有一個類型碼，它會影響類的行為，但你無法通過繼承消除它

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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;// 傭金.private int commission;// 獎金.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;} }

重構：以狀態對象取代類型碼

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;// 傭金.private int commission;// 獎金.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;} }

再繼續Replace Conditional with Polymorphism（以多態取代條件表達式）

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;// 傭金.private int commission;// 獎金.private int bonus;public Employee(int type) {this.type = type;}public int payAmount() {return getType().payAmount(this);}public EmpoyeeType getType() {return type;} }

總結

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