how to prevent a client from directly instantiating your concrete classes?

Question

How would you prevent a client from directly instantiating your concrete classes?

Answer 1

Try something like this

public class Record {
    private static final int MAX_INSTANCES = 1;
    private static volatile int instanceCounter = 0;

    private Record() {
        if (instanceCounter >= MAX_INSTANCES)
            throw new RuntimeException("max instances exceeded");

        instanceCounter ++;
    }
}

In the above example you can instantiate max 1 time, change the value of MAX_INSTANCES accordingly as per need.

I hope I understood your question correctly.