public interface Tools{
	public void write();
}

public class Pencil implements Tools{
	@Override
	public void write(){
		System.out.println("By pencil");
	}
}

public class BallpointPen implements Tools{
	@Override
	public void write(){
		System.out.println("By BallpointPen");
	}
}

public class FountainPen implements Tools{
	@Override
	public void write(){
		System.out.println("By FountainPen");
	}
}

public class Student{
	private Tools tools;

	public void change(Tools tools){
		this.tools = tools;
	}

	public void write(){
		if(tools == null){
			System.out.println("without tools");
		}else{
			tools.write();
		}
	}

}

public class main{
	publi static void main(String[] args){
		Student student = new Student();
		student.write();   //without tools

		student.change(new Pencil());
		student.write();   //by pencil

		student.change(new BallpointPen());
		student.write();   //by ballpointpen
	}
}