數(shù)據(jù)訪問對象接口 - 此接口定義要對模型對象執(zhí)行的標(biāo)準(zhǔn)操作。
數(shù)據(jù)訪問對象具體類 - 此類實(shí)現(xiàn)上述接口。這個類負(fù)責(zé)從數(shù)據(jù)源獲取數(shù)據(jù)，數(shù)據(jù)源可以是數(shù)據(jù)庫/xml或任何其他存儲機(jī)制。
模型對象或值對象 - 此對象是簡單的POJO，包含用于存儲使用DAO類檢索的數(shù)據(jù)的get/set方法。

實(shí)現(xiàn)實(shí)例

在這個將創(chuàng)建一個作為Model或Value對象的Student對象。 StudentDao是數(shù)據(jù)訪問對象接口。 StudentDaoImpl是實(shí)現(xiàn)數(shù)據(jù)訪問對象接口的具體類。 DaoPatternDemo這是一個演示類，將使用StudentDao演示使用數(shù)據(jù)訪問對象模式。

數(shù)據(jù)訪問對象模式示例的結(jié)構(gòu)如下圖所示 -

第1步

創(chuàng)建值對象，其代碼如下 -

Student.java

public class Student {
   private String name;
   private int rollNo;

   Student(String name, int rollNo){
      this.name = name;
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }
}

第2步

創(chuàng)建數(shù)據(jù)訪問對象接口，其代碼如下 -

StudentDao.java

import java.util.List;

public interface StudentDao {
   public List<Student> getAllStudents();
   public Student getStudent(int rollNo);
   public void updateStudent(Student student);
   public void deleteStudent(Student student);
}

第3步

創(chuàng)建實(shí)現(xiàn)上面的接口的具體類，其代碼如下 -

StudentDaoImpl.java

import java.util.ArrayList;
import java.util.List;

public class StudentDaoImpl implements StudentDao {

   //list is working as a database
   List<Student> students;

   public StudentDaoImpl(){
      students = new ArrayList<Student>();
      Student student1 = new Student("Robert",0);
      Student student2 = new Student("John",1);
      students.add(student1);
      students.add(student2);
   }
   @Override
   public void deleteStudent(Student student) {
      students.remove(student.getRollNo());
      System.out.println("Student: Roll No " + student.getRollNo() + ", deleted from database");
   }

   //retrive list of students from the database
   @Override
   public List<Student> getAllStudents() {
      return students;
   }

   @Override
   public Student getStudent(int rollNo) {
      return students.get(rollNo);
   }

   @Override
   public void updateStudent(Student student) {
      students.get(student.getRollNo()).setName(student.getName());
      System.out.println("Student: Roll No " + student.getRollNo() + ", updated in the database");
   }
}

第4步

使用StudentDao演示數(shù)據(jù)訪問對象模式的使用。
DaoPatternDemo.java

public class DaoPatternDemo {
   public static void main(String[] args) {
      StudentDao studentDao = new StudentDaoImpl();

      //print all students
      for (Student student : studentDao.getAllStudents()) {
         System.out.println("Student: [RollNo : " + student.getRollNo() + ", Name : " + student.getName() + " ]");
      }


      //update student
      Student student =studentDao.getAllStudents().get(0);
      student.setName("Michael");
      studentDao.updateStudent(student);

      //get the student
      studentDao.getStudent(0);
      System.out.println("Student: [RollNo : " + student.getRollNo() + ", Name : " + student.getName() + " ]");        
   }
}

第5步

驗(yàn)證輸出，執(zhí)行上面的代碼得到以下結(jié)果 -

Student: [RollNo : 0, Name : Robert ]
Student: [RollNo : 1, Name : John ]
Student: Roll No 0, updated in the database
Student: [RollNo : 0, Name : Michael ]

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