Question

Can any one suggest me which collection to use for this scenario:

Every student has a payment history with details of payment made by the student / family. The system should ensure that no duplicate payments are there against a student account. The program should be able to add payment details for a student, and ensure that duplicate payment details are not getting registered.

Answer 1

Perhaps a Map<Student, Set<Payment>> would do.

(A Set won’t allow for duplicates.)

If you override equals properly (and hashCode) you can do something like

Map<Student, Set<Payment>> studentPayments =new HashMap<Student, Set<Payment>>();

public void addStudentPayment(Student student, Payment payment) {

    if (!studentPayments.containsKey(student))
        studentPayments.put(student, new HashSet<Payment>());

    if (studentPayments.get(student).add(payment))
        System.out.println("Payment was added");
    else
        System.out.println("Duplicate found. Payment not added.");
}