StudentId.java

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class StudentId {
    private static final Pattern REPR_PATTERN = Pattern.compile("S(\\d{1,4})");
    private final int studentId;

    public StudentId(String repr) {
        Matcher m = REPR_PATTERN.matcher(repr);
        if (!m.matches())
            throw new IllegalArgumentException("Invalid representation: " + repr);
        studentId = Integer.parseInt(m.group(1));
    }

    @Override
    public String toString() {
        return "S" + studentId;
    }
}