[EDIT] interfaceにメソッドの宣言を追加しました。

src/main/java/com/example/demo/app/domain/model/CategoryModel.java

@@ -0,0 +1,30 @@
+package com.example.demo.app.domain.model;
+
+import java.util.List;
+
+import com.example.demo.app.infra.entity.CategoryEntity;
+
+import lombok.AllArgsConstructor;
+import lombok.Getter;
+
+@Getter
+@AllArgsConstructor
+public class CategoryModel {
+
+	private Long categoryId;
+	private String categoryName;
+	private Integer transactionType;
+	
+	
+	public static CategoryModel convertFrom(CategoryEntity categoryEntity) {
+		return new CategoryModel(
+				categoryEntity.getCategoryId(),
+				categoryEntity.getCategoryName(),
+				categoryEntity.getTransactionType());
+	}
+
+	public static List<CategoryModel> convertFrom(List<CategoryEntity> categoryEntityList) {
+		return categoryEntityList.stream().map(CategoryModel::convertFrom).toList();
+	}
+	
+}

src/main/java/com/example/demo/app/domain/service/CategoryService.java

@@ -0,0 +1,12 @@
+package com.example.demo.app.domain.service;
+
+import java.util.List;
+
+import com.example.demo.app.domain.model.CategoryModel;
+
+public interface CategoryService {
+
+	public List<CategoryModel> findAll();
+	
+	public CategoryModel findOne(Long categoryId);
+}

src/main/java/com/example/demo/app/domain/service/impl/CategoryServiceImpl.java

@@ -0,0 +1,39 @@
+package com.example.demo.app.domain.service.impl;
+
+import java.util.List;
+import java.util.Optional;
+
+import org.springframework.stereotype.Service;
+
+import com.example.demo.app.domain.model.CategoryModel;
+import com.example.demo.app.domain.service.CategoryService;
+import com.example.demo.app.infra.entity.CategoryEntity;
+import com.example.demo.app.infra.repository.CategoryRepository;
+
+import lombok.RequiredArgsConstructor;
+
+@Service
+@RequiredArgsConstructor
+public class CategoryServiceImpl implements CategoryService{
+
+	private final CategoryRepository categoryRepository;
+	
+	public List<CategoryModel> findAll() {
+		
+		return CategoryModel.convertFrom(categoryRepository.findAll());
+		
+	}
+	
+	public CategoryModel findOne(Long categoryId) {
+		
+		Optional<CategoryEntity> optionalCategoryModel = categoryRepository.findById(categoryId);
+		
+		if (optionalCategoryModel.isEmpty()) {
+			return null;
+		}
+		
+		return CategoryModel.convertFrom(optionalCategoryModel.get());
+		
+	}
+	
+}

src/main/java/com/example/demo/app/presentation/form/CategoryForm.java

@@ -0,0 +1,28 @@
+package com.example.demo.app.presentation.form;
+
+import java.util.List;
+
+import com.example.demo.app.domain.model.CategoryModel;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+@Data
+@AllArgsConstructor
+@NoArgsConstructor
+public class CategoryForm {
+	
+	private Long categoryId;
+	private String categoryName;
+	private Integer transactionType;
+	
+	public static CategoryForm convertFrom(CategoryModel categoryModel) {
+		return new CategoryForm(categoryModel.getCategoryId(), categoryModel.getCategoryName(), categoryModel.getTransactionType());
+	}
+	
+	public static List<CategoryForm> convertFrom(List<CategoryModel> categoryModelList){
+		return categoryModelList.stream().map(CategoryForm::convertFrom).toList();
+	}
+	
+}