Example of Interface

Main.java

import java.io.BufferedReader;
import java.util.Scanner;

import javax.swing.JOptionPane;


interface Dialog {
	void   say(String message);
	String ask(String question);
}

class ConsoleDialog implements Dialog {
	
	Scanner scanner = new Scanner(System.in);

	@Override
	public void say(String message) {
		System.out.println(message);
	}

	@Override
	public String ask(String question) {
		System.out.print(question + " >> ");
		return scanner.nextLine();
	}
	
}

class GUIDialog implements Dialog {

	@Override
	public void say(String message) {
		JOptionPane.showMessageDialog(null, message);
	}

	@Override
	public String ask(String question) {
		return JOptionPane.showInputDialog(question);
	}
	
}

public class Main {

	public static void main(String[] args) {
		
		Dialog d;
		
		if (JOptionPane.showConfirmDialog(
				null, "Do you want to use the GUI?", "Select Dialog", JOptionPane.YES_NO_OPTION) == JOptionPane.YES_OPTION) {
			d = new GUIDialog();
		} else {
			d = new ConsoleDialog();
		}
		
		d.say("Hello");
		String x = d.ask("What is your name?");
		d.say("Hello, " + x);
		
	}

}