);

                System.out.println(b4);

                boolean b5 = set2.retainAll(set1);

                System.out.println(b5);

                set2.clear();

               

                // Other

                Object o8 = set1.pollFirst();// Returns the first element, removing the element in the process

                System.out.println(set1);

                Object o9 = set1.pollLast();// Returns the last element, removing the element in the process

                System.out.println(set1);

                int i = set1.size();

                System.out.println(i);

                boolean b7 = set1.equals(set2);

                System.out.println(b7);

                boolean b8 = set1.isEmpty();

                System.out.println(b8);

        }

}

Output:

Contents of TreeSet set1: [10, 20, 30, 40]

Contents of TreeSet set2: [10, 20, 30, 40]

Contents of TreeSet set3: [10, 20, 30, 40]

10

30

10

40

true

true

30

20

30

20

[10, 20, 30]

[40]

[20, 30]

[10, 20, 30]

[40]

[20, 30]

true

true

true

[20, 40]

[20]

1

false

false

HashSet

    • HashSet stores the elements in a hash table. 

    • It uses the hash code of the object being inserted. 

    • Hash table is also a data structure which can be used for storing values. 

    • The values in the hash table are always stored after generating the hash code for that value. 

    • The mechanism of generating the hash code for a value is called hashing technique. In java hashCode() method provides the support for generating the hash code. 

    • HashSet defines no any extra methods other than those inherited from Collection Interface. 

java-set-2
 

LinkedHashSet

    • LinkedHashSet has a combined implementation of hash table and linked list. 

    • LinkedHashSet stores elements in a hash table and links them with double linked list. 

    • Values are stored in the same order as inserted and are unsorted. 

    • LinkedHashSet defines no any extra methods other than those inherited from Collection Interface. 

java-set-3
 
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