Example Dependency Inversion Principle

Contact

interface Mailer {
	public void send();
}

class Gmail implements Mailer {
	public void send() {
		//....send mail
	}
}

class Hotmail implements Mailer {
	public void send(){
		//..send mail
	}
}

class Contact  {
	Mailer mail;

	public void setMailer(Mailer m) {
		mail = m;
	}

	public void send() {
		mail.send();
	}

}