spring-boot-tutorial-3e.java

@RestController
@RequestMapping(produces = MediaType.APPLICATION_JSON_VALUE)
public class ProductController {

    private final List<Product> productDB = new ArrayList<>();

    @PostConstruct
    private void initDB() {
        productDB.add(new Product("B0001", "Android Development (Java)", 380));
        productDB.add(new Product("B0002", "Android Development (Kotlin)", 420));
        productDB.add(new Product("B0003", "Data Structure (Java)", 250));
        productDB.add(new Product("B0004", "Finance Management", 450));
        productDB.add(new Product("B0005", "Human Resource Management", 330));
    }

    @GetMapping("/products/{id}")
    public ResponseEntity<Product> getProduct(@PathVariable("id") String id) {
        Optional<Product> productOp = productDB.stream()
                .filter(p -> p.getId().equals(id))
                .findFirst();

        if (!productOp.isPresent()) {
            return ResponseEntity.notFound().build();
        }

        Product product = productOp.get();
        return ResponseEntity.ok().body(product);
    }

}