Add Injector with Component

src/main/java/mate.academy/Main.java

@@ -6,12 +6,13 @@
 import mate.academy.service.ProductService;
 
 public class Main {
+    private static final String FILE_PATH = "src/main/java/resources/products.txt";
 
     public static void main(String[] args) {
         // Please test your Injector here. Feel free to push this class as a part of your solution
         Injector injector = Injector.getInjector();
-        ProductService productService = null;
-        List<Product> products = productService.getAllFromFile("products.txt");
+        ProductService productService = (ProductService) injector.getInstance(ProductService.class);
+        List<Product> products = productService.getAllFromFile(FILE_PATH);
         products.forEach(System.out::println);
     }
 }

src/main/java/mate.academy/lib/Component.java

@@ -1,5 +1,9 @@
 package mate.academy.lib;
 
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+@Retention(RetentionPolicy.RUNTIME)
 public @interface Component {
 
 }

src/main/java/mate.academy/lib/Inject.java

@@ -1,5 +1,8 @@
 package mate.academy.lib;
 
-public @interface Inject {
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
 
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Inject {
 }

src/main/java/mate.academy/lib/Injector.java

@@ -1,13 +1,74 @@
 package mate.academy.lib;
 
+import java.lang.reflect.Field;
+import java.util.HashMap;
+import java.util.Map;
+import mate.academy.service.FileReaderService;
+import mate.academy.service.ProductParser;
+import mate.academy.service.ProductService;
+import mate.academy.service.impl.FileReaderServiceImpl;
+import mate.academy.service.impl.ProductParserImpl;
+import mate.academy.service.impl.ProductServiceImpl;
+
 public class Injector {
     private static final Injector injector = new Injector();
+    private final Map<Class<?>, Class<?>> componentMap = Map.of(
+            ProductService.class, ProductServiceImpl.class,
+            ProductParser.class, ProductParserImpl.class,
+            FileReaderService.class, FileReaderServiceImpl.class);
+    private final Map<Class<?>, Object> instances = new HashMap<>();
 
     public static Injector getInjector() {
         return injector;
     }
 
     public Object getInstance(Class<?> interfaceClazz) {
-        return null;
+        if (instances.containsKey(interfaceClazz)) {
+            return instances.get(interfaceClazz);
+        }
+        validateImplementationPresent(interfaceClazz);
+        Class<?> implementationClass = componentMap.get(interfaceClazz);
+        validateComponentPresent(implementationClass);
+        Object instance = newInstance(implementationClass);
+        Field[] fields = implementationClass.getDeclaredFields();
+        for (Field field : fields) {
+            if (!field.isAnnotationPresent(Inject.class)) {
+                continue;
+            }
+            Object fieldValue = getInstance(field.getType());
+            setFieldValue(instance, field, fieldValue);
+        }
+        return instance;
+    }
+
+    private static void setFieldValue(Object instance, Field field, Object fieldValue) {
+        try {
+            field.setAccessible(true);
+            field.set(instance, fieldValue);
+        } catch (IllegalAccessException e) {
+            throw new RuntimeException("Failed to initialize field value", e);
+        }
+    }
+
+    private static void validateComponentPresent(Class<?> implementationClass) {
+        if (!implementationClass.isAnnotationPresent(Component.class)) {
+            throw new RuntimeException("Target class = ["
+                    + implementationClass.getSimpleName() + "] should have @Component present");
+        }
+    }
+
+    private void validateImplementationPresent(Class<?> interfaceClazz) {
+        if (!componentMap.containsKey(interfaceClazz)) {
+            throw new RuntimeException(String.format("No implementation for interface %s",
+                    interfaceClazz.getName()));
+        }
+    }
+
+    private Object newInstance(Class<?> implementationClass) {
+        try {
+            return implementationClass.getDeclaredConstructor().newInstance();
+        } catch (ReflectiveOperationException e) {
+            throw new RuntimeException("An error occurred while creating a new instance", e);
+        }
     }
 }

src/main/java/mate.academy/service/impl/FileReaderServiceImpl.java

@@ -4,8 +4,10 @@
 import java.io.IOException;
 import java.nio.file.Files;
 import java.util.List;
+import mate.academy.lib.Component;
 import mate.academy.service.FileReaderService;
 
+@Component
 public class FileReaderServiceImpl implements FileReaderService {
     @Override
     public List<String> readFile(String fileName) {

src/main/java/mate.academy/service/impl/ProductParserImpl.java

@@ -1,9 +1,11 @@
 package mate.academy.service.impl;
 
 import java.math.BigDecimal;
+import mate.academy.lib.Component;
 import mate.academy.model.Product;
 import mate.academy.service.ProductParser;
 
+@Component
 public class ProductParserImpl implements ProductParser {
     public static final int ID_POSITION = 0;
     public static final int NAME_POSITION = 1;

src/main/java/mate.academy/service/impl/ProductServiceImpl.java

@@ -2,13 +2,18 @@
 
 import java.util.List;
 import java.util.stream.Collectors;
+import mate.academy.lib.Component;
+import mate.academy.lib.Inject;
 import mate.academy.model.Product;
 import mate.academy.service.FileReaderService;
 import mate.academy.service.ProductParser;
 import mate.academy.service.ProductService;
 
+@Component
 public class ProductServiceImpl implements ProductService {
+    @Inject
     private ProductParser productParser;
+    @Inject
     private FileReaderService fileReaderService;
 
     @Override