Collections

<?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>12collections-pair</artifactId>
                <version>1.0.0</version>
                <relativePath>../pom.xml</relativePath>
            </parent>
            <artifactId>collections-modern</artifactId>
            <packaging>jar</packaging>
            <name>Collections 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.collections.modern;

import java.util.*;

// Design Pattern: Snapshot
// Zweck: Gibt unveränderliche Momentaufnahmen statt interner mutabler Collections zurück.
public final class Inventory {
    private final LinkedHashMap<String, Integer> stock = new LinkedHashMap<>();
    public void add(String sku, int quantity) {
        Objects.requireNonNull(sku);
        if (quantity <= 0) throw new IllegalArgumentException("quantity");
        stock.merge(sku, quantity, Integer::sum);
    }

    public int quantity(String sku) {
        return stock.getOrDefault(sku, 0);
    }

    public Map<String, Integer> snapshot() {
        return Collections.unmodifiableMap(new LinkedHashMap<>(stock));
    }

    public List<String> orderedSkus() {
        return List.copyOf(stock.sequencedKeySet());
    }
}

package at.aydin.lab.collections.modern;

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

class InventoryTest {
    @Test
    void exposesImmutableSnapshot() {
        var i = new Inventory();
        i.add("A", 2);
        assertEquals(2, i.snapshot().get("A"));
        assertThrows(UnsupportedOperationException.class, () -> i.snapshot().put("B", 1));
    }
}