Incompatible types - Found Object when using generics

Question

I'm having an issue where the following loop is throwing an error Incompatible types: Required: Workstation, Found: java.lang.Object .

With everything being typed, I can't understand why it's finding an Object instead of the typed object.

private static WorkflowProcess getWorkflowProcessByWorkstationRecursive(Collection<WorkflowProcess> workflowProcesses) {
    for (WorkflowProcess workflowProcess : workflowProcesses) {
        for (Workstation workstation : workflowProcess.getWorkstations()) //ERROR IS HERE

WorkflowProcess.java

abstract public class WorkflowProcess<WorkstationType extends Workstation> {

    private ArrayList<WorkstationType> workstationList = new ArrayList<WorkstationType>();

    public ArrayList<WorkstationType> getWorkstations() {
        return workstationList;
    }

ServerWorkflowProcess.java

abstract public class ServerWorkflowProcess extends WorkflowProcess<ServerWorkstation> {

ServerWorkstation.java

abstract public class ServerWorkstation extends Workstation<ServerWorkflowProcess> {

It gets a little complicated, but essentially the class hierarchy looks like this:

SpecificWorkflowProcess extends TypeOfWorkflowProcess<TypeOfWorkstaton> extends WorkflowProcess

SpecificWorkstation extends TypeOfWorkstation<TypeOfWorkflowProcess> extends Workstation

WorkflowProcesses to Workstation = One to many

Answer 1

The problem is that you're using the raw type WorkflowProcess . That means that the API you get is effectively the erasure of the normal API, so the return type of getWorkstations becomes just ArrayList .

You can fix this easily as:

private static WorkflowProcess getWorkflowProcessByWorkstationRecursive(
    Collection<WorkflowProcess<?>> workflowProcesses) {
  for (WorkflowProcess<?> workflowProcess : workflowProcesses) {
    ..
  }
}

By using the wildcarding, you're basically saying "I know that generics are involved here, but I don't actually mind what the WorkstationType type parameter is in the collection".

Answer 2

You would have to add the generic parameter to the type of workflowProcess in the second line:

private static WorkflowProcess getWorkflowProcessByWorkstationRecursive(Collection<WorkflowProcess> workflowProcesses) {
    for (WorkflowProcess<Workstation> workflowProcess : workflowProcesses) {
        for (Workstation workstation : workflowProcess.getWorkstations())