有一个不可变的类型码，它会影响类的行为

public class Employee {static final int ENGINNER = 0;static final int SALESMAN = 1;static final int MANAGER = 2;private int type;public Employee(int type) {this.type = type;}public int getType() {return type;}
}

重构：以子类取代类型码

public class Employee {static final int ENGINNER = 0;static final int SALESMAN = 1;static final int MANAGER = 2;private int type;public Employee(int type) {this.type = type;}public static Employee create(int type) {switch(type) {case ENGINNER: return new Engineer();case SALESMAN: return new Salesman();case MANAGER: return new Manager();default:throw new IllegalArgumentException("Incorrect type code value.");}}public abstract int getType();
}public class Engineer extends Employee {public int getType() {return Employee.ENGINNER;}
}