First solution, basic tasks solved

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

@@ -10,7 +10,14 @@ 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);
+
+        // Checking the singleton pattern (on AI mentor request)
+        Injector injectorSecond = Injector.getInjector();
+        if (!injector.equals(injectorSecond)) {
+            throw new RuntimeException("The singleton pattern doesn't work correctly");
+        }
+
         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;
 
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.TYPE)
+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 +1,67 @@
 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.impl.ImplementationFinder;
+
 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;
+        if (interfaceClazz.isAnnotationPresent(Component.class)) {
+            Object clazzImplementationInstance = null;
+            Class<?> clazz = findImplementation(interfaceClazz);
+            Field[] declaredFields = clazz.getDeclaredFields();
+            if (clazzImplementationInstance == null) {
+                clazzImplementationInstance = createNewInstance(clazz);
+            }
+            clazzImplementationInstance = createNewInstance(clazz);
+            for (Field field : declaredFields) {
+                if (field.isAnnotationPresent(Inject.class)) {
+                    Object fieldInstance = getInstance(field.getType());
+                    field.setAccessible(true);
+                    try {
+                        field.set(clazzImplementationInstance, fieldInstance);
+                    } catch (IllegalAccessException e) {
+                        throw new RuntimeException("Can't initialize field value. Class: "
+                                + clazz.getName() + ". Field: " + field.getName(), e);
+                    }
+                }
+            }
+            return clazzImplementationInstance;
+        } else {
+            throw new RuntimeException("This instance of "
+                    + interfaceClazz + " class, cannot be created, because "
+                    + "class has no annotation '@Component'");
+        }
+    }
+
+    private Object createNewInstance(Class<?> clazz) {
+        if (instances.containsKey(clazz)) {
+            return instances.get(clazz);
+        }
+        Constructor<?> constructor = null;
+        try {
+            constructor = clazz.getConstructor();
+            Object object = constructor.newInstance();
+            instances.put(clazz, object);
+            return object;
+        } catch (ReflectiveOperationException e) {
+            throw new RuntimeException("Can't create new instance of: " + clazz.getName(), e);
+        }
+    }
+
+    private Class<?> findImplementation(Class<?> interfaceClazz) {
+        if (interfaceClazz.isInterface()) {
+            return ImplementationFinder.getImplementation(interfaceClazz);
+        }
+        return interfaceClazz;
     }
 }

src/main/java/mate.academy/service/FileReaderService.java

@@ -1,7 +1,9 @@
 package mate.academy.service;
 
 import java.util.List;
+import mate.academy.lib.Component;
 
+@Component
 public interface FileReaderService {
     List<String> readFile(String fileName);
 }

src/main/java/mate.academy/service/ProductParser.java

@@ -1,7 +1,9 @@
 package mate.academy.service;
 
+import mate.academy.lib.Component;
 import mate.academy.model.Product;
 
+@Component
 public interface ProductParser {
     /**
      * Parse the productInfo and return created {@link Product} object.

src/main/java/mate.academy/service/ProductService.java

@@ -1,8 +1,10 @@
 package mate.academy.service;
 
 import java.util.List;
+import mate.academy.lib.Component;
 import mate.academy.model.Product;
 
+@Component
 public interface ProductService {
     List<Product> getAllFromFile(String filePath);
 }

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/ImplementationFinder.java

@@ -0,0 +1,18 @@
+package mate.academy.service.impl;
+
+import java.util.Map;
+import mate.academy.service.FileReaderService;
+import mate.academy.service.ProductParser;
+import mate.academy.service.ProductService;
+
+public class ImplementationFinder {
+    private static final Map<Class<?>, Class<?>> IMPLEMENTATION_MAP = Map.of(
+            FileReaderService.class, FileReaderServiceImpl.class,
+            ProductParser.class, ProductParserImpl.class,
+            ProductService.class, ProductServiceImpl.class
+    );
+
+    public static Class<?> getImplementation(Class<?> clazz) {
+        return IMPLEMENTATION_MAP.get(clazz);
+    }
+}

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

@@ -2,20 +2,26 @@
 
 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 {
-    private ProductParser productParser;
-    private FileReaderService fileReaderService;
+    @Inject
+    private ProductParser txtProductParser;
+
+    @Inject
+    private FileReaderService txtFileReaderService;
 
     @Override
     public List<Product> getAllFromFile(String filePath) {
-        return fileReaderService.readFile(filePath)
+        return txtFileReaderService.readFile(filePath)
                 .stream()
-                .map(productParser::parse)
+                .map(txtProductParser::parse)
                 .collect(Collectors.toList());
     }
 }