Added application layer and blockchain implementation

src/main/java/com/rviewer/skeletons/application/adapter/CartController.java

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+package com.rviewer.skeletons.application.adapter;
+
+import com.rviewer.skeletons.application.exception.CartAlreadyExistsException;
+import com.rviewer.skeletons.application.exception.CartNotFoundException;
+import com.rviewer.skeletons.domain.model.Cart;
+import com.rviewer.skeletons.domain.model.Item;
+import com.rviewer.skeletons.domain.service.CartService;
+import com.rviewer.skeletons.application.request.CreateCartReq;
+import com.rviewer.skeletons.application.request.UpdateCartReq;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.DeleteMapping;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PatchMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import javax.validation.Valid;
+import java.util.UUID;
+
+@RestController
+@RequestMapping("/carts")
+public class CartController {
+
+    private final CartService cartService;
+
+    public CartController(CartService cartService) {
+        this.cartService = cartService;
+    }
+
+    @GetMapping("/{id}")
+    public ResponseEntity<Cart> getCart(@PathVariable String id) {
+        return ResponseEntity.status(HttpStatus.OK).body(cartService.get(id).orElseThrow(CartNotFoundException::new));
+    }
+
+    @PostMapping("/{id}")
+    public ResponseEntity<HttpStatus> saveCart(@PathVariable String id, @Valid @RequestBody CreateCartReq createCartReq) {
+        if (cartService.get(id).isPresent()) {
+            throw new CartAlreadyExistsException();
+        } else {
+            cartService.save(new Cart(UUID.fromString(id), createCartReq.getItems()
+                    .stream()
+                    .map(item -> new Item(item.getId(), item.getName(), item.getQuantity(), item.getPrice()))
+                    .toList()));
+            return ResponseEntity.status(HttpStatus.CREATED).build();
+        }
+    }
+
+    @PatchMapping("/{id}")
+    public ResponseEntity<HttpStatus> updateCart(@PathVariable String id, @Valid @RequestBody UpdateCartReq updateCartReq) {
+        Cart cart = cartService.get(id).orElseThrow(CartNotFoundException::new);
+        cart.setItems(updateCartReq.getItems());
+        cartService.save(cart);
+        return ResponseEntity.status(HttpStatus.OK).build();
+    }
+
+    @DeleteMapping("/{id}")
+    public ResponseEntity<HttpStatus> deleteCart(@PathVariable String id) {
+        if (cartService.get(id).isEmpty()) {
+            throw new CartNotFoundException();
+        } else {
+            cartService.delete(id);
+            return ResponseEntity.status(HttpStatus.OK).build();
+        }
+    }
+
+}

src/main/java/com/rviewer/skeletons/application/exception/CartAlreadyExistsException.java

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+package com.rviewer.skeletons.application.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(HttpStatus.CONFLICT)
+public class CartAlreadyExistsException extends RuntimeException {
+    private static final String CART_ALREADY_EXISTS = "Invalid identifier.";
+
+    public CartAlreadyExistsException() {
+        super(CART_ALREADY_EXISTS);
+    }
+}

src/main/java/com/rviewer/skeletons/application/exception/CartNotFoundException.java

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+package com.rviewer.skeletons.application.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(HttpStatus.NOT_FOUND)
+public class CartNotFoundException extends RuntimeException {
+    private static final String CART_NOT_FOUND = "Cart not found for the given ID";
+
+    public CartNotFoundException() {
+        super(CART_NOT_FOUND);
+    }
+}

src/main/java/com/rviewer/skeletons/application/exception/ControllerExceptionHandler.java

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+package com.rviewer.skeletons.application.exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.http.converter.HttpMessageNotReadableException;
+import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.annotation.ControllerAdvice;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ControllerAdvice
+public class ControllerExceptionHandler {
+
+    public static final String INVALID_BODY = "Invalid body provided, check the payload.";
+
+    @ExceptionHandler(CartNotFoundException.class)
+    public ResponseEntity<Object> handleCartNotFound(CartNotFoundException e) {
+        return new ResponseEntity<>(e.getMessage(), HttpStatus.NOT_FOUND);
+    }
+
+    @ExceptionHandler(CartAlreadyExistsException.class)
+    public ResponseEntity<Object> handleCartAlreadyExists(CartAlreadyExistsException e) {
+        return new ResponseEntity<>(e.getMessage(), HttpStatus.CONFLICT);
+    }
+
+    @ExceptionHandler({MethodArgumentNotValidException.class, HttpMessageNotReadableException.class})
+    @ResponseStatus(HttpStatus.BAD_REQUEST)
+    public ResponseEntity<Object> handleInvalidBody() {
+        return new ResponseEntity<>(INVALID_BODY, HttpStatus.BAD_REQUEST);
+    }
+
+}

src/main/java/com/rviewer/skeletons/application/request/CreateCartReq.java

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+package com.rviewer.skeletons.application.request;
+
+import com.rviewer.skeletons.domain.model.Item;
+import lombok.Getter;
+
+import javax.validation.constraints.NotNull;
+import java.util.List;
+
+@Getter
+public class CreateCartReq {
+    @NotNull
+    private List<Item> items;
+}

src/main/java/com/rviewer/skeletons/application/request/UpdateCartReq.java

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+package com.rviewer.skeletons.application.request;
+
+import com.rviewer.skeletons.domain.model.Item;
+import lombok.Getter;
+
+import javax.validation.constraints.NotNull;
+import java.util.List;
+
+@Getter
+public class UpdateCartReq {
+    @NotNull
+    private List<Item> items;
+}