I have the following domain classes:
public class Task {
private Project project;
}
public class Project{
private int id;
}
And I have a list of tasks called tasks.
I would like to group Task objects by project's ids. The resulting type of the
I tried the code shown below. But it doesn't group objects by ids, but rather groups based on whether the key is the same object or not. I found duplicates, I tried to get rid of them, but I couldn't make it work.
tasks.stream().collect(Collectors.groupingBy(task::getProject));
I want something like that
tasks.stream().collect(Collectors.groupingBy(task::getProject::getId));
But unfortunately, I couldn't do that.
As @n247s has pointed out in the comments, the syntax of the Method reference you're using is incorrect - you can't chain method reference (for more details, refer to the official tutorial).
The keyExtractor Function you need can be expressed only using a Lambda expression because in order to create a Method reference to an instance method of a Particular Object you need to have a reference to it, you don't have it (it's not possible in this case).
Here's the only option:
task -> task.getProject().getId()
It would be possible to create a Method reference of this type using a method of the nested object if you had a reference to the enclosing object stored somewhere. For instance, like that:
Task foo = new Task();
Function<Task, Integer> taskToId = foo.getProject()::getId;
But since you're dialing with a stream element, it not feasible.