NullObject Pattern

NPE 를 피하기 위한 디자인 패턴

public abstract class AbstractCustomer {
    protected String personName;

    public abstract boolean isNull();

    public abstract String getCustomer();
}

public class RealCustomer extends AbstractCustomer {
    public RealCustomer(String customerName) {
        this.personName = customerName;
    }

    @Override
    public boolean isNull() {
        return false;
    }

    @Override
    public String getCustomer() {
        return this.personName;
    }
}

public class NullCustomer extends AbstractCustomer {
    @Override
    public boolean isNull() {
        return true;
    }

    @Override
    public String getCustomer() {
        return "No customer";
    }
}

public class Database {

    private List<String> customerNames;

    public Database() {
        this.customerNames = new ArrayList<>();
        this.customerNames.add("A");
        this.customerNames.add("B");
        this.customerNames.add("C");
        this.customerNames.add("D");
    }

    public boolean existsCustomer(String name) {
        return customerNames.contains(name);
    }
}

public class CustomerFactory {

    private Database database;

    public CustomerFactory() {
        this.database = new Database();
    }

    public AbstractCustomer getCustomer(String name) {
        if (checkName(name)) {
            return new RealCustomer(name);
        }
        return new NullCustomer();
    }

    private boolean checkName(String name) {
        return database.existsCustomer(name);
    }
}

결론적으로 Customer 객체를 추상화해 isNull() 메서드를 사용하게 된다.
CustomerFactory 를 거쳐 customer를 가져와 NPE를 피할 수 있다.