added logic with dependency injection

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

@@ -1,5 +1,12 @@
 package mate.academy.lib;
 
+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.TYPE)
 public @interface Component {
 
 }

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

@@ -1,5 +1,12 @@
 package mate.academy.lib;
 
+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 +1,58 @@
 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 Map<Class<?>, Class<?>> interfaceClazzMap =
+            Map.of(ProductService.class, ProductServiceImpl.class,
+                    ProductParser.class, ProductParserImpl.class,
+                    FileReaderService.class, FileReaderServiceImpl.class);
+
+    private Map<Class<?>, Object> instances = new HashMap<>();
+
     public static Injector getInjector() {
         return injector;
     }
 
     public Object getInstance(Class<?> interfaceClazz) {
-        return null;
+        Class<?> clazz = interfaceClazzMap.get(interfaceClazz);
+
+        if (clazz == null) {
+            clazz = interfaceClazz;
+        }
+
+        if (!clazz.isAnnotationPresent(Component.class)) {
+            throw new RuntimeException("Injection failed, missing "
+                    + "@Component annotation on the class " + clazz.getName());
+        }
+
+        Object instance = instances.get(clazz);
+        if (instance != null) {
+            return instance;
+        }
+        try {
+            instance = clazz.getDeclaredConstructor().newInstance();
+            instances.put(clazz, instance);
+            for (Field field : clazz.getDeclaredFields()) {
+                if (field.isAnnotationPresent(Inject.class)) {
+                    Object fieldInstance = getInstance(field.getType());
+                    field.setAccessible(true);
+                    field.set(instance, fieldInstance);
+                }
+            }
+            return instance;
+        } catch (ReflectiveOperationException e) {
+            throw new RuntimeException("Cant create an instance of " + clazz.getName(), e);
+        }
     }
 }

src/main/java/mate.academy/model/Product.java

@@ -1,7 +1,9 @@
 package mate.academy.model;
 
 import java.math.BigDecimal;
+import mate.academy.lib.Component;
 
+@Component
 public class Product {
     private Long id;
     private String name;

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 +2,19 @@
 
 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