Adapter¶

The Adapter pattern is used to adapt a certain object to the target interface. It requires introducing an intermediate layer that acts as a link between these objects. This connector should also implement the target interface.

We can use this pattern when:

we adjust the application that uses the old API to the new one.
we are integrating a new library which assumes that our objects implement a certain interface (which they do not, and we do not want or cannot rewrite the implementation of these objects).
we integrate many systems into one and we want certain groups of objects with a slightly different representation to use the same abstraction.

adapter

Example¶

The following example shows a simple adapter implementation. In this example, we can see:

the Student interface, which is our target interface (i.e. the one we want to work with)
the Pupil class, which is an object that does not match theStudent interface
the PupilAdapter class, which is an adapter, i.e. a connector, between theStudent and Pupil objects

Note that the object that has to be adapted comes as a constructor argument to the class representing the adapter. In the adapter, for the methods implementing the target interface, the programmer's task is to come up with a way to adjust the data structure to the expected format.

import java.util.Collection;

public interface Student {
  String getFullName();
  String getContactDetails();
  boolean isAdult();
  Collection<Integer> getResults();
}

import java.util.List;

public class Pupil {
  private final String firstName;
  private final String lastName;
  private final String email;
  private final Integer age;
  private final List<Integer> grades;

  public Pupil(final String firstName, final String lastName, final String email, final Integer age, final List<Integer> grades) {
    this.firstName = firstName;
    this.lastName = lastName;
    this.email = email;
    this.age = age;
    this.grades = grades;
  }

  public String getFirstName() {
    return firstName;
  }

  public String getLastName() {
    return lastName;
  }

  public String getEmail() {
    return email;
  }

  public Integer getAge() {
    return age;
  }

  public List<Integer> getGrades() {
    return grades;
  }
}

import java.util.Collection;

public class PupilAdapter implements Student {

  private final Pupil pupil;

  public PupilAdapter(final Pupil pupil) {
    this.pupil = pupil;
  }

  @Override
  public String getFullName() {
    return pupil.getFirstName() + " " + pupil.getLastName();
  }

  @Override
  public String getContactDetails() {
    return pupil.getEmail();
  }

  @Override
  public boolean isAdult() {
    return pupil.getAge() >= 18;
  }

  @Override
  public Collection<Integer> getResults() {
    return pupil.getGrades();
  }
}

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

public class AdapterUsage {
  public static void main(String[] args) {
    List<Student> students = new ArrayList<>();
    students.add(new PupilAdapter(new Pupil("Andrzej", "Nowak", "anowak@sda.pl", 19, List.of(3, 4, 5))));
    for (final var student : students) {
      System.out.println(student.getFullName()); // Andrzej Nowak
      System.out.println(student.getContactDetails()); // anowak@sda.pl
      System.out.println(student.getResults()); // [3, 4, 5]
      System.out.println(student.isAdult()); // true
    }
  }
}