Solution

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

@@ -8,9 +8,8 @@
 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(ProductService.class);
         List<Product> products = productService.getAllFromFile("products.txt");
         products.forEach(System.out::println);
     }

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

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

@@ -0,0 +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

@@ -0,0 +1,9 @@
+package mate.academy.lib;
+
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Inject {
+
+}

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

@@ -0,0 +1,74 @@
+package mate.academy.lib;
+
+import java.lang.reflect.Constructor;
+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<?>, Object> instances = new HashMap<>();
+
+    public static Injector getInjector() {
+        return injector;
+    }
+
+    public Object getInstance(Class<?> interfaceClazz) {
+        Object clazzImplementationInstance = null;
+        Class<?> clazz = findImplementation(interfaceClazz);
+        Field[] declaredFields = clazz.getDeclaredFields();
+        if (!clazz.isAnnotationPresent(Component.class)) {
+            throw new RuntimeException("Injection failed, missing @Component "
+                    + "annotation on the class "
+                    + interfaceClazz.getName());
+        }
+        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 "
+                            + field.getName() + " in " + clazz.getName(), e);
+                }
+            }
+        }
+        if (clazzImplementationInstance == null) {
+            clazzImplementationInstance = createNewInstance(clazz);
+        }
+        return clazzImplementationInstance;
+    }
+
+    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 (ReflectiveOperationException e) {
+            throw new RuntimeException("Can`t create new instance of " + clazz.getName(), e);
+        }
+    }
+
+    private Class<?> findImplementation(Class<?> interfaceClazz) {
+        Map<Class<?>, Class<?>> interfaceImplementations = new HashMap<>();
+        interfaceImplementations.put(ProductParser.class, ProductParserImpl.class);
+        interfaceImplementations.put(FileReaderService.class, FileReaderServiceImpl.class);
+        interfaceImplementations.put(ProductService.class, ProductServiceImpl.class);
+        if (interfaceClazz.isInterface()) {
+            return interfaceImplementations.get(interfaceClazz);
+        }
+        return interfaceClazz;
+    }
+}