Java Collections – LinkedHashSet Example – Developers Corner

The LinkedHashSet is a hash table and Linked List based implementation of the Set interface. A LinkedHashSet is different than the HashSet because it maintains a linked list which allows it to maintain the order in which the elements were inserted into the set (insertion-order).

Big-O Notation

According to the Javadocs, this implementation provides constant-time O(1) performance for the basic operations (add and remove).

Creating a LinkedHashSet

This was the old-way prior to Generics.

Set employees = new LinkedHashSet();
LinkedHashSet employees = new LinkedHashSet();

Generics

If you look closely, you will notice that I am using Generics to limit the type to String in the Set. Generics add stability to your code, by having the computer detect type incompatibilities during compile-time. These runtime bugs would be more difficult to debug if left unchecked.

Set<String> employees = new LinkedHashSet<String>();
LinkedHashSet<String> employees = new LinkedHashSet<String>();

LinkedHashSet Points

LinkedHashSet will maintain the insertion order. This will allow us to iterate through the set in the same order as the elements were inserted
Better than HashSet in that you know the ordering sequence
No duplicates contained in Set
Faster implementation than that of TreeSet
LinkedHashSet are not thread safe, you may need to wrap in Collections.synchronizedSet()

java_collections

Adding elements

Adding elements to the employees Set is done by using the add(Object obj) method.

No Duplicates Allowed

When adding elements, we need not worry about duplicates as any dups added during the processing will not appear in the set.

scores.add(7); 
scores.add(18);
scores.add(7);  // duplicate
scores.add(2);
scores.add(64);
scores.add(7);  // duplicate

Auto-Boxing and Unboxing

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes. For example, converting an int to an Integer and vice-versa without the need to cast. In this case, Java is performing the boxing when we put elements into the set (converting int to Integer wrapper class) and unboxing when we get() when iterating through the elements from the Set (converting Integer to int primitive).

employees.add("John");
employees.add("David");
employees.add("James");
employees.add("Danielle");
employees.add("Jeff");
employees.add("Chris");
employees.add("Mary");
employees.add("Tina");

Removing elements

Removing elements is just a matter of calling the remove(Object obj) method.

employees.remove("Jeff");
employees.remove("Mary");

Checking elements using contains

You can see if object is in set by calling the contains(Object obj) method.

employees.contains("Danielle");
employees.contains("Amaury");

Size of Collection

Returning the number of elements in a LinkedHashSet is as easy as calling the size() method.

employees.size();

Iterating through the Collection

Java 1.5 and above provides a foreach loop, which makes it much easier to iterate over the entire collection. This is my preferred way of doing it.

// Loop through the collection of employees
for (String emp : employees) {
  System.out.println(emp);
}

Iterating through the Collection with Generics

LinkedHashSet<String> employees = new LinkedHashSet<String>();
    
Iterator<String> iter = employees.iterator();
while (iterator.hasNext()) {
  String e = iter.next();
  System.out.println(e);
}

Full Program Listing

package com.avaldes.tutorials;

import java.util.LinkedHashSet;
import java.util.Set;

public class LinkedHashSetExample {

  public static void main(String[] args) {
    LinkedHashSet<String> employees = new LinkedHashSet<String>();
    LinkedHashSet<Integer> scores = new LinkedHashSet<Integer>();
    
    // Populate the employee LinkedHashSet  -- Auto-Boxing 
    employees.add("John");
    employees.add("David");
    employees.add("James");
    employees.add("Danielle");
    employees.add("Jeff");
    employees.add("Chris");
    employees.add("Mary");
    employees.add("Tina");
    
    scores.add(12);
    scores.add(45);
    scores.add(23);
    scores.add(7);
    scores.add(87);
    scores.add(37);
    scores.add(29);
    scores.add(7);  // duplicate
    scores.add(18);
    scores.add(6);
    scores.add(2);
    scores.add(64);
    scores.add(7);  // duplicate
    
    System.out.println("Display All Employees -- prior to delete operation...");
    System.out.println(employees + " size=" + employees.size());

    // Let's remove employee Jeff & Mary 
    System.out.println("nRemoving Jeff from employees");
    employees.remove("Jeff");
    System.out.println("Removing Mary from employees");
    employees.remove("Mary");
    System.out.println(employees + " size=" + employees.size());
    
    // Is Danielle in this set? 
    System.out.println("nIs Danielle in this set? " + employees.contains("Danielle"));
    // Is  in this set? 
    System.out.println("Is Amaury in this set? " + employees.contains("Amaury"));
    
    System.out.println("nDisplay All Employees and Scores...");
    System.out.println(employees + " size=" + employees.size());
    System.out.println(scores + " size=" + scores.size());
    
    System.out.println("nDisplaying the Employees..");
    for (String emp : employees) {
      System.out.println(emp);
    }
  }
}

Output

Display All Employees -- prior to delete operation...
[John, David, James, Danielle, Jeff, Chris, Mary, Tina] size=8

Removing Jeff from employees
Removing Mary from employees
[John, David, James, Danielle, Chris, Tina] size=6

Is Danielle in this set? true
Is Amaury in this set? false

Display All Employees and Scores...
[John, David, James, Danielle, Chris, Tina] size=6
[12, 45, 23, 7, 87, 37, 29, 18, 6, 2, 64] size=11

Displaying the Employees..
John
David
James
Danielle
Chris
Tina