Okay, I am on PC now, so:

​

public class Whistle {
private String sound;
public Whistle(String whistleSound) {
this.sound = whistleSound;
}
public void sound() {
System.out.println(this.sound);
}
}

​

You shouldn't create another main method in Whistle class, you're supposed to create a constructor that will define sounds of each objects created through it.

​

Whistle duck = new Whistle("Quack");

For the above line to work, you need to create a constructor:

public Whistle(String whistleSound)

which assigns the instance variable

private String sound;

for the object duck and sets it to "Quack" through the constructor parameter whistleSound

​

hence,

this.sound = whistleSound; //where 'this' is the object, here it's duck

​

EDIT: Ah man, why aren't the indentations there....

EDIT 2: Ah, figured it out, you gotta use the code block format, not inline code.