HashSet LinkedHashSet

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layout: default title: Set Implementation Classes - HashSet and LinkedHashSet

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To create a Set, we need an implementation class.

HashSet is a general-purpose implementation of the Set interface. * A HashSet contains unique objects whose order is not guaranteed. * The hashCode() and equals() methods work in combination to ensure that only unique objects are in the set.

The class below represents an Album, with equals and hashCode methods.

package com.example.setinterface.examples;

public class Album {
  private int id;
  private String artist;
  private String title;

  public Album(int i, String a, String t) {
      id = i;
      artist = a;
      title = t;
  }

  @Override
  public String toString() {
      return "Album [id=" + id + ", artist=" + artist + ", title="
              + title + "]";
  }

  @Override
  public int hashCode() {
      final int prime = 31;
      int result = 1;
      result = prime * result
              + ((artist == null) ? 0 : artist.hashCode());
      result = prime * result + id;
      result = prime * result
              + ((title == null) ? 0 : title.hashCode());
      return result;
  }

  @Override
  public boolean equals(Object obj) {
      if (this == obj)
          return true;
      if (obj == null)
          return false;
      if (getClass() != obj.getClass())
          return false;
      Album other = (Album) obj;
      if (artist == null) {
          if (other.artist != null)
              return false;
      }
      else if (!artist.equals(other.artist))
          return false;
      if (id != other.id)
          return false;
      if (title == null) {
          if (other.title != null)
              return false;
      }
      else if (!title.equals(other.title))
          return false;
      return true;
  }
}

Duplicate Albums are created, and we attempt to store them in a HashSet.

package com.example.setinterface.examples;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class SetTest {

  public static void main(String[] args) {
    SetTest st = new SetTest();
    st.launch();
  }

  public void launch() {
    Album al1 = new Album(1, "The Beatles", "The Beatles 1");
    Album al2 = new Album(2, "Prince", "The Very Best of Prince");
    Album al3 = new Album(3, "AC/DC", "Let There Be Rock");
    Album al4 = new Album(3, "AC/DC", "Let There Be Rock");

    Set<Album> albumCollection = new HashSet<>();

    // Add all Albums
    albumCollection.add(al1);
    albumCollection.add(al2);
    albumCollection.add(al3);
    albumCollection.add(al4);  // duplicate Album

    printAlbums(albumCollection);
  }

  private void printAlbums(Set<Album> collection) {
    Iterator<Album> it = collection.iterator();  // Get Iterator object
    while (it.hasNext()) {
      System.out.println(it.next());
    }
  }
}

Drill

SetInterface/com.example.setinterface.drills.SetTest

• Place each call to add in a System.out.println() and run the application.

LinkedHashSet

LinkedHashSet extends HashSet. * LinkedHashSet guarantees that the order of iteration will be insertion-order. * HashSet makes no guarantee about order.

Drill

SetInterface/com.example.setinterface.drills.SportsTeamApp

• Change addTeam() to add the object to the Set.
• Iterate through the contents of the Set.
• If the results are not as expected, fix SportsTeam.

(Solution: SportsTeamApp.java) * Change SportsTeamApp so that the Set is a LinkedHashSet, and run the program again.

(Solution: SportsTeamApp2.java)

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