設計模式之Null 物件(Null Object)模式

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通過引用Null Object，用於取消商務邏輯中對對象的為空白判斷

類圖:

Nullable:

package com.demo.user;public interface Nullable {boolean isNull();}

Group:

package com.demo.user;public class Group implements Nullable{private int groupId;private String groupName;public int getGroupId() {return groupId;}public void setGroupId(int groupId) {this.groupId = groupId;}public String getGroupName() {return groupName;}public void setGroupName(String groupName) {this.groupName = groupName;}public boolean isNull() {return true;}}

NullGroup:

package com.demo.user;public class NullGroup extends Group {private NullGroup() {}@Overridepublic int getGroupId() {return 9999999;}@Overridepublic boolean isNull() {return false;}static Group createNull(){return new NullGroup();}}

User:

package com.demo.user;public class User {private int userId ;private String userName;private Group group;public int getUserId() {return userId;}public void setUserId(int userId) {this.userId = userId;}public String getUserName() {return userName;}public void setUserName(String userName) {this.userName = userName;}public Group getGroup() {//判斷返回一個NullGroup對象return group==null?NullGroup.createNull():group;}public void setGroup(Group group) {this.group = group;}}

UserService:

package com.demo.user;public class UserService {public void add(User user){//擷取groupIdint  groupId=user.getGroup().getGroupId();//doing something}}

設計模式之Null 物件(Null Object)模式