Ben Ben - 2 years ago 107
Java Question

Chaining tasks using generics

I am trying to create an API for chaining tasks.

First, I designed a very simple interface defining a task:

public interface Task<U, V> {
U execute(V input);

is the input type and
the output type of the task.

What I whish to create is a
class that would chain a list of

For instance, I would like to execute:
Task1<TypeA, TypeB> -> Task2<TypeB, TypeC> -> Task3<TypeC, TypeD>

class would thus be a
Task<TypeA, TypeD>

So I wrote this code, which does not compile :

public class Chain<U, V> implements Task<U, V> {
List<Task<?, ?>> taskList;

public Chain() {
taskList = new LinkedList<Task<?, ?>>();

public U execute(V input) {
V currentInput = input;
U output = null;
for (Task<?, ?> task : taskList) {
output = task.execute(currentInput);
// Compile error because currentInput is of type V
// and output of type U
currentInput = output; // Compile error as well
return output;

// Other methods to add and remove tasks in the list

I do understand why it cannot compile but I have no idea how I could implement something that works and answers my problem.

Has anyone ever faced this kind of problem?

Kind regards,


Answer Source

Your Task class is pretty much the same as the Guava class Function. You could use that instead of defining your own one.

An advantage of using Function would be that you could use the helper methods in the Functions class, one of which is compose():

Function<String,MyObject1> stringToMyObject1 = ...;
Function<MyObject1, AnotherObject> myObject1ToAnotherObject = ...;
Function<String,AnotherObject> stringtoAnotherObject = Functions.compose(myObject1ToAnotherObject, stringToMyObject1);

Deeper chaining can be achieved by repeated calls to compose().

Even if you don't want to use Guava for some reason, you can take inspiration from this approach: chaining any two functions/task with known type arguments creates a new function/task with easily-calculated type arguments.

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