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Java设计模式七大原则之迪米特法则详解

2024-04-02 19:55

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定义

迪米特法则（Law of Demeter, LoD）是1987年秋天由lan holland在美国东北大学一个叫做迪米特的项目设计提出的，它要求一个对象应该对其他对象有最少的了解，所以迪米特法则又叫做最少知识原则。

案例

需求

有一个公司，下属有各个部门，现要求打印出公司员工的ID和部门员工的ID

方案一

定义公司员工类


@Data
public class CompanyEmployee {
    private String id;
}

定义公司管理类


public class CompanyManager {

    public List<CompanyEmployee> getCompanyEmployee(){
        List<CompanyEmployee> list = new ArrayList<CompanyEmployee>();
        for (int i = 0; i < 5; i++) {
            CompanyEmployee companyEmployee = new CompanyEmployee();
            companyEmployee.setId("公司员工，id是" + i);
            list.add(companyEmployee);
        }
        return list;
    }
}

定义部门员工类


@Data
public class DempartEmployee {
    private String id;
}

定义部门管理类


public class DepartmentManager {

    
    public List<DempartEmployee> getDempartEmployee(){
        List<DempartEmployee> list = new ArrayList<DempartEmployee>();
        for (int i = 0; i < 5; i++) {
            DempartEmployee dempartEmployee = new DempartEmployee();
            dempartEmployee.setId("部门员工，id是" + i);
            list.add(dempartEmployee);
        }
        return list;
    }

    public void pringAllEmployee(CompanyManager companyManager){
        List<DempartEmployee> dempartEmployeeList = getDempartEmployee();
        for (DempartEmployee dempartEmployee : dempartEmployeeList) {
            System.out.println(dempartEmployee.getId());
        }
        System.out.println("--------------------------------");
        List<CompanyEmployee> companyEmployeeList = companyManager.getCompanyEmployee();
        for (CompanyEmployee companyEmployee : companyEmployeeList) {
            System.out.println(companyEmployee.getId());
        }
    }

}

定义测试类


public class Test1 {
    public static void main(String[] args) {
        new DepartmentManager().pringAllEmployee(new CompanyManager());
    }
}

查看结果

方案二

该方案与方案一的区别只有两个地方，具体如下：修改后的公司管理类，新增打印公司员工id的方法


public class CompanyManagerNew {

    
    public List<CompanyEmployee> getCompanyEmployee(){
        List<CompanyEmployee> list = new ArrayList<CompanyEmployee>();
        for (int i = 0; i < 5; i++) {
            CompanyEmployee companyEmployee = new CompanyEmployee();
            companyEmployee.setId("公司员工，id是" + i);
            list.add(companyEmployee);
        }
        return list;
    }

    
    public void printCompanyEmployee(){
        List<CompanyEmployee> list = getCompanyEmployee();
        for (CompanyEmployee companyEmployee: list) {
            System.out.println(companyEmployee.getId());
        }
    }
}

修改后的部门管理类


public class DepartmentManagerNew {

    
    public List<DempartEmployee> getDempartEmployee(){
        List<DempartEmployee> list = new ArrayList<DempartEmployee>();
        for (int i = 0; i < 5; i++) {
            DempartEmployee dempartEmployee = new DempartEmployee();
            dempartEmployee.setId("部门员工，id是" + i);
            list.add(dempartEmployee);
        }
        return list;
    }

    public void pringAllEmployee(CompanyManagerNew companyManagerNew){
        List<DempartEmployee> dempartEmployeeList = getDempartEmployee();
        for (DempartEmployee dempartEmployee : dempartEmployeeList) {
            System.out.println(dempartEmployee.getId());
        }
        System.out.println("--------------------------------");
        companyManagerNew.printCompanyEmployee();
    }

}

测试类


public class Test2 {
    public static void main(String[] args) {
        new DepartmentManagerNew().pringAllEmployee(new CompanyManagerNew());
    }
}

结果