JAVA CERTIFICATION QUESTION: When are Java objects eligible for garbage collection?

public class GCDemo {
    public static ArrayList<Object> l = new ArrayList<>();    
    public void doIt() {
        HashMap<String, Object> m = new HashMap<>();     
        Object o1 = new Object(); // line n1
        Object o2 = new Object();
        m.put("o1", o1);
        o1 = o2; // line n2
        o1 = null; // line n3
        l.add(m);
        m = null; // line n4
        System.gc();// line n5
    }
}

GCDemo demo = new GCDemo();
demo.doIt();
demo = null;  // line n6