Mine

public class Test {

	private Integer id = null;
	
	public Test() {
	}
	
	public int compareTo(Test test) {
		System.out.println("in compareTo");
		return id != null ? id.compareTo(test.id) : test != null ? test.compareTo(this) : 0;
	}
	
	public static void main(String[] args) {
		Test test = new Test();
		Test test2 = new Test();
		test.compareTo(test2);
	}
}