当我声明和初始化类的对象时，如何自动增加类中的字段？

Question

I have 3 classes in my Java application, One representing a Medical Policy:

public class MedicalPolicy implements PolicyType {
    private int id;
    // and some other fields

    public MedicalPolicy(final LocalDate effective,final LocalDate expiry,final ArrayList<Beneficiary> beneficiaries){
          //Constructor that does NOT set/initialize the id field
    }
}

Another class which represents the Motor Policy:

public class MotorPolicy implements  PolicyType{

    private int id;
    // and some other fields

    public MotorPolicy(final LocalDate effective,final LocalDate expiry,final double vehiclePrice){
        //Constructor that does NOT set/initialize the id field
    }
}

And the 3rd class contains the main() method.

Every time I declare and initialize a Medical Policy object in my main() I want the id of the Medical Policy to increment by 1, the same for the Motor Policy, ie if I declare 2 Medical Policy objects in my main() I want the value of 1st Medical Policy to have the id equal to 1 and the 2nd Medical Policy to have the id 's value set to 2, and if I declare 3 Motor Policy, I want the 1st to have it's id equal to 1, the 2nd's id equal to 2 and the 3rd's id equal to 3.

How can I accomplish that?

PS: There is no prob if I have to set/initialize it from the constructor, what's important is that I shouldn't pass it's value as a constructor parameter.

Answer 1

As I have suggested in my comment, I would add an AtomicInteger to count the number of constructor calls:

public class MedicalPolicy implements PolicyType {
    private static final AtomicInteger counter = new AtomicInteger();

    private final int id;

    public MedicalPolicy(
            final LocalDate effective,
            final LocalDate expiry,
            final ArrayList<Beneficiary> beneficiaries){
        id = counter.incrementAndGet();
    }
}

The reason for using an AtomicInteger is thread-safety.

Notice, however, that this implementation does only count how often the constructor was called. To get a count of reachable instances of a given class there are two solutions coming to my mind. One involves overriding finalize() (which is deprecated in Java 13). The other uses a List of PhantomReference s to track reachable instances.

Answer 2

You can use AtomicInteger , which is used in applications such as atomically incremented counters.

public class MedicalPolicy implements PolicyType {
  private static final AtomicInteger count = new AtomicInteger(); 
    // and some other fields

    public MedicalPolicy(final LocalDate effective,final LocalDate expiry,final ArrayList<Beneficiary> beneficiaries){
          //Constructor that does NOT set/initialize the id field
         count.incrementAndGet(); 
    }
}

Answer 3

Use a private static int as an instance variable

Inside the constructor you set the id to the serial and add one to the serial.

public class MedicalPolicy implements PolicyType {
    private static int serial = 1;
    private int id;
    // and some other fields

    public MedicalPolicy(final LocalDate effective,
                         final LocalDate expiry,
                         final ArrayList<Beneficiary> beneficiaries){
      this.id = serial++;
      // Other constructor stuff
    }
}

If your class needs to be thread safe, use an AtomicInteger as Turing85 mentioned.

Answer 4

Could you not create a List, then each object of that class would be associated to its incrementing location in the index. eg. listName.get(obj num-1) since the first object will be stored at index of 0. Also listName.size() would return a value so you could track how many you had.