implemented an injector using annotations.

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

@@ -4,13 +4,15 @@
 import mate.academy.lib.Injector;
 import mate.academy.model.Product;
 import mate.academy.service.ProductService;
+import mate.academy.service.impl.ProductServiceImpl;
 
 public class Main {
 
     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;
+        ProductService productService = (ProductService) injector
+                .getInstance(ProductServiceImpl.class);
         List<Product> products = productService.getAllFromFile("products.txt");
         products.forEach(System.out::println);
     }

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

@@ -1,5 +1,11 @@
 package mate.academy.lib;
 
-public @interface Component {
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Component {
 }

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

@@ -1,5 +1,11 @@
 package mate.academy.lib;
 
-public @interface Inject {
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
 
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.FIELD)
+public @interface Inject {
 }

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

@@ -1,13 +1,80 @@
 package mate.academy.lib;
 
+import java.lang.reflect.Constructor;
+import java.lang.reflect.Field;
+import java.lang.reflect.InvocationTargetException;
+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<?>, Object> instances = new HashMap<>();
 
     public static Injector getInjector() {
         return injector;
     }
 
     public Object getInstance(Class<?> interfaceClazz) {
-        return null;
+        Class<?> clazz = findImplementation(interfaceClazz);
+        isClassValid(clazz);
+        Object clazzImplementationInstance = null;
+        Field[] declaredFields = interfaceClazz.getDeclaredFields();
+        for (Field field : declaredFields) {
+            if (field.isAnnotationPresent(Inject.class)) {
+                Object fieldInstance = getInstance(field.getType());
+                clazzImplementationInstance = createNewInstance(clazz);
+                try {
+                    field.setAccessible(true);
+                    field.set(clazzImplementationInstance, fieldInstance);
+                } catch (IllegalAccessException e) {
+                    throw new RuntimeException("Can't initialize field value. Class: "
+                            + clazz.getName() + ". Field: " + field.getName());
+                }
+            }
+        }
+        if (clazzImplementationInstance == null) {
+            clazzImplementationInstance = createNewInstance(clazz);
+        }
+        return clazzImplementationInstance;
+    }
+
+    private void isClassValid(Class<?> clazz) {
+        if (!clazz.isAnnotationPresent(Component.class)) {
+            throw new RuntimeException("Class: " + clazz.getName()
+                    + " isn't valid, because isn't declared as @Component");
+        }
+    }
+
+    private Object createNewInstance(Class<?> clazz) {
+        if (instances.containsKey(clazz)) {
+            return instances.get(clazz);
+        }
+        try {
+            Constructor<?> constructor = clazz.getConstructor();
+            Object instance = constructor.newInstance();
+            instances.put(clazz, instance);
+            return instance;
+        } catch (NoSuchMethodException | InvocationTargetException
+                 | InstantiationException | IllegalAccessException e) {
+            throw new RuntimeException("Can't create new instance of " + clazz.getName());
+        }
+
+    }
+
+    private Class<?> findImplementation(Class<?> interfaceClazz) {
+        Map<Class<?>, Class<?>> interfaceImplementation = new HashMap<>();
+        interfaceImplementation.put(ProductService.class, ProductServiceImpl.class);
+        interfaceImplementation.put(ProductParser.class, ProductParserImpl.class);
+        interfaceImplementation.put(FileReaderService.class, FileReaderServiceImpl.class);
+        if (interfaceClazz.isInterface()) {
+            return interfaceImplementation.get(interfaceClazz);
+        }
+        return interfaceClazz;
     }
 }

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