StudentManager.java

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);
        }
    }
}