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HashMaps, HashSets, Collections, SQL Student Lessons

Posted Dec 13, 2023
By Aliya W. Tang
18 min read

Collections

What are collections?

Collections are any group of individual objects that are represented as a single unit is known as a Java Collection of Objects. In Java, a separate framework named the “Collection Framework” has been defined in JDK 1.2 which holds all the Java Collection Classes and Interface in it. It works as an interface that defines the highest-level of shared collection behavior, and extends Iterable which allows usage of ForEach loop.

Types of Collections

  • List:
    • Ordered collection
    • Allows duplicate elements
    • Example: ArrayList and LinkedList
  • Set:
    • Unordered collection
    • No duplicates
    • Example: HashSet and TreeSet
  • Map:
    • Key-pair values
    • Each key must be unique
    • Example: HashMap and TreeMap
  • Queue:
    • Follows First-In-First-Out order
    • Example: LinkedList and PriorityQueue
  • Deque:
    • Double ended queue
    • Insertion and removal at both ends
    • Example: ArrayDeque

Popcorn Hack!

Come up with a real world example in which collections are used! Write your answer below:

Students standing line to leave a classroom, the first people in the line get to leave first.

HashMaps

What are they?

Hash Maps are a data structure with keys and values; very similar to a Python dictionary. The keys in Hash Maps have a single value assigned to them, which can be accessed by calling the key:

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import java.util.HashMap;  //Hashmaps are part of the java.util package

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

        HashMap<String, Double> hashMap = new HashMap<>();

        // key-value pairs, where the key is the planet (a string) and the value is the double assigned to each key
        hashMap.put("Mercury", 0.39);
        hashMap.put("Venus", 0.72); //adding an element
        hashMap.put("Earth", 1.00);
        hashMap.remove("Earth", 1.00); //removing an element

        // the value 0.39 can be accessed by calling the key "Mercury"
        double value = hashMap.get("Mercury");
        boolean exists = hashMap.containsKey("Mercury"); //you can also check if a key exists
        System.out.println("Mercury is " + value + " astronomical units away from the Sun");
    }
}

PlanetDistances.main(null);

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Mercury is 0.39 astronomical units away from the Sun

As shown above, the data type of the keys and values must be defined when creating the Hashmap. You cannot use data types for keys or values that are different from the ones assigned to it. Also, keys must be non-null objects. However, values can be null!


Keys in a Hashmap must be unique. Otherwise the previous values of the key get overwritten.

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import java.util.HashMap;  

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

        HashMap<String, Double> hashMap = new HashMap<>();

        hashMap.put("Nike", 41.97);
        hashMap.put("Nike", 80.97); //this value replaces the previous value 
        hashMap.put("Adidas", 69.99);
        hashMap.put("Vans", 55.00);

        double value = hashMap.get("Nike");
        System.out.println("A Nike shoe would cost " + value);
    }
}

ShoePrices.main(null);

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A Nike shoe would cost 80.97

Popcorn hack: what can we do if we want to assign multiple values to a single key? Do it below!:

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import java.util.HashMap;
import java.util.ArrayList;

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

        HashMap<String, Double> hashMap = new HashMap<>();
        HashMap<String, ArrayList<Double>> hashMap2 = new HashMap<>();

        ArrayList<Double> NikeVals = new ArrayList<>();
        NikeVals.add(80.97);
        NikeVals.add(40.13);
        
        hashMap2.put("Nike", NikeVals);
        hashMap.put("Adidas", 69.99);
        hashMap.put("Vans", 55.00);

        ArrayList<Double> GotNikeVals = hashMap2.get("Nike");
        for (int i=0; i<GotNikeVals.size(); i++){
            System.out.println(GotNikeVals.get(i));
        }
    }
}

ShoePrices.main(null);

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80.97
40.13

Note that collisions can still occur when two keys produce the same hash code. Hash codes are assigned to each key through a hash function (this is called hashing). They are used to determine what the key-value pair’s index should be iwthin the Hashmaps. A bad hash function may cause collisions to occur, in which case the function would need to be adjusted.

HashMaps do not store the input order of the key-value pairs, and there is no guarantee the pairs will always be stored in the same order. Therefore if something needs to be found within a HashMap, iteration must be used. This can be done through keySet(), values(), or entrySet()


keySet() – provides access to the set of keys. Useful for if you only need to access keys w/out values

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import java.util.HashMap;
import java.util.Set;

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

        HashMap<String, Integer> oscarWinnersYear = new HashMap<>();

        oscarWinnersYear.put("Nomadland", 2020);
        oscarWinnersYear.put("Parasite", 2019);
        oscarWinnersYear.put("Green Book", 2018);
        oscarWinnersYear.put("The Shape of Water", 2017);

        Set<String> oscarWinners = oscarWinnersYear.keySet(); //using keySet to get keys 

        System.out.println("List of Oscar winners " + oscarWinners);
    }
}

OscarWinners.main(null);

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List of Oscar winners [Nomadland, Parasite, Green Book, The Shape of Water]

values() – returns set of values, without any keys

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import java.util.HashMap;
import java.util.Set;

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

        HashMap<String, Integer> oscarWinnersYear = new HashMap<>();

        oscarWinnersYear.put("Nomadland", 2020);
        oscarWinnersYear.put("Parasite", 2019);
        oscarWinnersYear.put("Green Book", 2018);
        oscarWinnersYear.put("The Shape of Water", 2017);

        Collection<Integer> oscarWinnerYears = oscarWinnersYear.values(); //using values to get values 

        System.out.println("Years movies won Oscars " + oscarWinnerYears);
    }
}

OscarWinners.main(null);

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Years movies won Oscars [2020, 2019, 2018, 2017]

entryset() – returns keys and values in the form of objects. Useful for when working with key-value pair relationships

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import java.util.HashMap;
import java.util.Set;

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

        HashMap<String, Integer> oscarWinnersYear = new HashMap<>();

        oscarWinnersYear.put("Nomadland", 2020);
        oscarWinnersYear.put("Parasite", 2019);
        oscarWinnersYear.put("Green Book", 2018);
        oscarWinnersYear.put("The Shape of Water", 2017);

        Set<Map.Entry<String, Integer>> oscarWinnersAndYears = oscarWinnersYear.entrySet(); //using values to get values 

        System.out.println("Oscars and their years " + oscarWinnersAndYears);
    }
}

OscarWinners.main(null);

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Oscars and their years [Nomadland=2020, Parasite=2019, Green Book=2018, The Shape of Water=2017]

HashMaps have a time complexity of O(1) on average!

HashSet

What is a hashset?

  • A hashset is a data structure that stores only unique values and does not allow duplicates. You can also think of it as a set of keys with no values.
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import java.util.HashSet;
import java.util.Set;

public class Colors{
    static Set<String> colors = new HashSet<>();

    public static void main(String[] args) {
        colors.add("Red");
        colors.add("Blue");
        colors.add("Green");
        System.out.println(colors);
    }
}

Colors.main(null);

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[Red, Blue, Green]

What if we add duplicates?

  • If we try add duplicates, the hashset will simply not add the duplicate value and return false as seen bellow.
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public class ColorsButDifferent{
    static Set<String> colors = new HashSet<>();

    public static void main(String[] args) {
        System.out.println(colors.add("Blue"));
        System.out.println(colors.add("Green"));
        System.out.println(colors.add("Green"));
        System.out.println(colors);
    }
}

ColorsButDifferent.main(null);

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true
true
false
[Blue, Green]

Order?

  • Hashsets are not ordered and do not store the order in which the values were added that means that if values are removed their order will not be preserved.
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public class Fruits{
    static Set<String> fruits = new HashSet<>();

    public static void main(String[] args) {
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        System.out.println(fruits);

        fruits.remove("Banana");

        System.out.println(fruits);
    }
}

Fruits.main(null);

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[Apple, Orange, Banana]
[Apple, Orange]

How do you know if a value is in a hashset?

  • You can use the contains method to check if a value is in a hashset.
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public class Fruits{
    static Set<String> fruits = new HashSet<>();

    public static void main(String[] args) {
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");

        System.out.println(fruits.contains("Apple"));
    }
}

Fruits.main(null);

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true

Other methods

  • clear() - removes all values from the hashset
  • isEmpty() - returns true if the hashset is empty
  • size() - returns the number of values in the hashset
  • toArray() - returns an array of the values in the hashset
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public class Fruits{
    static Set<String> fruits = new HashSet<>();

    public static void main(String[] args) {
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        System.out.println(fruits);

        System.out.println();
        System.out.println("Length of Hashmap: " + fruits.size());
        
        System.out.println();
        System.out.println("Array Version of Hashmap: " + fruits.toArray());

        System.out.println();
        fruits.clear();
        System.out.println("Hashmap After Clearing " + fruits);

    }
}

Fruits.main(null);

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[Apple, Orange, Banana]

Length of Hashmap: 3

Array Version of Hashmap: [Ljava.lang.Object;@665e264a

Hashmap After Clearing []

Iterations

  • You can iterate through a hashset using a for each loop or an iterator. The order of the values is not guaranteed and can change as values are removed or added.
  • These two methods are can be called using the iterator() and forEach() methods. For the forEach() method you can pass in a lambda expression (a lambda expression is an anonymous function) or a method reference.
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// foreach loop to iterate through a set

public class Fruits{
    static Set<String> fruits = new HashSet<>();

    public static void main(String[] args) {
        fruits.add("Apple");
        fruits.add("Banana");
        fruits.add("Orange");
        System.out.println(fruits);

        // Using foreach loop to iterate through the set and print each fruit as it goes
        fruits.forEach(fruit -> System.out.println(fruit));
    }
}

Fruits.main(null);


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[Apple, Orange, Banana]
Apple
Orange
Banana

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// Different version of foreach loop to iterate through a set

public class Cheeses{
    static Set<String> cheeses = new HashSet<>();

    public static void main(String[] args) {
        cheeses.add("Cheddar");
        cheeses.add("Brie");
        cheeses.add("Gouda");
        System.out.println(cheeses);

        cheeses.forEach(cheese -> {
            if(cheese.equals("Brie")) {
                System.out.println("I love " + cheese);
            } else {
                System.out.println("I like " + cheese);
            }
        });
    }
}

Cheeses.main(null);

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[Brie, Cheddar, Gouda]
I love Brie
I like Cheddar
I like Gouda

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// using iterator to iterate through a set

import java.util.Iterator;

public class Games{
    static Set<String> games = new HashSet<>();

    public static void main(String[] args) {
        games.add("Monopoly");
        games.add("Scrabble");
        games.add("Sorry");
        System.out.println(games);
        System.out.println();

        Iterator<String> iterator = games.iterator();
        while(iterator.hasNext()) {
            System.out.println(iterator.next());
        }
    }
}

Games.main(null);

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[Sorry, Monopoly, Scrabble]

Sorry
Monopoly
Scrabble

How to check for equality?

  • You can check if two hashsets are equal by using the equals() method. This method will return true if the hashsets contain the same values and false if they do not.
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public class Pokemon {
    public static void main(String[] args) {
        Set<String> pokemonSet1 = new HashSet<>();
        Set<String> pokemonSet2 = new HashSet<>();

        // Populate the sets
        pokemonSet1.add("Pikachu");
        pokemonSet1.add("Charmander");
        pokemonSet1.add("Pikaman");

        pokemonSet2.add("Pikachu");
        pokemonSet2.add("Charmander");
        pokemonSet2.add("Pikaman");

        // Check equality
        boolean areEqual = pokemonSet1.equals(pokemonSet2);
        System.out.println("Are the sets equal? " + areEqual);
    }
}

Pokemon.main(null);


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Are the sets equal? true

Other kinds of hashsets

  • There are other kinds of hashsets that are similar to the hashset but have some differences. These include the linkedhashset and the treeset.
  • The linkedhashset is similar to the hashset but it maintains the order in which the values were added.
  • The treeset is similar to the hashset but it sorts the values in ascending order.
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// LinkedHashSet

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

public class LinkedHashSetExample {
    public static void main(String[] args) {
        Set<String> linkedHashSet = new LinkedHashSet<>();

        // Add elements
        linkedHashSet.add("Pikachu");
        linkedHashSet.add("Charmander");
        linkedHashSet.add("Pikaman");

        // Iterate through the LinkedHashSet
        System.out.println("LinkedHashSet: ");
        linkedHashSet.forEach(pokemon -> System.out.println(pokemon));
        System.out.println();

        linkedHashSet.remove("Pikachu");
        System.out.println("New hashset: " + linkedHashSet);
    }
}

LinkedHashSetExample.main(null);

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LinkedHashSet: 
Pikachu
Charmander
Pikaman

New hashset: [Charmander, Pikaman]

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// TreeSet
import java.util.Set;
import java.util.TreeSet;

public class TreeSetExample {
    public static void main(String[] args) {
        Set<String> treeSet = new TreeSet<>();

        // Add elements
        treeSet.add("Alberquerque, New Mexico");
        treeSet.add("Pikachu");
        treeSet.add("Charmander");
        treeSet.add("Pikaman");

        // Iterate through the TreeSet
        treeSet.forEach(pokemon -> System.out.println(pokemon));
        // The values for the TreeSet are sorted alphabetically
    }
}

TreeSetExample.main(null);

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Alberquerque, New Mexico
Charmander
Pikachu
Pikaman

SQL

SQL is a programming language focused on managing and manipulating relational (table-based) databases. It acts as the backbone for many of this class’s project backends, allowing users to create, read, update and delete data efficiently. To get a better idea of how data is formatted with JPA to be stored in an SQL database, we’ll be looking at the Person object in the lesson backend. Follow along in this notebook.

Person Object Setup

(see …/mvc/person/Person.java)

After the first three tags, which specify to Lombok that @AllArgsConstructor and @NoArgsConstructor methods should be created

The @Entity tag indicates that Person objects will be stored as entities in a database. Generally, entities’ attributes are represented by values in columns in the SQLite table.

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@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
@Convert(attributeName ="person", converter = JsonType.class)
public class Person //...

The @Id tag specifies to that this value will be used as the unique identifier for each object in the database, and @GeneratedValue(strategy = GenerationType.AUTO) allows these ID’s to be automatically generated when a new Person is created. In order for data to be most easily differentiated and manipulated in a database, IDs are deeply important to use.

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@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

Person JPA Repository Setup

(see …/mvc/person/PersonJpaRepository.java)

JPA is a great asset when creating an SQL database because it can provide methods that help you modify its contents. If a specific object has a JpaRepository interface made that extends the base JPA repository, it allows you to access many useful JPA methods.

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import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

// ...

public interface PersonJpaRepository extends JpaRepository<Person, Long> // ...

Popcorn Hack!

List all of the JPA methods that you see in the PersonJpaRepository.java file and what their purpose is.

findByEmail: find a person by their email findAllByOrderByNameAsc: retrieve every person in ascending order based on name findByNameContainingIgnoreCaseOrEmailContainingIgnoreCase: search for name substring in email, not case sensitive findByEmailAndPassword: find person by matching both email and password findByLikeTermNative: find person by seeing if name or email contains term

Custom JPA Queries

You can create custom JPA queries that utilize SQL to serve a specific purpose outside of the existing JPA methods. See the example below from PersonJpaRepository.java:

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@Query(
        value = "SELECT * FROM Person p WHERE p.name LIKE ?1 or p.email LIKE ?1",
        nativeQuery = true)
List<Person> findByLikeTermNative(String term);

Going step-by-step:

  • The @Query indicates a custom query is being created
  • value is a String containing an SQL query that will be run with the method findByLikeTermNative
    • ?1 is a placeholder for a parameter (in this case the String term from the method declaration)
  • nativeQuery is set to true, indicating that the value uses native SQL rather than JPQL (Java Persistence Query Language)

Using a custom SQL query like this would be a great way to show understanding on the homework.

Many-to-Many Relationship

(see …/mvc/person/Person.java)

A “Many-to-Many” relationship (shown in the @ManyToMany tag) indicates that entities on both sides of a relationship can have multiple connections with each other.

This condition allows for one object to be related to multiple different objects, and those different objects on the other side of the relationship can have their own relationship to other objects.

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@ManyToMany(fetch = EAGER)
private Collection<PersonRole> roles = new ArrayList<>();

(fetch = EAGER) specifies that, whenever a Person object is loaded, its corresponding PersonRole objects should be loaded simultaneously.

Objects in a “many-to-many” relationship often use “join tables” to represent the connections between these objects.

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| person_id | role_id |
|-----------|---------|
| 1         | 1       |
| 1         | 2       |
| 2         | 2       |
| 3         | 1       |
| 3         | 3       |

This is what a “join table” may look like with multiple interconnected people and roles. There is a table in the backend repository’s SQLite.db called person_roles that acts as a join table for the two roles, but you’ll notice that it’s empty. If you can show it filled up, that sounds like a good reason to give extra points.

Popcorn Hack!

Explain in your own words what the relationship between Person objects and PersonRole objects is. Why is this relevant to collections? (Hint: In the code above, multiple PersonRole objects are stored within a Person object’s roles attribute.)

The role is given to the person. One person can have multiple roles, but each role is associated with only one person. One-to-many relationship.

Many-to-One Relationship

On the other hand, there is also a “Many-to-One” relationship that can exist between two objects. It means that multiple instances of one entity (the “many” side) are associated with a single instance of another entity (the “one” side). This can be seen applied to the Note object in relation to the Person object.

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@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "tutorial_id", nullable = false) // notice this!
@OnDelete(action = OnDeleteAction.CASCADE)
@JsonIgnore
private Person person;

Notice the @JoinColumn(name = "tutorial_id", nullable = false) tag. Then, check the note table in the sqlite.db file.

Popcorn Hack!

Where do you see “tutorial_id” on the table? What does it represent in relation to the leftmost “id” column?

I see tutorial_id on the very right of the table. id is the primary key and tutorial_id references primary key of another table. Many-to-one relationship.

Storing HashMaps with jsonb

JSONB is a binary representation of JSON data. It is a data type used in some relational databases (such as SQL databases) to store JSON documents in a more efficient and flexible way compared to traditional JSON. Within SQL databases, this data from the JSON can often be filtered, searched and/or extracted using provided functions.

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@JdbcTypeCode(SqlTypes.JSON)
@Column(columnDefinition = "jsonb")
private Map<String,Map<String, Object>> stats = new HashMap<>();

JSONB allows for the storage of nested and dynamic data structures. In the context of Person object stats, it means that each person can store activity data with the formatted date as the key, and the structure of this JSON data can be modified or extended without altering the database schema.

You can view the way this data is structured in the person sqlite.db table.

Homework

  • Complete all popcorn hacks throughout this lesson
  • Using the Person object as inspiration, create your own UNIQUE SQL database with at least 3 object entries that incorporates either a “many-to-many” relationship with another object (hint: Person and PersonRole) OR uses the JSONB column definition to store more complex data (such as a HashMap) as an attribute (hint: stats in Person object).
    • Show a clear screenshot of your SQLite table (using SQLite viewer) on your blog for credit.
    • Using past group project materials is valid for this homework as long as the expectations are met.

Main employee table hashmapimg1

Many to many employee table hashmapimg2

Ideas for 1.0/1.0

  • Incorporating both a “many-to-many” relationship and a JSONB column with information would be great.
  • Implementing JPA repository methods (think CRUD methods, custom queries, etc.) would show interest in modifying SQL database data.
  • Take extra notes on this lesson that show deeper research into Collections and SQL.
Java, Tri 2
Notebook Java Collections Lesson
This post is licensed under CC BY 4.0 by the author.
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