feat: EntityManager implement practice with Leopold

src/main/java/com/yourssu/Application.java

@@ -1,10 +1,37 @@
 package com.yourssu;
 
 import com.yourssu.controller.Controller;
+import com.yourssu.controller.EntityManager;
+import com.yourssu.controller.StudentManager;
+import com.yourssu.model.Student;
+
+import java.util.HashMap;
+import java.util.Map;
 
 public class Application {
     public static void main(String[] args) {
-//        Controller controller = new BoardGameController();
-//        controller.run();
+        final Map<Long, Student> studentDB = new HashMap<>();
+
+        Student ducks = new Student(1L, "Ducks", "Computer Science");
+        Student emin = new Student(2L, "Emin", "Mathematics");
+        Student citrus = new Student(3L, "Citrus", "Physics");
+
+        EntityManager studentManager = new StudentManager(studentDB);
+
+        studentManager.persist(ducks);
+        studentManager.persist(emin);
+        studentManager.persist(citrus);
+
+        System.out.println("before flush - current DB members:");
+        for (Student student : studentDB.values()) {
+            System.out.println("ID: " + student.getId());
+        }
+
+        studentManager.flush();
+
+        System.out.println("after flush - current DB members:");
+        for (Student student : studentDB.values()) {
+            System.out.println("ID: " + student.getId() + ", Name: " + student.getName() + ", Major: " + student.getMajor());
+        }
     }
 }

src/main/java/com/yourssu/controller/EntityManager.java

@@ -0,0 +1,11 @@
+package com.yourssu.controller;
+
+import com.yourssu.model.Student;
+
+public interface EntityManager {
+    void persist(Student entity);
+    Student merge(Student entity);
+
+    void flush();
+}
+

src/main/java/com/yourssu/controller/SpringDataRepository.java

@@ -0,0 +1,5 @@
+package com.yourssu.controller;
+
+public interface SpringDataRepository {
+    void save(Object entity);
+}

src/main/java/com/yourssu/controller/StudentManager.java

@@ -0,0 +1,58 @@
+package com.yourssu.controller;
+
+import com.yourssu.model.Student;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class StudentManager implements EntityManager{
+    private final Map<Long, Student> studentCache;
+    private final Map<Long, Student> studentDB;
+    private final Map<Long, Boolean> flushFlag;
+
+    public StudentManager(Map<Long, Student> studentDB) {
+        studentCache = new HashMap<>();
+        flushFlag = new HashMap<>();
+        this.studentDB = studentDB;
+    }
+
+    @Override
+    public void persist(Student entity) {
+        if (!studentCache.containsKey(entity.getId())) {
+            studentCache.put(entity.getId(), entity);
+            flushFlag.put(entity.getId(), false);
+        }
+    }
+
+    @Override
+    public Student merge(Student entity) {
+        // cache nonempty
+        if (studentCache.get(entity.getId()) != null) {
+            Student cachedStudent = studentCache.get(entity.getId());
+            flushFlag.put(entity.getId(), false);
+            return cachedStudent;
+        }
+
+        // cache is empty
+        if (studentDB.get(entity.getId()) != null) {
+            Student dbStudent = studentDB.get(entity.getId());
+            studentCache.put(entity.getId(), dbStudent);
+            flushFlag.put(entity.getId(), false);
+            return dbStudent;
+        }
+
+        persist(entity);
+        return new Student(entity);
+    }
+
+    @Override
+    public void flush() {
+        for (Student student : studentCache.values()) {
+            if (flushFlag.get(student.getId())) {
+                continue;
+            }
+            studentDB.put(student.getId(), student);
+            flushFlag.put(student.getId(), true);
+        }
+    }
+}

src/main/java/com/yourssu/model/Student.java

@@ -0,0 +1,43 @@
+package com.yourssu.model;
+
+public class Student {
+    private final Long id;
+    private String name;
+    private String major;
+
+    public Student(Long id, String name, String major) {
+        this.id = id;
+        this.name = name;
+        this.major = major;
+    }
+
+    public Student(Student student) {
+        this.id = student.id;
+        this.name = student.name;
+        this.major = student.major;
+    }
+
+    public boolean equals(Student student) {
+        return this.id.equals(student.id);
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public void setMajor(String major) {
+        this.major = major;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public String getMajor() {
+        return major;
+    }
+}