有一个类型码，它会影响类的行为，但你无法通过继承消除它

 

public class Employee {static final int ENGINNER = 0;static final int SALESMAN = 1;static final int MANAGER = 2;private int type;// 月薪.private int montylySalary;// 佣金.private int commission;// 奖金.private int bonus;public Employee(int type) {this.type = type;}public int payAmount() {switch(getType()) {case ENGINNER: return montylySalary;case SALESMAN: return montylySalary + commission;case MANAGER: return montylySalary + bonus;default:throw new IllegalArgumentException("Incorrect Employee.");}}public int getType() {return type;}
}

重构：以状态对象取代类型码

public abstract class EmpoyeeType {static final int ENGINNER = 0;static final int SALESMAN = 1;static final int MANAGER = 2;public abstract int getTypeCode();public static EmpoyeeType newType(int code) {switch (code) {case ENGINNER: return new Engineer();case SALESMAN: return new Salesman();case MANAGER: return new Manager();default:throw new IllegalArgumentException("Incorrect Employee Code.");}}
}public class Engineer extends EmpoyeeType {public int getTypeCode() {return Employee.ENGINNER;}
}public class Manager extends EmpoyeeType {public int getTypeCode() {return Employee.MANAGER;}
}public class Salesman extends EmpoyeeType {public int getTypeCode() {return Employee.SALESMAN;}
}public class Employee {private EmpoyeeType type;// 月薪.private int montylySalary;// 佣金.private int commission;// 奖金.private int bonus;public Employee(int type) {this.type = type;}public int payAmount() {switch(getType()) {case EmpoyeeType.ENGINNER: return montylySalary;case EmpoyeeType.SALESMAN: return montylySalary + commission;case EmpoyeeType.MANAGER: return montylySalary + bonus;default:throw new RuntimeException("Incorrect Employee.");}}public EmpoyeeType getType() {return type;}
}

再继续Replace Conditional with Polymorphism（以多态取代条件表达式）

public abstract class EmpoyeeType {static final int ENGINNER = 0;static final int SALESMAN = 1;static final int MANAGER = 2;public abstract int getTypeCode();public abstract int payAmount(Employee employee);public static EmpoyeeType newType(int code) {switch (code) {case ENGINNER: return new Engineer();case SALESMAN: return new Salesman();case MANAGER: return new Manager();default:throw new IllegalArgumentException("Incorrect Employee Code.");}}
}public class Engineer extends EmpoyeeType {public int getTypeCode() {return Employee.ENGINNER;}public int payAmount(Employee employee) {return employee.getMontylySalary();}
}public class Manager extends EmpoyeeType {public int getTypeCode() {return Employee.MANAGER;}public int payAmount(Employee employee) {return employee.getMontylySalary() + employee.getBonus();}
}public class Salesman extends EmpoyeeType {public int getTypeCode() {return Employee.SALESMAN;}public int payAmount(Employee employee) {return employee.getMontylySalary() + employee.getCommission();}
}public class Employee {private EmpoyeeType type;// 月薪.private int montylySalary;// 佣金.private int commission;// 奖金.private int bonus;public Employee(int type) {this.type = type;}public int payAmount() {return getType().payAmount(this);}public EmpoyeeType getType() {return type;}
}