Detect defects in code without executing the Runner class

In the project, there are two classes — Runner and ComputerShop. It is required to analyze their source codes and identify potential issues without running the main method of the Runner class.

public class Runner {
    public static void main(String[] args) {
        ComputerShop computerShop = new ComputerShop();
        PhoneShop phoneShop = new PhoneShop();

        Computer computer = computerShop.sellComputer();
        Phone phone = phoneShop.sellPhone();

        computer.start();
        phone.call();
    }
}

public class ComputerShop {
    private List<Computer> computers;

    public void buyComputers(List<Computer> computers) {
    }

    public Computer sellComputer() {
        if (computers.size() == 0) {
            throw new RuntimeException("Computers are sold out!");
        }
        return computers.remove(0);
    }
}