Generics

<?xml version="1.0" encoding="UTF-8"?>
        <project xmlns="http://maven.apache.org/POM/4.0.0"
                 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
            <modelVersion>4.0.0</modelVersion>
            <parent>
                <groupId>at.aydin.academy</groupId>
                <artifactId>13generics-pair</artifactId>
                <version>1.0.0</version>
                <relativePath>../pom.xml</relativePath>
            </parent>
            <artifactId>generics-modern</artifactId>
            <packaging>jar</packaging>
            <name>Generics Modern</name>

            <dependencies>

<dependency>
    <groupId>org.junit.jupiter</groupId>
    <artifactId>junit-jupiter</artifactId>
    <scope>test</scope>
</dependency>

            </dependencies>
            <build>
                <plugins>
                    <plugin>
                        <groupId>org.apache.maven.plugins</groupId>
                        <artifactId>maven-compiler-plugin</artifactId>
                    </plugin>

                </plugins>
            </build>
        </project>

package at.aydin.lab.generics.modern;

// Design Pattern: Generic Container
// Zweck: Der gespeicherte Typ wird zur Compile-Zeit festgelegt; Laufzeit-Casts entfallen.
public final class TypedBox<T> {
    private T value;
    public void set(T value) {
        this.value = value;
    }

    public T get() {
        return value;
    }
}

package at.aydin.lab.generics.modern;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class TypedBoxTest {
    @Test
    void preservesTheDeclaredType() {
        TypedBox<String> box = new TypedBox<>();
        box.set("text");
        assertEquals("text", box.get());
    }
}

package at.aydin.lab.generics.modern;

public interface Identifiable<ID> {
    ID id();
}

package at.aydin.lab.generics.modern;

import java.util.*;

// Design Pattern: Generic Repository
// Zweck: Wiederverwendbarer, typsicherer Datenzugriffsvertrag für unterschiedliche Entitäten.
public interface Repository<ID, T extends Identifiable<ID>> {
    T save(T entity);
    Optional<T> findById(ID id);
    List<T> findAll();
}

package at.aydin.lab.generics.modern;

import java.util.*;

public final class InMemoryRepository<ID, T extends Identifiable<ID>> implements Repository<ID, T> {
    private final Map<ID, T> data = new LinkedHashMap<>();
    public T save(T entity) {
        data.put(entity.id(), entity);
        return entity;
    }

    public Optional<T> findById(ID id) {
        return Optional.ofNullable(data.get(id));
    }

    public List<T> findAll() {
        return List.copyOf(data.values());
    }
}

package at.aydin.lab.generics.modern;

public record Customer(Long id, String name) implements Identifiable<Long> {
}

package at.aydin.lab.generics.modern;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class InMemoryRepositoryTest {
    @Test
    void staysTypeSafe() {
        Repository<Long, Customer> r = new InMemoryRepository<>();
        r.save(new Customer(1L, "Test"));
        assertEquals("Test", r.findById(1L).orElseThrow().name());
    }
}