Added streams.

streams/.project

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<projectDescription>
+	<name>streams</name>
+	<comment></comment>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
+	<natures>
+		<nature>org.eclipse.jdt.core.javanature</nature>
+	</natures>
+	<filteredResources>
+		<filter>
+			<id>1742997445640</id>
+			<name></name>
+			<type>30</type>
+			<matcher>
+				<id>org.eclipse.core.resources.regexFilterMatcher</id>
+				<arguments>node_modules|\.git|__CREATED_BY_JAVA_LANGUAGE_SERVER__</arguments>
+			</matcher>
+		</filter>
+	</filteredResources>
+</projectDescription>

streams/.settings/org.eclipse.core.resources.prefs

@@ -0,0 +1,2 @@
+eclipse.preferences.version=1
+encoding/<project>=UTF-8

streams/.settings/org.eclipse.jdt.core.prefs

@@ -0,0 +1,11 @@
+eclipse.preferences.version=1
+org.eclipse.jdt.core.compiler.codegen.targetPlatform=21
+org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve
+org.eclipse.jdt.core.compiler.compliance=21
+org.eclipse.jdt.core.compiler.debug.lineNumber=generate
+org.eclipse.jdt.core.compiler.debug.localVariable=generate
+org.eclipse.jdt.core.compiler.debug.sourceFile=generate
+org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=disabled
+org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=warning
+org.eclipse.jdt.core.compiler.release=enabled
+org.eclipse.jdt.core.compiler.source=21

streams/src/streams/Main.java

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+package streams;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+import tester.Tester;
+
+class Customer {
+    private final int id;
+    private final String name;
+
+    public Customer(int id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    public int getId() { return this.id; }
+    public String getName() { return this.name; }
+}
+
+class Inventory {
+    private final int id;
+    private final String description;
+    private final int costPerUnit;
+
+    public Inventory(int id, String description, int costPerUnit) {
+        this.id = id;
+        this.description = description;
+        this.costPerUnit = costPerUnit;
+    }
+
+    public int getId() { return this.id; }
+    public String getDescription() { return this.description; }
+    public int getCostPerUnit() { return this.costPerUnit; }
+}
+
+class Purchase {
+    private final int id;
+    private final List<LineItem> lineItems;
+    private final int customerIdOfPurchaser;
+    private final int numberOfDaysAgo;
+
+    public Purchase(
+        int id, List<LineItem> lineItems, int customerIdOfPurchaser,
+        int numberOfDaysAgo
+    ) {
+        this.id = id;
+        this.lineItems = new ArrayList<>(lineItems);
+        this.customerIdOfPurchaser = customerIdOfPurchaser;
+        this.numberOfDaysAgo = numberOfDaysAgo;
+    }
+
+    public int getId() { return this.id; }
+    public List<LineItem> getLineItems() { return this.lineItems; }
+    public int getCustomerIdOfPurchaser() { return this.customerIdOfPurchaser; }
+    public int getNumberOfDaysAgo() { return this.numberOfDaysAgo; }
+}
+
+class LineItem {
+    private final int inventoryId;
+    private final int numberOfItemsPurchased;
+
+    public LineItem(int inventoryId, int numberOfItemsPurchased) {
+        this.inventoryId = inventoryId;
+        this.numberOfItemsPurchased = numberOfItemsPurchased;
+    }
+
+    public int getInventoryId() { return this.inventoryId; }
+    public int getNumberOfItemsPurchased() {
+        return this.numberOfItemsPurchased;
+    }
+}
+
+interface CustomerDB {
+    Optional<Customer> getCustomerById(int id);
+    List<Customer> getAllCustomers();
+}
+
+interface InventoryDB {
+    Optional<Inventory> getInventoryById(int id);
+    List<Inventory> getAllInventory();
+}
+
+interface PurchaseDB {
+    Optional<Purchase> getPurchaseById(int id);
+    // get purchases made numberOfDaysAgo, numberOfDaysAgo - 1... today
+    List<Purchase> getPurchasesSince(int numberOfDaysAgo);
+    List<Purchase> getPurchasesFor(int customerId);
+    List<Purchase> getPurchasesFor(List<Integer> customerIds);
+    List<Purchase> getPurchasesForSince(int customerId, int numberOfDaysAgo);
+    List<Purchase>
+    getPurchasesForSince(List<Integer> customerIds, int numberOfDaysAgo);
+}
+
+class Examples {}