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# (DD02) Business Logic — ProductService.findInStock() [5 LOC]

| Field | Value |
|-------|-------|
| Fully Qualified Name | `com.example.service.ProductService` |
| Layer | Service |
| Module | `service` (Package: `com.example.service`) |

## 1. Role

### ProductService.findInStock()

This method provides the service-layer entry point for retrieving only the products that are currently available for sale or consumption. It reads the full product collection from the service’s in-memory store, applies an availability filter, and returns a new list containing only in-stock items. In business terms, it acts as a stock-availability selector rather than a data mutator, ensuring that callers receive only products that can be presented to users as purchasable or selectable.

The implementation follows a simple filtering/delegation pattern: the service delegates the availability decision to `Product.isInStock()` and aggregates the qualifying entities into a list. There are no conditional branches, loops, or fallback paths in this method; the business rule is singular and deterministic. In the larger system, this method is used as a reusable query operation by presentation-layer code, such as controllers, to populate screens with currently available products.

## 2. Processing Pattern (Detailed Business Logic)

```mermaid
flowchart TD
START(["findInStock()"])
READ_PRODUCTS["Read products.values() from in-memory store"]
STREAM["Create stream from product collection"]
FILTER["Filter by Product.isInStock()"]
COLLECT["Collect matching products into List<Product>"]
END_NODE(["Return List<Product>"])
START --> READ_PRODUCTS
READ_PRODUCTS --> STREAM
STREAM --> FILTER
FILTER --> COLLECT
COLLECT --> END_NODE
```

## 3. Parameter Analysis

| No | Parameter Name | Type | Business Description |
|----|---------------|------|---------------------|
| - | (none) | - | This method accepts no input parameters. It operates entirely on the service’s internal product collection and returns the subset of products that are currently in stock. |

**Instance fields / external state read by the method:** `products` (the in-memory product store managed by `ProductService`).

## 4. CRUD Operations / Called Services

| CRUD | SC / CBS | SC Code | Entity / DB | Operation Description |
|------|----------|---------|-------------|----------------------|
| R | `findInStock()` | N/A | `products` in-memory collection | Reads all products from the service store and filters them down to the subset that is currently in stock. |
| R | `Product.isInStock()` | N/A | `Product` entity state | Evaluates each product’s availability flag/state to determine whether it should be included in the result list. |

## 5. Dependency Trace

| # | Caller (Screen/Batch) | Call Chain (Full Path to this Method) | Terminal (SC / CRUD / Entity) |
|---|----------------------|--------------------------------------|-------------------------------|
| 1 | Screen: `ProductController` | `ProductController.listAvailable()` -> `ProductService.findInStock()` | `findInStock() [R] products` |

## 6. Per-Branch Detail Blocks

**Block 1** — [SEQUENCE] (L19-L23)

> Retrieves the current product set, filters for stock availability, and returns the resulting list.

| # | Type | Code |
|---|------|------|
| 1 | EXEC | `products.values()` // obtain all stored product records |
| 2 | EXEC | `products.values().stream()` // convert the product collection into a stream |
| 3 | EXEC | `Product::isInStock` // evaluate each product’s in-stock status |
| 4 | EXEC | `filter(Product::isInStock)` // keep only products that satisfy the stock condition |
| 5 | EXEC | `collect(Collectors.toList())` // aggregate filtered products into a `List<Product>` |
| 6 | RETURN | `return products.values().stream().filter(Product::isInStock).collect(Collectors.toList());` |

## 7. Glossary

| Term | Type | Business Meaning |
|------|------|------------------|
| `Product` | Entity | Business item being managed by the service, representing a product that may be available or unavailable. |
| `products` | Field | In-memory product store used by `ProductService` as the source collection for lookups. |
| `findInStock` | Method | Service query that returns only products currently marked as in stock. |
| `in stock` | Business term | Availability state indicating that a product can be shown to users as selectable or purchasable. |
| `isInStock` | Method | Product-level availability check used to determine whether a product should be included in the result set. |
| `stream` | Technical term | Java Stream API pipeline used to process the product collection functionally. |
| `collect(Collectors.toList())` | Technical term | Terminal stream operation that materializes filtered results into a list. |
| `ProductController` | Controller | Presentation-layer component that requests the in-stock product list for display or response generation. |
| `listAvailable` | Method | Controller action that exposes available products to callers. |
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