一而再，再而三地检查对象是否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();
}

重构：将null值替换为null对象

注意：空对象一定是常量，它们的任何成分都不会发生变化。因此可以使用Singleton 模式实现。

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();