How to ensure finalize() is always called (Thinking in Java exercise)

I'm slowly working through Bruce Eckel's Thinking in Java 4th edition, and the following problem has me stumped:

Create a class with a finalize( ) method that prints a message. In main( ), create an object of your class. Modify the previous exercise so that your finalize( ) will always be called.

This is what I have coded:

public class Horse {
    boolean inStable;
    Horse(boolean in){
        inStable = in;
    }   
    public void finalize(){
        if (!inStable) System.out.print("Error: A horse is out of its stable!");
    }
}
public class MainWindow {
    public static void main(String[] args) {
        Horse h = new Horse(false);
        h = new Horse(true);
        System.gc();
    }
}

It creates a new Horse object with the boolean inStable set to false. Now, in the finalize() method, it checks to see if inStable is false. If it is, it prints a message.

Unfortunately, no message is printed. Since the condition evaluates to true, my guess is that finalize() is not being called in the first place. I have run the program numerous times, and have seen the error message print only a couple of times. I was under the impression that when System.gc() is called, the garbage collector will collect any objects that aren't referenced.

Googling a correct answer gave me this link, which gives much more detailed, complicated code. It uses methods I haven't seen before, such as System.runFinalization(), Runtime.getRuntime(), and System.runFinalizersOnExit().

Is anybody able to give me a better understanding of how finalize() works and how to force it to run, or walk me through what is being done in the solution code?