How to use a "contains(object)" method in ArrayList?

First of all, I am not a native English speaker, so.... understand my poor writing plz :)

I am making a Grade Calculator myself.

I just found on the Internet that "contains" method easily works when you using ArrayList<String> or ArrayList<Integer>.

But I have to use my own class... for instance.. ArrayList<Grade> - this class receives 3 valuables in constructor -

"contains" doesn't work in my own class... I already tested.

If I want to find duplication, do I have to use Iterator?

I will show you parts of Grade and DAO class

Java Code:

public Grade(String subjectName, String category, double score) {
                this.subjectName = subjectName;
                this.score = score;
                this.category = category;
        }

public int addGrade(Grade tempGrade) {
                /*
                Iterator<Grade> it = gradeList.iterator();
                while(it.hasNext()){
                        Grade temp = it.next();
                }
                */

                if (gradeList.contains(tempGrade)) {
                        return DUPLICATION;
                }
                gradeList.add(tempGrade);
                return SUCCESS;
        }