* The dependency inversion principle: depend upon Abstractions. Do not
depend upon implementations.

5) What is the motivation behind this principle?
- Concretions change much faster.

6) Code a program that reads characters from the input, and then print
these characters in the output.
public class Control {
  public void copy(KeyboardInput i, PrinterOutput p) {
    int d;
    while ((d = i.readNextKey()) != -1) {
      p.printData(d);
    }
  }
  public static void main(String args[]) {
    Control c = new Control();
    c.copy(new KeyboardInput(), new PrinterOutput());
  }
}
public class KeyboardInput {
  public int readNextKey() {
    return 0;
  }
}
public class PrinterOutput {
  public void printData(int d) {
    System.out.println(d);
  }
}

6.1) We now have an object called ScreenOutput, which has a method
printOnScreen(int). Modify the Controller to handle this object too.

6.2) We now have an object called FileInput, which a method readNextChar().
Modify the Controller to handle this object.

6.3) What is the best design for this problem?

* The interface segregation principle: many client specific interfaces
are better than one general purpose interface.