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Accept generic List as parameter and use it base on its type

I'm developing a website for my company, and I use Spring as my backend.
There is a situation now, where I need to use one of my Utils method twice, but for different DAOs.
In order to avoid code duplication, I was wondering how can I use Java Generics in order to make this method usable for both cases. The method just count one of the fields which is common for both DAOs.
Util method :
SeverityCount calculateSeveritiesCount(List<?> events){
if(null == events){
return new SeverityCount();
}
if(events.get(1) instanceof EventDAO){
events = (List<EventDAO>)events;
}
else if (events.get(1) instanceof EventsByAreaDAO) {
events = (List<EventsByAreaDAO>)events;
}
Map<String, Long> severityCountMap = events.stream().collect(
Collectors.groupingBy(
EventDAO::getSeverity, //It should be EventDAO or EventsByAreaDAO. both has severity field.
Collectors.counting())
);
return mapper.convertValue(severityCountMap, SeverityCount.class);
}
Event DAO:
#Data
#NoArgsConstructor
#AllArgsConstructor
#Entity
#Table(name = "events")
public class EventDAO {
#Id #Column(name = "uid")
private String uID;
private String date;
private String severity;
}
Area DAO:
#Data
#NoArgsConstructor
#AllArgsConstructor
#Entity
#Table(name = "events")
public class EventsByRegionDAO {
#Id #Column(name = "uid")
private String uID;
private String date;
private String type;
private String severity;
private String area;
private String server;
}
This is how I call it from the service:
SeverityCount severitiesCount = Utils.calculateSeveritiesCount(eventsList); //EventsList could be list of EventDAO or EventsByAreaDAO

You can change the method to
SeverityCount calculateSeveritiesCount(List<? extends SeverityCalculable> events)
where SeverityCalculable
interface SeverityCalculable {
String getSeverity(); // implemente getter in all subclasses
}
Have all your relevant classes implement this interface.
public class EventDAO implements SeverityCalculable {
// ...
#Override
public String getSeverity() {
return this.severity;
}
}
Now in your method, remove the casts and it should become something like this:
SeverityCount calculateSeveritiesCount(List<? extends SeverityCalculable> events) {
if(null == events){
return new SeverityCount();
}
Map<String, Long> severityCountMap = events.stream().collect(
Collectors.groupingBy(
SeverityCalculable::getSeverity,
Collectors.counting()
)
);
return mapper.convertValue(severityCountMap, SeverityCount.class);
}

Since both DAO's have a severity property, they could potentially implement a common interface, say SeverityAware:
public interface SeverityAware {
public String getSeverity();
}
public class EventsByRegionDAO implements SeverityAware { .. }
public class EventDAO implements SeverityAware { .. }
Further, your method could now accept subtypes of this interface:
SeverityCount calculateSeveritiesCount(List<? extends SeverityAware> events){
if (null == events){
return new SeverityCount();
}
Map<String, Long> severityCountMap = events.stream().collect(Collectors.groupingBy(
SeverityAware::getSeverity,Collectors.counting())
);
return mapper.convertValue(severityCountMap, SeverityCount.class);
}

How to transform entities extending a generic class to another entity extending another generic class

I'm developing a service oriented platform for retrieving, creating and updating entities from DB.
The point here is that every single java entity extends AbstractEntity, so for example, I have,
MyCar extends AbstractEntity implements Serializable
This AbstractEntity has common fields (such as id or audit ones).
So I have already developed a GenericReadService that, receiving a classname and a parameterMap, can read any entity and creates a EntityActionResult<T> including a List<T extends AbstractEntity>.
My problem comes when trying to transform that T type into something like <K extends GenericDTO>, as the client asking doesn't know AbstractEntity (obvious) but only GenericDTO. Doing this for safety and modularization.
So, now, I'm stuck on transforming the ListResponse<T> to a ReturnList<K extends GenericDTO>, as I don't find the way for knowing which K class should apply for each T.
This is what I actually have written:
private GenericEntityActionResult transform (EntityActionResult result) {
AsnGenericEntityActionResult r = new AsnGenericEntityActionResult();
if (result.getCode() == EntityActionResult.Code.ENTITY || result.getCode() == EntityActionResult.Code.ENTITY_LIST ) {
r.setCode(AsnGenericEntityActionResult.Code.OK);
List <? extends GenericDTO> list = new ArrayList<>();
if (result.getEntity() != null) {
//transform this entity into DTO
//<b>SOMETHING LIKE list.add(result.getEntity());</b>
} else if (result.getList() != null && !result.getList().isEmpty()) {
for (AbstractEntity a:result.getList()) {
//transform this entity into DTO
//<b>SOMETHING LIKE list.add(result.getEntity());</b>
//list.add(result.getEntity());
}
}
r.setList(list);
}
return r;
I´m obviously stuck on the SOMETHING LIKE, but can't find a proper way of doing so.
I thought about creating a abstract <T extends GenericDTO> transformToDTO() method on AbstractEntity, but can't do it since there are lots (and i mean hundreds) of Entities extending AbstractEntity and this client-service approach is a new development for some Entities, not the whole system.
Any clever ideas?

You can try to use the Java Introspection API or some more robust library on top of this API like apache commons beanutils or even more powerful bean mapping library like Dozer or something newer see
Following example demonstrating the basic technique, only raw introspection and reflection with two compatible POJO beans.
import java.beans.BeanInfo;
import java.beans.IntrospectionException;
import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.io.Serializable;
import java.lang.reflect.InvocationTargetException;
import java.util.Arrays;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;
class Tranformation {
public static void main(String[] args) {
MyCar car = new MyCar();
car.setId("id01");
car.setName("Komodo");
car.setMadeIn("Jakarta");
CarDTO dto = toDto(CarDTO.class, car);
System.out.println(dto);
}
public <E extends AbstractEntity, D extends GenericDTO> D toDto(Class<D> dtoClass, E entity) {
if (null == entity) {
throw new NullPointerException("Entity can not be null");
}
try {
final D ret = dtoClass.newInstance();
BeanInfo dtoBeanInfo = Introspector.getBeanInfo(dtoClass);
final Map<String, PropertyDescriptor> mapping = Arrays.stream(dtoBeanInfo.getPropertyDescriptors())
.collect(Collectors.toMap(PropertyDescriptor::getName, Function.identity()));
final BeanInfo entityBeanInfo = Introspector.getBeanInfo(entity.getClass());
Arrays.stream(entityBeanInfo.getPropertyDescriptors()).forEach(src -> {
if (!"class".equals(src.getName())) {
PropertyDescriptor dst = mapping.get(src.getName());
if (null != dst) {
try {
dst.getWriteMethod().invoke(ret, src.getReadMethod().invoke(entity, null));
} catch (IllegalAccessException | IllegalArgumentException | InvocationTargetException e) {
throw new IllegalStateException(e);
}
}
}
});
return ret;
} catch (InstantiationException | IntrospectionException | IllegalAccessException e) {
throw new IllegalStateException(e);
}
}
public static class GenericDTO {
private String id;
private String name;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public static class CarDTO extends GenericDTO {
private String madeIn;
public String getMadeIn() {
return madeIn;
}
public void setMadeIn(String madeIn) {
this.madeIn = madeIn;
}
#Override
public String toString() {
return String.format("CarDTO [id=%s, name=%s, madeIn=%s]", getId(), getName(), madeIn);
}
}
public static class AbstractEntity implements Serializable {
private static final long serialVersionUID = 70377433289079231L;
private String id;
private String name;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
public static class MyCar extends AbstractEntity {
private static final long serialVersionUID = 8702011501211036271L;
private String madeIn;
public String getMadeIn() {
return madeIn;
}
public void setMadeIn(String madeIn) {
this.madeIn = madeIn;
}
}
}
Outputs:
CarDTO [id=id01, name=Komodo, madeIn=Jakarta]

Java EE: Interface with a method that returns an instance of a helper class. Bad idea?

I have an interface that defines a method that returns an instance of a helper class and I was wondering if there's a better way of going about what I'm trying to do.
For example, say I have a Tag class:
public class Tag {
private long id;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
}
You can assign Tags to other objects.
The assignment is represented by TagAssign:
public abstract class TagAssign {
private Tag tag;
public Tag getTag() {
return tag;
}
public void setTag(Tag tag) {
this.tag = tag;
}
}
A new type of assignment will extend TagAssign and define what the Tag's being assigned to.
For example, a Tag assigned to a User would look like the following:
public class UserTag extends TagAssign {
private long id;
private User user;
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
}
An object that can have a tag assigned to it will implement the TagAssignable interface:
public interface TagAssignable {
public TagAssignableHelper getTagAssignableHelper();
}
TagAssignable defines a method that returns an instance of TagAssignableHelper.
At the moment, TagAssignableHelper just defines a method that assigns the Tag to the TagAssignable, returning a TagAssign.
public interface TagAssignableHelper<A extends TagAssignable, T extends TagAssign> {
public T assignTag(A assignable, Tag tag);
}
Here's what a User class that implements TagAssignable would look like:
public class User implements TagAssignable {
private long id;
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
#Override
public UserTagAssignableHelper getTagAssignableHelper() {
return new UserTagAssignableHelper();
}
}
And UserTagAssignableHelper looks like:
public class UserTagAssignableHelper implements TagAssignableHelper<User, UserTag> {
#Override
public UserTag assignTag(User assignable, Tag tag) {
UserTag userTag = new UserTag();
userTag.setUser(assignable);
userTag.setTag(tag);
return userTag;
}
}
The code in use:
public class Main {
public static void main(String[] args) {
List<TagAssignable> assignables = new ArrayList<>();
assignables.add(new User());
Tag tag = new Tag();
List<TagAssign> assignments = new ArrayList<>();
for (TagAssignable assignable : assignables) {
TagAssign assignment = assignable.getTagAssignableHelper().assignTag(assignable, tag);
assignments.add(assignment);
}
}
}
I basically want each type of TagAssignable to define how it's used inside the framework. When a developer marks a class as TagAssignable, they have to define how the TagAssign class is created, because at some point in the framework it's going to try and do just that.
I'm trying to avoid doing something like this:
if(assignable instanceof User) {
User user = (User) assignable;
UserTag userTag = new UserTag();
userTag.setTag(tag);
userTag.setUser(user);
return userTag;
}
// followed by more ifs for each type of TagAssignable
I'm open to all feedback and suggestions.

I think introducing the TagAssign class hierarchy complicates your design. What I would try to achieve is to change the TagAssignable interface to only have Set<Tag> getTags(), add(Tag tag) and remove(Tag tag) tags. If you want to achieve immutability of objects with tags, you could change the interface to have with(Tag tag) and without(Tag tag) methods that return mutated instances. This would remove the need for separate assignment classes and their corresponding helpers.

Which pattern to use to avoid code duplication with object value transformer

I want to get rid of the following code duplication within the MyFacadeBean. Consider the following situation:
public class FacadeBean implements Facade {
#EJB
private CrudService crudService;
#Inject
private FirstAssembler firstAssembler;
#Inject
private SecondAssembler secondAssembler;
#Inject
private ThirdAssembler thridAssembler;
#Inject
private FourthAssembler fourthAssembler;
#Override
public void save(FirstValue value) {
FirstEntity entity = this.firstAssembler.transformToEntity(value);
this.crudService.persist(entity);
}
#Override
public void save(SecondValue value) {
SecondEntity entity = this.secondAssembler.transformToEntity(value);
this.crudService.persist(entity);
}
#Override
public void save(ThirdValue value) {
ThirdEntity entity = this.thirdAssembler.transformToEntity(value);
this.crudService.persist(entity);
}
#Override
public void save(FourthValue value) {
FourthEntity entity = this.fourthAssembler.transformToEntity(value);
this.crudService.persist(entity);
}
}
public interface MyFacade {
void save(FirstValue value);
void save(SecondValue value);
}
With the CrudService:
public interface CrudService {
void persist(Object entity);
}
#Stateless
#Local(CrudService.class)
#TransactionAttribute(TransactionAttributeType.MANDATORY)
public class CrudServiceBean implements CrudService {
public static final String PERSISTENCE_UNIT_NAME = "my_persistence_unit";
private EntityManager entityManager;
#PersistenceContext(unitName = PERSISTENCE_UNIT_NAME)
public void setEntityManager(EntityManager entityManager) {
this.entityManager = entityManager;
}
#Override
public void persist(Object entity) {
this.entityManager.persist(entity);
}
}
With the following assemblers:
public class FirstAssembler extends AbstractAssembler<FirstEntity> {
public FirstEntity transformToEntity(FirstValue value) {
if (value == null)
return null;
FirstEntity entity = new FirstEntity();
transformAbstractValueToAbstractObject(value, entity);
entity.setFixedRate(value.getFixedRate());
entity.setStartDate(value.getStartDate());
return entity;
}
}
public class SecondAssembler extends AbstractAssembler<SecondEntity> {
public SecondEntity transformToEntity(SecondValue value) {
if (value == null)
return null;
SecondEntity entity = new SecondEntity();
transformAbstractValueToAbstractObject(value, entity);
entity.setTransactionType(value.getTransactionType());
entity.setValueDate(value.getValueDate());
return entity;
}
}
public abstract class AbstractAssembler<T extends AbstractEntity> {
protected void transformAbstractValueToAbstractObject(AbstractValue value, T object) {
object.setUniqueId(value.getUniqueId());
object.setNominalAmountValue(value.getNominalAmountValue());
}
}
With the following entities:
#Entity
public class FirstEntity extends AbstractEntity {
private static final long serialVersionUID = 1L;
#Id
#Column(name = "ID")
private Long id;
#Column(name = "START_DATE")
#Temporal(TemporalType.DATE)
private Date startDate;
#Column(name = "FIXED_RATE")
#Digits(integer = 1, fraction = 10)
private BigDecimal fixedRate;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public Date getStartDate() {
return startDate;
}
public void setStartDate(Date startDate) {
this.startDate = startDate;
}
public BigDecimal getFixedRate() {
return fixedRate;
}
public void setFixedRate(BigDecimal fixedRate) {
this.fixedRate = fixedRate;
}
}
#Entity
public class SecondEntity extends AbstractEntity {
private static final long serialVersionUID = 1L;
#Id
#Column(name = "ID")
private Long id;
#Column(name = "VALUE_DATE")
#Temporal(TemporalType.DATE)
private Date valueDate;
#Column(name = "TRANSACTION_TYPE")
#Enumerated(EnumType.STRING)
private TransactionType transactionType;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public Date getValueDate() {
return valueDate;
}
public void setValueDate(Date valueDate) {
this.valueDate = valueDate;
}
public TransactionType getTransactionType() {
return transactionType;
}
public void setTransactionType(TransactionType transactionType) {
this.transactionType = transactionType;
}
}
#MappedSuperclass
public abstract class AbstractEntity implements Serializable {
private static final long serialVersionUID = 1L;
#Column(name = "TRANSACTION_NOM_AMOUNT_VALUE")
#Digits(integer = 18, fraction = 5)
#Min(0)
private BigDecimal nominalAmountValue;
public BigDecimal getNominalAmountValue() {
return nominalAmountValue;
}
public void setNominalAmountValue(BigDecimal nominalAmountValue) {
this.nominalAmountValue = nominalAmountValue;
}
}
I tried the following approach:
public class FacadeBean implements Facade {
#Inject
private Assembler assembler;
#Inject
private AssemblerFactory assemblerFactory;
#Override
public <T extends AbstractValue> void save(T value) {
Assembler assembler = assemblerFactory.createAssembler(value);
AbstractEntity entity = assembler.transformToEntity(value);
this.crudService.persist(entity);
}
}
Problems are the AssemblerFactoryImpl and the AssemblerImpl in which I have to do instanceOf checks and castings...
Another idea would be to let the value know which transformer to use (or how to transform). But I want the value to be "dumb".
#Glenn Lane
public AbstractValue save(AbstractValue value) {
AbstractAssembler<AbstractValue, AbstractEntity> assembler = new FirstAssembler();
AbstractEntity entity = assembler.transformToEntity(value);
AbstractValue result = assembler.transformToValue(entity);
return result;
}
does not work, because of
Type mismatch: cannot convert from FirstAssembler to AbstractAssembler

I'm posting this as a separate answer, since I don't really think there's anything wrong with having a save method for every AbstractValue type.
First we'll establish your base value class for this example. I'm using an interface just so we don't muddy the waters. Your AbstractValue interface:
public interface AbstractValue
{
int getUniqueId();
double getNominalValue();
<T> T accept(AbstractValueVisitor<T> visitor);
}
And the "visitor interface":
public interface AbstractValueVisitor<T>
{
T visit(FirstValue value);
T visit(SecondValue value);
T visit(ThirdValue value);
T visit(FourthValue value);
}
I know you don't want intelligence baked into AbstractValue, but we are going to add one specification... that all concrete implementations of AbstractValue (all four) implement the accept method exactly this way:
#Override
public <T> T accept(AbstractValueVisitor<T> visitor)
{
return visitor.visit(this);
}
So that method is implemented four times: in all four value classes, exactly the same way. Because the visitor interface is aware of all concrete implementations, the appropriate method will be called for each particular value type. All three of these parts put together is the "visitor pattern".
Now we'll make an entity factory. Its job is to create the appropriate AbstractEntity when provided an AbstractValue:
public class AbstractEntityFactory
implements AbstractValueVisitor<AbstractEntity>
{
private static final AbstractEntityFactory INSTANCE;
static
{
INSTANCE = new AbstractEntityFactory();
}
// Singleton pattern
private AbstractEntityFactory()
{
}
public static AbstractEntity create(AbstractValue value)
{
if (value == null)
{
return null;
}
AbstractEntity e = value.accept(INSTANCE);
e.setNominalValue(value.getNominalValue());
e.setUniqueId(value.getUniqueId());
return e;
}
#Override
public AbstractEntity visit(FirstValue value)
{
FirstEntity entity = new FirstEntity();
// Set all properties specific to FirstEntity
entity.setFixedRate(value.getFixedRate());
entity.setStartDate(value.getStartDate());
return entity;
}
#Override
public AbstractEntity visit(SecondValue value)
{
SecondEntity entity = new SecondEntity();
// Set all properties specific to SecondEntity
entity.setTransactionType(value.getTransactionType());
entity.setValueDate(value.getValueDate());
return entity;
}
#Override
public AbstractEntity visit(ThirdValue value)
{
ThirdEntity entity = new ThirdEntity();
// Set all properties specific to ThirdEntity
return entity;
}
#Override
public AbstractEntity visit(FourthValue value)
{
FourthEntity entity = new FourthEntity();
// Set all properties specific to FourthEntity
return entity;
}
}
Now your facade implementation takes an AbstractValue, and you got that one save method you're looking for:
public class FacadeBean implements Facade
{
#EJB
private CrudService crudService;
#Override
public void save(AbstractValue value)
{
AbstractEntity entity = AbstractEntityFactory.create(value);
crudService.persist(entity);
}
}
Because your AbstractValue now follows the visitor pattern, you can do all sorts of polymorphic behavior. Such as:
public class AbstractValuePrinter implements AbstractValueVisitor<Void>
{
private final Appendable out;
public AbstractValuePrinter(Appendable out)
{
this.out = out;
}
private void print(String s)
{
try
{
out.append(s);
out.append('\n');
}
catch (IOException e)
{
throw new IllegalStateException(e);
}
}
#Override
public Void visit(FirstValue value)
{
print("I'm a FirstValue!");
print("Being a FirstValue is groovy!");
return null;
}
#Override
public Void visit(SecondValue value)
{
print("I'm a SecondValue!");
print("Being a SecondValue is awesome!");
return null;
}
#Override
public Void visit(ThirdValue value)
{
print("I'm a ThirdValue!");
print("Meh.");
return null;
}
#Override
public Void visit(FourthValue value)
{
print("I'm a ThirdValue!");
print("Derp.");
return null;
}
}
In this example, this visitor isn't returning anything... it's "doing" something, so we'll just set the return value as Void, since it's non-instantiatable. Then you print the value simply:
// (value must not be null)
value.accept(new AbstractValuePrinter(System.out));
Finally, the coolest part of the visitor pattern (in my opinion): you add FifthValue. You add the new method to your visitor interface:
T visit(FifthValue value);
And suddenly, you can't compile. You must address the lack of this handling in two places: AbstractEntityFactory and AbstractValuePrinter. Which is great, because you should consider it in those places. Doing class comparisons (with either instanceof or rinde's solution of a class-to-factory map) is likely to "miss" the new value type, and now you have runtime bugs... especially if you're doing 100 different things with these value types.
Anyhoo, I didn't want to get into this, but there you go :)

Use a generic method with a bound type parameter in order to spare yourself the repetition:
public <T extends AbstractValue> T save(T value) {...}
Within the method body, you'll be able to reference the argument value with all methods pertaining to AbstractValue.
Notes
Since your save methods seem to be overrides in this example, you might need to change the design of the parent class or interface too.
You could also use a generic class to start with (instead of a generic method in a non-necessarily generic class), depending on your use case.

I think a problem in your code is that the generic type of AbstractAssembler is that of the output of the transform method, not the input. If you change it as follows:
public abstract class AbstractAssembler<T extends AbstractValue> {
protected void transformAbstractValueToAbstractObject(AbstractEntity entity, T value) {
entity.setUniqueId(value.getUniqueId());
entity.setNominalAmountValue(value.getNominalAmountValue());
}
public abstract AbstractEntity transformToEntity(T value);
}
Then you can change the FacadeBean to the following.
public class FacadeBean {
#EJB
private CrudService crudService;
final Map<Class<?>, AbstractAssembler<?>> knownAssemblers;
FacadeBean() {
knownAssemblers = new LinkedHashMap<>();
knownAssemblers.put(FirstValue.class, new FirstAssembler());
knownAssemblers.put(SecondValue.class, new SecondAssembler());
// add more assemblers here
}
public <T extends AbstractValue> void save(T value, Class<T> type) {
#SuppressWarnings("unchecked") // safe cast
final AbstractAssembler<T> assembler =
(AbstractAssembler<T>) knownAssemblers.get(type);
final AbstractEntity entity = assembler.transformToEntity(value);
this.crudService.persist(entity);
}
}
Notice that I changed the signature of the save(..) method such that we have the type of the object that needs to be saved. With this type we can simply lookup the right assembler that should be used. And because the assembler is now generic on its input type, we can do a safe cast (be careful to keep your map consistent).
This implementation avoids duplication of code as you only need one save method. The use of the instanceof operator is prevented by changing the generic type of AbstractAssembler and storing all assemblers in a map.
The assemblers can look like this:
public class FirstAssembler extends AbstractAssembler<FirstValue> {
#Override
public FirstEntity transformToEntity(FirstValue value) {
final FirstEntity entity = new FirstEntity();
// do transformational stuff
super.transformAbstractValueToAbstractObject(entity, value);
entity.setFixedRate(value.getFixedRate());
entity.setStartDate(value.getStartDate());
return entity;
}
}
public class SecondAssembler extends AbstractAssembler<SecondValue> {
#Override
public SecondEntity transformToEntity(SecondValue value) {
final SecondEntity entity = new SecondEntity();
// do transformational stuff
super.transformAbstractValueToAbstractObject(entity, value);
return entity;
}
}
Note: I'm not familiar with Java beans so you probably have to adapt the code a little if you want to use the #Injected assemblers instead of calling the constructors directly.

You're getting close to gold-plating here, but there is a bit of reduction you can do, specifically the null-check and calling the common field-setting method from each extension.
public abstract class AbstractAssembler<V extends AbstractValue, E extends AbstractEntity>
{
public final E transformToEntity(V value)
{
if (value == null)
{
return null;
}
E entity = createEntity(value);
entity.setUniqueId(value.getUniqueId());
entity.setNominalAmountValue(value.getNominalAmountValue());
return entity;
}
/**
* #return
* Appropriate entity object, with the fields not common to all AbstractEntity
* already set
*/
protected abstract E createEntity(V value);
}
And then the extended assembler:
public class FirstAssembler extends AbstractAssembler<FirstValue, FirstEntity>
{
#Override
protected FirstEntity createEntity(FirstValue value)
{
FirstEntity entity = new FirstEntity();
entity.setFixedRate(value.getFixedRate());
entity.setStartDate(value.getStartDate());
return entity;
}
}
If you really want a single factory class to handle all your values/entities, I would look into the visitor pattern, enhanced with a generic type parameter on the visitor interface (and the entity/value accept methods return a type based on the visiting interface). I won't show an example here simply because I don't think it's warranted in your case.

You can have one save method from the point of view of the classes that save those values, but you still have to implement three individual save methods.
Implement a class with all three save methods. For example:
public class ValuePersister {
#Inject
private Assembler1 assembler1;
#Inject
private Assembler2 assembler2;
#Inject
private Assembler3 assembler3;
public Value1 save(Value1 value1, CrudService crudService) {
Entity1 entity1 = assembler1.transformToObject(value1);
crudService.persist(entity1);
return assembler1.transformToValue(entity1);
}
public Value2 save(Value2 value2, CrudService crudService) {
Entity2 entity2 = assembler2.transformToObject(value2);
crudService.persist(entity2);
return assembler2.transformToValue(entity2);
}
public Value3 save(Value3 value3, CrudService crudService) {
Entity3 entity3 = assembler3.transformToObject(value3);
crudService.persist(entity3);
return assembler3.transformToValue(entity3);
}
}
Add an abstract method to AbstractValue:
public abstract AbstractValue save(ValuePersister valuePersister, CrudService crudService);
Implement that method in each class that extends AbstractValue:
#Override
public AbstractValue save(ValuePersister valuePersister, CrudService crudService) {
return valuePersister.save(this, crudService);
}
Inject ValuePersister and implement your original generic save method:
#Inject
private ValuePersister valuePersister;
#Override
public AbstractValue save(AbstractValue value) {
return value.save(valuePersister, crudService)
}

How to forbid creation of two class objects with the same parameters?

friends and colleagues. I have following class:
public class Department {
private String departmentName;
private int moneyForDepartment;
public Department(String departmentName, int moneyForDepartment){
if (moneyForDepartment < 0){
throw new IllegalArgumentException("invalid value");
}
this.departmentName = departmentName;
this.moneyForDepartment = moneyForDepartment;
}
public Department(){
}
public String getDepartmentName() {
return departmentName;
}
public void setDepartmentName(String departmentName) {
this.departmentName = departmentName;
}
public int getMoneyForDepartment() {
return moneyForDepartment;
}
public void setMoneyForDepartment(int moneyForDepartment) {
this.moneyForDepartment = moneyForDepartment;
}
}
and for example i will create two objects of this class:
Department dep1 = new Department("Storage", 100000);
Department dep2 = new Department("Storage", 200000);
Please tell me how can i forbid to create class object in case when object of this class with the same first parameter is already exists ?

It's probably not a good idea to do that globally across your entire application (in static state), because then testing becomes difficult. Instead, create a factory for your Department class, maybe looking something like this:
public class Department {
private Department(String departmentName, int moneyForDepartment) { ... }
...
public static class Factory {
private Set<String> seenDepartmentNames = new HashSet<>();
public Department create(String departmentName, int moneyForDepartment) {
if (!seenDepartmentNames.add(departmentName)) {
throw new IllegalArgumentException("Department already created");
}
return new Department(departmentName, moneyForDepartment);
}
}
}
This forces all construction to go through the Factory, since the Department constructor is private. Just use one Factory across your application, and you can still create new ones for testing without running into problems.

You could create a static field in the Department class and keep a Set with the department names already used:
public class Department {
private static final Set<String> usedDepartmentNames = new HashSet<>();
...
}
Then, in the constructor, do:
public Department(String departmentName, int moneyForDepartment) {
if (usedDepartmentNames.contains(departmentName)) {
throw new IllegalArgumentException("Department already exists");
}
if (moneyForDepartment < 0){
throw new IllegalArgumentException("invalid value");
}
this.departmentName = departmentName;
this.moneyForDepartment = moneyForDepartment;
usedDepartmentNames.add(departmentName);
}