java.lang.ClassCastException: Entity A incompatible with Entity B - spring-boot

Question

I'm trying to get proficient in generics in Java. I have some 100 entities that use the same findBy method in JPA interface. Almost all of them require a call to AwrSnapDetails so instead of adding
#ManyToOne private AwrSnapDetails awrSnapDetails; to each Entity, I've created a HelperEntity class and using #Embedded annotation. Now I have gotten to the point in coding where I can't figure out what I am doing wrong and how to go about resolving this error.
Entity
#Entity
public class AwrMemStats {
String description;
double begin_;
double end_;
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
Long id;
#Embedded
private HelperEntity helperEntity;
public AwrMemStats() {
}
public AwrMemStats(String description, double begin_, double end_, AwrSnapDetails awrSnapDetails) {
this.description = description;
this.begin_ = begin_;
this.end_ = end_;
HelperEntity h = new HelperEntity(awrSnapDetails);
}
// getters/setters removed for clarity
}
Embedded Entity
#Embeddable
public class HelperEntity implements Serializable{
private static final long serialVersionUID = 1L;
#ManyToOne
AwrSnapDetails awrSnapDetails;
public HelperEntity() {
}
public HelperEntity(AwrSnapDetails awrSnapDetails) {
super();
this.awrSnapDetails = awrSnapDetails;
}
public AwrSnapDetails getAwrSnapDetails() {
return awrSnapDetails;
}
public AwrSnapDetails setAwrSnapDetails(AwrSnapDetails awrSnapDetails) {
return this.awrSnapDetails = awrSnapDetails;
}
}
Service Class
#Service
public class HelperService<T> {
#Autowired
private HelperRepository<T> repository;
public void add(T entity) {
repository.save(entity);
}
public void add(List<T> entities) {
repository.saveAll(entities);
}
public T get(T entity) {
T t = repository.findByHelperEntityAwrSnapDetailsStartSnapIdAndHelperEntityAwrSnapDetailsInstanceDetailDbNameAndHelperEntityAwrSnapDetailsInstanceDetailDbId(
((HelperEntity) entity).getAwrSnapDetails().getStartSnapId(),
((HelperEntity) entity).getAwrSnapDetails().getInstanceDetail().getDbName(),
((HelperEntity) entity).getAwrSnapDetails().getInstanceDetail().getDbId());
//((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getStartSnapId(),
//((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getInstanceDetail().getDbName(),
//((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getInstanceDetail().getDbId());
if (t!= null) {
return t;
}
return null;
}
}
Controller
#RestController
public class HelperController<T> {
#Autowired
private HelperService<T> service;
public void add(T entity) {
service.add(entity);
}
public void add(List<T> entities) {
service.add(entities);
}
public T get(T entity) {
return service.get(entity);
}
}
Execution
getAwrSnapDetails() initilized in HelperLoader
#Component
public class LoadAwrMemStats extends HelperLoader{
#Autowired
private HelperController<AwrMemStats> controller;
public void doThis() {
AwrMemStats profile = new AwrMemStats("a",1.0,1.0,getAwrSnapDetails());
AwrMemStats s = controller.get(profile);
ANd finally the ERROR message
Exception in thread "restartedMain" java.lang.reflect.InvocationTargetException
...
...
Caused by: java.lang.ClassCastException: net.mharoon.perfmon.awr.entities.AwrMemStats incompatible with net.mharoon.perfmon.awr.entities.HelperEntity
at net.mharoon.perfmon.awr.service.HelperService.get(HelperService.java:27)
at net.mharoon.perfmon.awr.controller.HelperController.get(HelperController.java:24)
...
...
Update this code works but only for given class AwrMemStats.
public List<T> get(T entity) {
List<T> ts = repository.findByHelperEntityAwrSnapDetailsStartSnapIdAndHelperEntityAwrSnapDetailsInstanceDetailDbIdAndHelperEntityAwrSnapDetailsInstanceDetailDbName(
//((HelperEntity) entity).getAwrSnapDetails().getStartSnapId(),
//((HelperEntity) entity).getAwrSnapDetails().getInstanceDetail().getDbName(),
//((HelperEntity) entity).getAwrSnapDetails().getInstanceDetail().getDbId());
((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getStartSnapId(),
((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getInstanceDetail().getDbId(),
((AwrMemStats) entity).getHelperEntity().getAwrSnapDetails().getInstanceDetail().getDbName());
if (!ts.isEmpty()) {
return ts;
}
return null;
}

Answer 1

The reason is because you are returning an Object that is not AwrMemStats and assigning it to AwrMemStats.
A simple work around is to replace
public T get(T entity)
with
public <T extends AwrMemStats> T get(T entity)
EDIT : Another solution (which is more generic) is..
replace
public class AwrMemStats
with
public class AwrMemStats extends HelperEntity
then replace
AwrMemStats s = controller.get(profile);
with
AwrMemStats s = (AwrMemStats) controller.get(profile);