Interfaces

Defining an interface

An interface specifies some methods which must be implemented by any implementing class. It does not actually implement the methods - that is left to the child classes.

public interface Protected {
  void setPassword(String password);
  boolean checkPassword(String attempt);
}

Implementing an interface

Now, any device which implements the interface must implement the interface's methods. Failing to do so will cause an error. Each implementation can decide its own behaviour.

SmartScreen.js

class SmartScreen implements Protected {
  private String password;

  public SmartScreen () {
    this.password = "default";
  }

  @Override
  public void setPassword(String password) {
    if (password.length < 6) {
      System.out.println("Password must have at least 6 characters.");
    } else {
      this.password = password;
    }
  }

  @Override
  public boolean checkPassword(String attempt) {
    return attempt == this.password;
  }

  // Other methods...
}

SmartThermostat.js

class SmartThermostat implements Protected {
  private String password;
  private int temperature;

  public SmartThermostat () {
    this.password = "admin"
    this.temperature = 18;
  }

  @Override
  public void setPassword(String password) {
    if (password.length < 10) {
      System.out.println("Password must have at least 10 characters.");
    } else {
      this.password = password;
    }
  }

  @Override
  public boolean checkPassword(String attempt) {
    return attempt == this.password
  }

  // Other methods...
}

TIP

When a class implements an interface, it can also freely add additional properties and methods - it isn't limited to the interface.

Inheritance and interfaces

A class can only inherit from one parent class, but it can implement many interfaces. It can both inherit and implement interfaces if it wishes to.

public class SmartScreen extends SmartDevice implements Lockable, Sleepable {
  // implementation
}