一而再，再而三地檢查對象是否null

public class Site {private Customer customer;public Customer getCustomer() {return customer;} }public class Customer {private String name;private BillingPlan plan;private PaymentHistory history;public String getName() {return name;}public String getPlan() {return plan;}public String getHistory() {return history;} }public class PaymentHistory {private Integer weeksDelingquent;public int getWeeksDelingquentInLastYear() {return weeksDelingquentInLastYear;} }Customer customer = site.getCustomer(); BillingPlan plan; if (customer == null) {plan = BillingPlan.basic(); } else {plan = customer.getPlan(); } //... String customerName; if (customer == null) {customerName = "occupant"; } else {customerName = customer.getName(); } //... int weeksDelingquent; if (customer == null) {weeksDelingquent = 0; } else {weeksDelingquent = customer.getHistory().getWeeksDelingquentInLastYear(); }

重構：將null值替換為null對象

注意：空對象一定是常量，它們的任何成分都不會發生變化。因此可以使用Singleton 模式實現。

public interface Nullable {public boolean isNull(); }public class Site {private Customer customer;public Customer getCustomer() {return customer == null ? Customer.newNull() : customer;} }public class Customer implements Nullable {private String name;private BillingPlan plan;private PaymentHistory history;public boolean isNull() {return false;}public static Customer newNull() {return new NullCustomer();}public String getName() {return name;}public String getPlan() {return plan;}public void setPlan(BillingPlan plan) {this.plan = plan;}public String getHistory() {return history;} }public class PaymentHistory implements Nullable {private Integer weeksDelingquent;public boolean isNull() {return false;}public int getWeeksDelingquentInLastYear() {return weeksDelingquentInLastYear;}public static PaymentHistory newNull() {return new NullPaymentHistory();} }public class NullCustomer extends Customer {public boolean isNull() {return true;}public String getName() {return "occupant";}public String getPlan() {return BillingPlan.basic();}public String getHistory() {return PaymentHistory.newNull();}}public class NullPaymentHistory extends PaymentHistory {public boolean isNull() {return true;}public int getWeeksDelingquentInLastYear() {return 0;}}Customer customer = site.getCustomer(); BillingPlan plan = customer.getPlan(); //... String customerName = customer.getName(); //... int weeksDelingquent = customer.getHistory().getWeeksDelingquentInLastYear();

總結

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