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.
Come up with a real world example in which collections are used! Write your answer below:
Like a DMV where people wait in line and some ppl have priority because htey have appointments.
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:
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);
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.
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);
A Nike shoe would cost 80.97
You can use a collection object: Arrays
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);
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);
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
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);
List of Oscar winners [Nomadland, Parasite, Green Book, The Shape of Water]
values() – returns set of values, without any keys
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);
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
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);
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!
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);
[Red, Blue, Green]
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);
true
true
false
[Blue, Green]
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);
[Apple, Orange, Banana]
[Apple, Orange]
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);
true
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);
[Apple, Orange, Banana]
Length of Hashmap: 3
Array Version of Hashmap: [Ljava.lang.Object;@665e264a
Hashmap After Clearing []
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.// 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);
[Apple, Orange, Banana]
Apple
Orange
Banana
// 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);
[Brie, Cheddar, Gouda]
I love Brie
I like Cheddar
I like Gouda
// 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);
[Sorry, Monopoly, Scrabble]
Sorry
Monopoly
Scrabble
equals() method. This method will return true if the hashsets contain the same values and false if they do not.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);
Are the sets equal? true
// 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);
LinkedHashSet:
Pikachu
Charmander
Pikaman
New hashset: [Charmander, Pikaman]
// 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);
Alberquerque, New Mexico
Charmander
Pikachu
Pikaman
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.
(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.
@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.
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
(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.
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;
// ...
public interface PersonJpaRepository extends JpaRepository<Person, Long> // ...
List all of the JPA methods that you see in the PersonJpaRepository.java file and what their purpose is.
findByEmail: finds the person by email findAllByOrderByNameAsc: finds the names in ascending order findByNameContainingIgnoreCaseOrEmailContainingIgnoreCase: find the name or email if it contains a string without caring about case findByEmailAndPassword: finding the person by email and password findByLikeTermNative: find by like term native
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:
@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:
@Query indicates a custom query is being createdvalue 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.
(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.
@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.
| 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.
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.)
You can connect with more than one role, i think thats what it means.
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.
@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.
Where do you see “tutorial_id” on the table? What does it represent in relation to the leftmost “id” column?
on the right side of the table. it represents the same number as the id column.
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.
@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.