LinkedHashSet

1. What It Is

2. How to Use It

public class LinkedHashSetTest
{
    public static void main(String[] args)
    {
        LinkedHashSet<Integer> set = new LinkedHashSet<>();
        set.add(1);
        set.add(2);
        set.remove(2);
        System.out.println(set.contains(1));
        System.out.println(set.contains(2));
        System.out.println(set.contains(3));

    }
}

3. Source Analysis

3.1. Constructor

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
    // Implemented using LinkedHashMap
    map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

3.2. Fields

public class LinkedHashSet<E>
    extends HashSet<E>// Extends HashSet, so its fields are the same as HashSet
    implements Set<E>, Cloneable, java.io.Serializable


// HashSet
public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    private transient HashMap<E,Object> map;
// Placeholder used as the value
    private static final Object PRESENT = new Object();
}

3.3. Other Methods

Same as LinkedHashMap.md.

4. Summary

It is implemented using LinkedHashMap underneath, with a newly created Object used as the value placeholder.