静的メンバーの回収の使用

public class Exec {
	public static void main(String[] args) {
		Book b1 = new Book();
		System.out.println("   " + Book.counter + "   ");
		Book b2 = new Book();
		System.out.println("   " + Book.counter + "   ");
		Book b3 = new Book();
		System.out.println("   " + Book.counter + "   ");
		
		b3 = null;
		System.gc();
		System.runFinalization();
		System.out.println("   " + Book.counter + "   ");
		b2 = null;
		System.gc();
		System.runFinalization();
		System.out.println("   " + Book.counter + "   ");
	}
}

class Book {
	public static int counter = 0;

	public Book() {
		++counter;
	}

	public void finalize() {
		--counter;
	}
}