Related
I'm trying to get all the records from 'Messages' table but the problem is 'roles' are saving as an array in dynamo DB and I need to get all records which having the "INSTRUCTOR" role of each record.
Available records are as follows in dynamoDB.
Record 1
{
"product": "Maths",
"messageSummary": "test message",
"roles": [
"ADMIN",
"INSTRUCTOR"
],
"title": "My course1",
"createdBy": 0,
"authorName": "test author",
"id": "1"
}
Record 2
{
"product": "Maths",
"messageSummary": "test message",
"roles": [
"STUDENT"
],
"title": "My course2",
"createdBy": 0,
"authorName": "test author",
"id": "2"
}
Record 3
{
"product": "Maths",
"messageSummary": "test message",
"roles": [
"INSTRUCTOR",
"STUDENT"
],
"title": "My course3",
"createdBy": 0,
"authorName": "test author",
"id": "3"
}
Message model class as follows which referring "Messages" table
#DynamoDBTable(tableName = "Messages")
public class Message {
#Id
#ApiModelProperty(accessMode = ApiModelProperty.AccessMode.READ_ONLY, position = 1)
private String id;
/* authorName, messageSummary, title .. attributes goes here */
#ApiModelProperty(required = true, allowableValues = "maths", position = 6)
private Product product;
#ApiModelProperty(allowableValues = "student, instructor, admin", position = 7)
private List<Role> roles;
// getters and setters
}
Message repository as follows which extends CrudRepository
#EnableScan
public interface MessageRepository extends CrudRepository<Message, String> {
List<Message> findMessageByProductAndRoles(Product product, List roles); // Need to filter by given role
}
Since I need to get all the records that have INSTRUCTOR role, Record 1 and Record 3 should be in the result list. However, I can filter it only using the product value but am not able to filter using roles.
Additionally, I tried this using some keywords such as Contains, Like, In etc. but those are not succeeded for me. as per my observation, those keywords are not supporting to filter a specific value from an array.
And getting this error:
{
"timestamp": "2022-01-27T08:42:54.786+0000",
"status": 500,
"error": "Internal Server Error",
"message": "Request processing failed; nested exception is com.amazonaws.services.dynamodbv2.model.AmazonDynamoDBException: One or more parameter values were invalid: ComparisonOperator CONTAINS is not valid for SS AttributeValue type (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ValidationException; Request ID: xxxxxxx)",
"path": "/api/xxx/my route"
}
How can I achieve that and implement such conditions using the CrudRepository?
Like you pointed out, you cannot use those operators (CONTAINS, LIKE, IN) in a filter expression.
One or more parameter values were invalid: ComparisonOperator CONTAINS is not valid for SS AttributeValue type
FilterExpression criteria follows that of the KeyConditionExpression, with the addition of the not-equals operator.
It seems like your current table structure is incompatible with this access pattern. DynamoDB only works as a solution when the base table is designed around the access patterns, extending the base table's capabilities when necessary with the addition secondary indexes.
One option would be to simply query by the partition key (and sort key if applicable) and let your application do the filtering. DynamoDB FilterExpressions are applied after the initial query (or scan) is completed anyway, so you pay for all the data you've read. The filter simply returns less than that to your application. There's no real performance or cost benefit besides whatever comes from offloading the burden from your application to DyanamoDB.
Another option, rather than adjusting your key structure, would be to leverage secondary indexes. More specifically, sparse indexes (multiple in your case, one for each role), but only if you have the flexibility break out that String Set attribute into several "key friendly" values. You cannot use Sets as keys. Keep in mind, the practicality of a secondary indexes entirely depends entirely on the nature of the access pattern.
When thinking about creating an index for your base table, you should always ask yourself:
Does this query happen frequently enough to justify the cost of a secondary index?
If the answer to that is yes, you should then also ask yourself:
Am I projecting enough of the right attributes to the new index so that I can avoid charges for updating unused projections, and for fetches of missing projections?
After those questions, there is plenty to more to consider, including Secondary Indexes fundamentals and best practices.
After trying many options to query for the database, I was able to get relevant records lists by implementing GSI in DynamoDB. Since I'm using java I have used QuerySpec to get them.
QuerySpec spec = new QuerySpec()
.withKeyConditionExpression("lifeCycle = :life_cycle AND startDate < :s_date")
.withFilterExpression("contains (#user_roles, :roles))
.withNameMap(expressionAttributeNames)
.withValueMap(new ValueMap()
.withString(":life_cycle", lifeCycle)
.withString(":roles", userRole)
.with(":s_date", startDate)
);
ItemCollection<QueryOutcome> items = index.query(spec);
[I want this function to work, i.e. change a lot of data at once][1]
I want to change a lot of data at once, how?
Under this DAO to change just one data, what if I want to change a lot?
#PutMapping("/roomstatus/update/{id}")
public ResponseEntity<RoomStatus> updatRoomStatus(#PathVariable(value="id") Integer empid,#Valid #RequestBody RoomStatus RsDetails){
RoomStatus emp=roomStatusDAO.findOne(empid);
if(emp==null) {
return ResponseEntity.notFound().build();
}
emp.setRoomNumber(RsDetails.getRoomNumber());
emp.setFloor(RsDetails.getFloor());
emp.setGuestName(RsDetails.getGuestName());
emp.setRoomType(RsDetails.getRoomType());
emp.setBedType(RsDetails.getBedType());
emp.setRoomStatus(RsDetails.getRoomStatus());
emp.setConditions(RsDetails.getConditions());
RoomStatus updateRoomStatus=roomStatusDAO.save(emp);
return ResponseEntity.ok().body(updateRoomStatus);
}
The simplest solution is to pass as #RequestBody RoomStatus collection.
#PatchMapping("/roomstatus/update")
public ResponseEntity<RoomStatus> updatRoomsStatus(#Valid #RequestBody List<RoomStatus> rsDetails){
List<RoomStatus> updated = rsDetails.stream().map(rs -> updateRoomStatus(rs)).collect(Collectors.toList());
return ResponseEntity.ok().body(updated);
}
private RoomStatus updateRoomStatus(RoomStatus emp) {
if(emp==null) {
return ResponseEntity.notFound().build();
}
emp.setRoomNumber(RsDetails.getRoomNumber());
emp.setFloor(RsDetails.getFloor());
emp.setGuestName(RsDetails.getGuestName());
emp.setRoomType(RsDetails.getRoomType());
emp.setBedType(RsDetails.getBedType());
emp.setRoomStatus(RsDetails.getRoomStatus());
emp.setConditions(RsDetails.getConditions());
return roomStatusDAO.save(emp);
}
But be careful of created heap size - all the objects can be really heavy.
Take note about that question - passed objects on List will not be validated, so you can follow these answers.
PS. If modification is the same for every object - just require custom object from RequestBody that contains a list od ID and modification. Then call custom JPQL query from spring data that calls DB once instead of making n calls.
Using ModelMapper you can change a lot of data check links
https://www.appsdeveloperblog.com/java-objects-mapping-with-modelmapper/
Or
https://www.baeldung.com/entity-to-and-from-dto-for-a-java-spring-application
if you want to update your resource completely use #PutMapping to update partially use #PatchMapping. For reference check here.
First of all, your question is not specific and not formulated well enough.
But as I have understood, empId is employeeId. Employee itself has a One-to-Many relation to RoomStatus. Your method takes employeeId and roomStatusDetails and for each RoomStatus associated with employeeId, you want to set roomStatusDetails. In that case you can do the following.
First, in your roomStatusDAO interface add a method to get a collection of RoomStatus by empId: findAllByEmpId(Integer empId) and provide its implementation in respective class.
Then, the solution will be like this:
#PutMapping("/roomstatus/update/{id}")
public ResponseEntity<RoomStatus> updatRoomStatus(#PathVariable(value="id") Integer empid,#Valid #RequestBody RoomStatus RsDetails){
List<RoomStatus> updatedRoomStatuses = roomStatusDAO.findAllByEmpId(empid).stream().map(rs -> setRsDetailsAndSave(rs, RsDetails)).collect(Collectors.toList());
return updatedRoomStatuses.isEmpty() ? ResponseEntity.notFound().build() : ResponseEntity.ok().body(updatedRoomStatus);
}
private RoomStatus setRsDetailsAndSave (RoomStatus rs, RoomStatus RsDetails) {
rs.setRoomNumber(RsDetails.getRoomNumber());
rs.setFloor(RsDetails.getFloor());
rs.setGuestName(RsDetails.getGuestName());
rs.setRoomType(RsDetails.getRoomType());
rs.setBedType(RsDetails.getBedType());
rs.setRoomStatus(RsDetails.getRoomStatus());
rs.setConditions(RsDetails.getConditions());
return roomStatusDAO.save(rs);
}
tl;dr
Atempting to add an ArrayList in which Object may be an ArrayList to Persistance.
Tried to add an AttributeConverter > Failed
Plz Help
I have no idea what I am doing.
How stupid am I?
The Problem
Dependencies
spring-boot-starter-data-jpa 2.0.0
spring-boot-starter-data-mongodb 2.0.0
eclipselink 2.7.1 <- Probably don't need this one, not sure.
So here is my problem I am trying to add persistence in a Spring Boot Application for a MongoDB in this case I am using tables, the problem comes exactly on the TableRaw bean (a striped down version of Table just for persistance).
Document(collection = "rule_tables")
public class TableRaw {
#Id
private String _id;
private String key;
private String name;
private String returns;
private ArrayList<AxisRaw> axis;
private ArrayList<Object> values = new ArrayList<>();
}
Everything else is just the default constructor (without _id) and getsetters.
So everything works fine with the exception of the values ArrayList. It works fine if it just a simple ArrayList with number and whatnot however in my case I want something like what I am inserting into the database (this is done every time it runs for testing purposes and the values inserted are using the MongoRepository, it works fine)
{
"_id":"5ac20c8b8ee6e6360c8947be",
"key":"1",
"name":"Table 1",
"returns":"Number",
"axis":[
{
"name":"potato",
"values":[
{
"_id":"BottomEdge","value":0
},{
"_id":"Range",
"value":[1,2]
},{
"_id":"TopEdge",
"value":3
}
]
}
],
"values":[
[1,2,3],
[1,2,3],
[1,2,3]
],
"_class":"pt.i2s.gm.gm.rulehandler.tables.model.TableRaw"
}
(For usage in the code the axis length and number of axis matters but in this case it is completely irrelevant.)
Anyway as stated previously it inserts fine into MongoDB but when attempting to get the value the following error is presented.
org.springframework.data.mapping.MappingException: Cannot convert [1, 2, 3] of type class java.util.ArrayList into an instance of class java.lang.Object! Implement a custom Converter<class java.util.ArrayList, class java.lang.Object> and register it with the CustomConversions. Parent object was: [empty]
First thing first I don't exactly know what Parent object was: [empty] means.
Second I tried creating an AttributeConverter as such:
#Component
#Converter(autoApply = true)
public class ArrayList2ObjectConverter implements
AttributeConverter<ArrayList<Object>,Object> {
#Override
public Object convertToDatabaseColumn(ArrayList<Object> attribute) {
return attribute;
}
#SuppressWarnings("unchecked") //If you don't like it suppress it
#Override
public ArrayList<Object> convertToEntityAttribute(Object dbData) {
System.out.println("Converting...");
return (ArrayList<Object>)dbData;
}
}
And adding #Convert(converter = ArrayList2ObjectConverter.class) above the values attribute. However this wasn't even called.
For some reason I couldn't find any answers to this problem, possibly due to my bad coding and making something that is just stupid to do so nobody would do it like this cause it doesn't work.
So how do I do this? And thank you for reading.
Update regarding the Axis and Value amounts
thomi sugested something that would work if I knew from the get go what type of values the table added. I apreciate the answere however some clarification should be made regarding this.
I do not know how many Axis, and therefore nested arrays I will have, it may be 1 it may be 30.
I do not know what the class type of objects will be, it may be numbers, Strings, Booleans, dates, etc. the options are limited but still extensive.
Possible Solution Which I Do Not don't want to use
I could simply create an Object that held a string and an ArrayList which would probably work fine, however I wanted to avoid this resolution, as I don't want to add irrelevant information to the database.
Adopted Solution
By request of #user_531 I will add the solution to this problem.
As this was not working I altered my aproach to the utilization of a new object called ValueList which is simply a wrapper class for a single Object
private ArrayList<ValueList> values;
ValueList Class
public class ValueList {
public Object value;
}
This allows me to add any type of object I want to the list, this does result however in tables looking like this:
{
"key":1,
...... (Same as above)
"values": [
{
"value": [
{
"value":1
},
{
"value":2
}
]
},
{
"value": [
{
"value":3
},
{
"value":4
}
]
}
]
}
Which does look hidious but it doesn't fail anymore and allows me to read values relativelly consistently by calling the "getValue()" method or "getValueList()" method acording to the result from "isValueList()".
I think you should not map something to an object. In your DB, you will surely have an idea of what datatype there will be in your Array, In your case, try and replace with:
#Document(collection = "rule_tables")
public class TableRaw {
#Id
private String _id;
private String key;
private String name;
private String returns;
private ArrayList<AxisRaw> axis;
private List<List<Integer>> values; // no initialization.
}
This should map your structure just fine.
Let's say I have multiple Objects to be stored:
Person ------------ Employee ------------ Sales Engineer
| |
Customer Field Engineer
So: Person, Customer, Employee, Sales Engineer, Field Engineer.
I need to keep track of all of these...what is the best way to store them? In an ArrayList? A custom ArrayList?
The way they are stored also may affect future expansion - in the future, these objects might be generated by fields from an SQL Server. (Also, this is an Android App - so that could be a factor.)
You'll want a List<Person>. Your diagram suggests inheritance, so you'll want to have a collection of the super class and let polymorphism do the rest.
Your code can do this:
List<Person> people = new ArrayList<Person>();
// Any class that extends person can be added
people.add(new Customer());
people.add(new FieldEngineer());
for (Person person : people) {
System.out.println(person);
}
Your design as expressed won't allow Engineers to be Customers, or Sales engineers to go into the Field, but that's the curse of inheritance in cases like yours.
A better design, if you need the flexibility, might be to keep the Person class and assign a Person a Role in decorator fashion.
A decorator would add behavior using composition rather than inheritance, like this:
public class Customer {
private Person person;
public Customer(Person p) { this.person = p; }
public void buyIt() { // do something customer like here }
}
public class FieldEngineer {
private Person person;
public FieldEngineer(Person p) { this.person = p; }
public void fixIt() { // do something field engineer like here }
}
Use a heterogenous list -- in java you can use generics like this List <Person>
If you are uncertain about how you will need to access objects in the future you may find that a HashTable <Person> affords a wide degree of flexibility.
Since it uses key-value pairs you can retrieve a specific object quickly and the .keys() method offers a means to traverse the entire set iteratively if you find that necessary.
I am assuming all of the objects are a part of a set?
Ideally, the Person should have a get/setCustomer and the Employee should have a get/setFieldEngineer and then the structure should be something like:
class CustomerRelationship{
public Employee employee;
public SalesEngineer salesEngineer;
public Person customer;
}
If the objects are not parts of a set, but are a list of Object, then you might want to reconsider your design. Or you could use instanceof everywhere ( bad ).
I'm struggling to come up with a good way of adding a bidirectional relation in OO model. Let's say there is a Customer who can place many Orders, that is to say there is a one-to-many association between Customer and Order classes that need to be traversable in both directions: for a particular customer it should be possible to tell all orders they have placed, for an order it should be possible to tell the customer.
Here is a snippet of Java code, although the question is largely language-agnostic:
class Customer {
private Set orders = new HashSet<Order> ();
public void placeOrder (Order o) {
orders.add(o);
o.setCustomer(this);
}
}
class Order {
private Customer customer;
public void setCustomer (Customer c) {
customer = c;
}
}
What buggers me is that given the model someone could easily call:
o.setCustomer(c);
instead of correct
c.placeOrder(o);
forming unidirectional link instead of bidirectional one.
Still learning OOP, could anyone please help with what would be an idiomatic and practical way of solving this problem without resorting to "reflection" or fancy frameworks (that would anyway rely on reflection).
P.S. There is a similar question: Managing bidirectional associations in my java model, however I don't feel it answers my plea.
P.S.S. Any links to source code of real-life projects implementing business model on top of db4o are greatly appreciated!
This is a very interesting question, which has profound implications on the theory and practice of OOP. First I will tell you the quick and dirty way to (almost) accomplish what you requested. In general I don't recommend this solution, but since nobody mentioned it and (if memory doesn't fail me) it is mentioned in a book from Martin Fowler (UML Distilled), it is probably worth talking about; you can change the definition of the setCustomer method from:
public void setCustomer (Customer c) {
customer = c;
}
to:
void setCustomer (Customer c) {
customer = c;
}
and make sure Customer and Order are in the same package. If you don't specify an access modifier, setCustomer defaults to package visibility, which means it will be only accessible from classes within the same package. Obviously this does not protect you from illegitimate access from classes other than Customer within the same package. Also, your code will break if you decide to move Customer and Order in two different packages.
Package visibility is largely tolerated in common programming practice in Java; I feel like within the C++ community the friend modifier is not as tolerated as package visibility in Java, despite the fact that it serves a similar purpose. I can't really understand why, because friend is much more selective: basically for each class you can specify other friend classes and functions which will be able to access the private members of the first class.
However, there are no doubts that neither Java's package visibility nor C++'s friend are good representatives of what OOP means, and not even of what Object-Based Programming means (OOP is basically OBP plus inheritance and polymorphism; I'll use the term OOP from now on). The core aspect of OOP is that there are entities called objects, and they communicate by sending messages to each other. Objects have an internal state, but this state can only be altered by the object itself. State is typically structured i.e. it is basically a collection of fields such as name, age and orders. In most languages messages are synchronous and they can't be dropped by mistake, like a mail or a UDP packet. When you write c.placeOrder(o) it means that sender, which is this, is sending a message to c. The contents of this message are placeOrder and o.
When an object receives a message it must handle it. Java, C++, C# and a lot of other languages assume that an object can handle a message only if its class defines a method with an appropriate name and list of formal parameters. The set of the methods of a class is called its interface, and languages such as Java and C# also have an appropriate construct, namely interface to model the concept of a set of methods. The handler for the message c.placeOrder(o) is the method:
public void placeOrder(Order o) {
orders.add(o);
o.setCustomer(this);
}
The body of the method is where you write the instructions that will alter the state of object c, if necessary. In this example the orders field is modified.
This is, in essence, what OOP means. OOP was developed in the context of simulations, in which you basically have a lot of black boxes that communicate with each other, and each box is responsible for its own internal state.
Most modern languages adhere perfectly to this scheme, but only if you restrict yourself to private fields and public/protected methods. There are a few gotchas, though. For instance, within a method of class Customer you could access the private fields, such as orders, of another Customer object.
The two answers on the page you linked are actually very good, and I upvoted both. However, I think, it is completely reasonable with respect to OOP, to have a real bidirectional association, as you described. The reason is that to send a message to someone, you must have a reference to him. That is why I'll try to outline what the problem is, and why we OOP programmers sometimes struggle with this. Long story short, real OOP is sometimes tedious, and very akin to a complex formal method. But it produces code that is easier to read, modify and extend, and in general saves you from a lot of headaches. I've been wanting to write this down for a while, and I think your question is a good excuse to do it.
The main problem with OOP techniques arises whenever a group of object must alter the internal state simultaneously, as a result of an external request, dictated by business logic. For instance, when a person is hired, lots of stuff happen. 1) The employee must be configured to point to his department; 2) he must be added to the list of hired employees in the department; 3) something else must be added somewhere else, like a copy of the contract (maybe even a scan of it), insurance information and so on. The first two actions that I cited are exactly an example of establishing (and maintaining, when the employee is fired or transferred) a bidirectional association, like the one you described between customers and orders.
In procedural programming Person, Department and Contract would be structures, and a global procedure like hirePersonInDepartmentWithContract associated to the click of a button in an user interface would manipulate 3 instances of these structures by the means of three pointers. The entire business logic is inside this function, and it must take into consideration every possible special case while updating the state of these three objects. For instance, there is the possibility that when you click the button to hire someone, he is already employed in another department, or even worse in the same. And computer scientists know that special cases are bad. Hiring a person is basically a very complex use case, with lots of extensions which don't happen very often, but that must be considered.
Real OOP mandates instead that objects must exchange messages to accomplish this task. The business logic is split among the responsibilities of several objects. CRC cards are an informal tool to study business logic in OOP.
To get from the valid state where John is unemployed, to the other valid state where he is a project manager at the R&D department, it is necessary to go through a number of invalid states, at least one. So there is an initial state, an invalid state and a final state, and at least two messages exchanged between a person and a department. You can also be sure that one message must be received by the department, to give it a chance of altering its internal state, and another one must be received by the person, for the same reason. The middle state is invalid in the sense that it doesn't really exist in the real world, or maybe exists but is of no importance. However, the logical model in your application must in a way keep track of it.
Basically the idea is that when the human resource guy fills the "New Employee" JFrame and clicks the "Hire" JButton, the selected department is retrieved from a JComboBox, which in turn may have been populated from a database, and a new Person is created based on the information inside the various JComponents. Maybe a job contract is created containing at least the name of the position and the salary. Finally there is appropriate business logic that wires all the objects together and triggers updates for all the states. This business logic is triggered by a method called hire defined in class Department, which takes as arguments a Person and a Contract. All of this may happen in the ActionListener of the JButton.
Department department = (Department)cbDepartment.getSelectedItem();
Person person = new Person(tfFirstName.getText(), tfLastName.getText());
Contract contract = new Contract(tfPositionName.getText(), Integer.parseInt(tfSalary.getText()));
department.hire(person, contract);
I would like to stress what's going on at line 4, in OOP terms; this (which in our case is the ActionListener, is sending a message to department, saying they must hire person under contract. Let's have a look at a plausible implementation of these three classes.
Contract is a very simple class.
package com.example.payroll.domain;
public class Contract {
private String mPositionName;
private int mSalary;
public Contract(String positionName, int salary) {
mPositionName = positionName;
mSalary = salary;
}
public String getPositionName() {
return mPositionName;
}
public int getSalary() {
return mSalary;
}
/*
Not much business logic here. You can think
about a contract as a very simple, immutable type,
whose state doesn't change and that can't really
answer to any message, like a piece of paper.
*/
}
Person is way more interesting.
package com.example.payroll.domain;
public class Person {
private String mFirstName;
private String mLastName;
private Department mDepartment;
private boolean mResigning;
public Person(String firstName, String lastName) {
mFirstName = firstName;
mLastName = lastName;
mDepartment = null;
mResigning = false;
}
public String getFirstName() {
return mFirstName;
}
public String getLastName() {
return mLastName;
}
public Department getDepartment() {
return mDepartment;
}
public boolean isResigning() {
return mResigning;
}
// ========== Business logic ==========
public void youAreHired(Department department) {
assert(department != null);
assert(mDepartment != department);
assert(department.isBeingHired(this));
if (mDepartment != null)
resign();
mDepartment = department;
}
public void youAreFired() {
assert(mDepartment != null);
assert(mDepartment.isBeingFired(this));
mDepartment = null;
}
public void resign() {
assert(mDepartment != null);
mResigning = true;
mDepartment.iResign(this);
mDepartment = null;
mResigning = false;
}
}
Department is quite cool.
package com.example.payroll.domain;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
public class Department {
private String mName;
private Map<Person, Contract> mEmployees;
private Person mBeingHired;
private Person mBeingFired;
public Department(String name) {
mName = name;
mEmployees = new HashMap<Person, Contract>();
mBeingHired = null;
mBeingFired = null;
}
public String getName() {
return mName;
}
public Collection<Person> getEmployees() {
return mEmployees.keySet();
}
public Contract getContract(Person employee) {
return mEmployees.get(employee);
}
// ========== Business logic ==========
public boolean isBeingHired(Person person) {
return mBeingHired == person;
}
public boolean isBeingFired(Person person) {
return mBeingFired == person;
}
public void hire(Person person, Contract contract) {
assert(!mEmployees.containsKey(person));
assert(!mEmployees.containsValue(contract));
mBeingHired = person;
mBeingHired.youAreHired(this);
mEmployees.put(mBeingHired, contract);
mBeingHired = null;
}
public void fire(Person person) {
assert(mEmployees.containsKey(person));
mBeingFired = person;
mBeingFired.youAreFired();
mEmployees.remove(mBeingFired);
mBeingFired = null;
}
public void iResign(Person employee) {
assert(mEmployees.containsKey(employee));
assert(employee.isResigning());
mEmployees.remove(employee);
}
}
The messages I defined have, at the very least, very pittoresque names; in a real application you might not want to use names like these, but in the context of this example they help to model the interactions between objects in a meaningful and intuitive way.
Department can receive the following messages:
isBeingHired: the sender wants to know whether a particular person is in the process of being hired by the department.
isBeingFired: the sender wants to know whether a particular person is in the process of being fired by the department.
hire: the sender wants the department to hire a person with a specified contract.
fire: the sender wants the department to fire an employee.
iResign: the sender is likely an employee, and is telling the department that he is resigning.
Person can receive the following messages:
youAreHired: the department sends this message to inform the person that he is hired.
youAreFired: the department sends this message to inform the employee that he is fired.
resign: the sender wants the person to resign from his current position. Note that an employee who was hired by another department can send the resign message to himself in order to quit the old job.
The fields Person.mResigning, Department.isBeingHired, Department.isBeingFired are what I use to encode the aforementioned invalid states: when either one of them is "non-zero", the application is in an invalid state, but is on its way to a valid one.
Also note that there are no set methods; this contrasts with the common practice of working with JavaBeans. JavaBeans are in essence very similar to C structures, because they tend to have a set/get (or set/is for boolean) pair for every private property. However they do allow for validation of set, for instance you can check that a String being passed to a set method is not-null and not empty and eventually raise an exception.
I wrote this little library in less than a hour. Then I wrote a driver program and it worked correctly with the JVM -ea switch (enable assertions) at the very first run.
package com.example.payroll;
import com.example.payroll.domain.*;
public class App {
private static Department resAndDev;
private static Department production;
private static Department[] departments;
static {
resAndDev = new Department("Research & Development");
production = new Department("Production");
departments = new Department[] {resAndDev, production};
}
public static void main(String[] args) {
Person person = new Person("John", "Smith");
printEmployees();
resAndDev.hire(person, new Contract("Project Manager", 3270));
printEmployees();
production.hire(person, new Contract("Quality Control Analyst", 3680));
printEmployees();
production.fire(person);
printEmployees();
}
private static void printEmployees() {
for (Department department : departments) {
System.out.println(String.format("Department: %s", department.getName()));
for (Person employee : department.getEmployees()) {
Contract contract = department.getContract(employee);
System.out.println(String.format(" %s. %s, %s. Salary: EUR %d", contract.getPositionName(), employee.getFirstName(), employee.getLastName(), contract.getSalary()));
}
}
System.out.println();
}
}
The fact that it worked is not the cool thing though; the cool thing is that only the hiring or firing department is authorized to send youAreHired and youAreFired messages to the person that is being hired or fired; in a similar way, only a resigning employee can send the iResign message to its department, and only to that department; any other illegitimate message sent from main would trigger an assertion. In a real program you would use exceptions instead of assertions.
Is all of this overkill? This example is admittedly a little extreme. But I feel like this is the essence of OOP. Objects must cooperate to achieve a certain goal i.e. changing the global state of the application according to predetermined pieces of business logic, in this case hiring, firing and resign. Some programmers think that business problems are not suited for OOP, but I disagree; business problems are basically workflows, and they are very simple tasks by themselves, but they involve a lot of actors (i.e. objects), which communicate through messages. Inheritance, polymorphism, and all the patterns are welcome extensions, but they are not the base of the object-oriented process. In particular, reference-based associations are often preferred to implementation inheritance.
Note that by using static analysis, design-by-contract and automatic theorem provers, you would be able to verify that your program is correct, for any possible input, without running it. OOP is the abstraction framework that enables you to think this way. It is not necessarily more compact than procedural programming, and it does not automatically lead to code reuse. But I insist that it is easier to read, modify and extend; let's have a look at this method:
public void youAreHired(Department department) {
assert(department != null);
assert(mDepartment != department);
assert(department.isBeingHired(this));
if (mDepartment != null)
resign();
mDepartment = department;
}
The business logic relevant to the use case is the assignment at the end; the if statement is an extension, a special case that only occurs when the person is already an employee in another department. The first three assertions describe forbidden special cases. If one day we want to forbid this automatic resign from the previous department we only need to modify this method:
public void youAreHired(Department department) {
assert(department != null);
assert(mDepartment == null);
assert(department.isBeingHired(this));
mDepartment = department;
}
We can also extend the application by making youAreHired a boolean function, which returns true only if the old department is ok with the new hiring. Obviously we may need to change something else, in my case I made Person.resign a boolean function, which in turn may require Department.iResign to be a boolean function:
public boolean youAreHired(Department department) {
assert(department != null);
assert(mDepartment != department);
assert(department.isBeingHired(this));
if (mDepartment != null)
if (!resign())
return false;
mDepartment = department;
return true;
}
Now the current employeer has the final word in determining whether an employee can be transferred to another department. The current department could delegate the responsibility of determining this to a Strategy which may in turn take into consideration the projects in which the employee is involved, their deadlines and various contractual constraints.
In essence, adding an order to a customer really is part of business logic. If a bidirectional association is required, and reflection is not an option, and none of the solutions proposed on this and the linked question are satisfactory, I think the only solution is something like this.
first, unless you plan on moving orders between customers, I think you shouldn't provide a setCustomer() method, the customer should be a parameter for the constructor and leave it unchanged.
then, the constructor shouldn't be accessible for the user, only use the factory method of Owner.
There is no single answer. It really depends on the classes involved. In your case, you obviously don't want to give people the option of doing something invalid so I would get rid of Order.SetCustomer.
That may not always be the case though. Like I said, it depends on the classes involved.
If you are maintaining the bidirectional relationship in Customer.placeOrder(Order), why don't you do the same thing in Order.setCustomer(Customer)?
class Order {
private Customer customer;
public void setCustomer (Customer c) {
customer = c;
c.getOrders().add(this);
// ... or Customer.placeOrder(this)
}
}
It seems like duplicating code but it solves the problem. The simpler thing to do though is to avoid bidirectional relationships where possible.
I think the best way in this case is to delegate the responsibility for wiring to another class:
class OrderManager {
void placeOrder(Customer c, Order o){
c.addOrder(o);
o.setCustomer(c);
}
}
class Customer {
private Set<Order> orders = new LinkedHashSet<Order>();
void addOrder(Order o){ orders.add(o); }
}
class Order {
private Customer customer;
void setCustomer(Customer c){ this.customer=c; }
}