java 8 - store method in HashMap and get return value from method in map

Question

I want to store methods pointer in map to execute them based on a string value. Аrom what I found, I can use Map<String, Runnable> to do it, but the problem is I want to get the return value from the method.

Say I have something like this:

private Map<String, Runnable> timeUnitsMap = new HashMap<String, Runnable>() {{
    timeUnitsMap.put("minutes", () -> config.getMinutesValues());
}}

The method config.getMinutesValues() is from another class.

How can I do int value = timeUnitsMap.get("minutes").run(); or to store something else in the map (instead of Runnable) in order to get the value from the function in the map?

Answer 1

Runnable doesn't return a value. You should use Supplier or Callable instead.

The primary difference between Supplier and Callable is that Callable allows you to throw a checked exception. You then have to handle the possibility of that exception everywhere you use the Callable. Supplier is probably simpler for your use case.

You would need to change your Map<String, Runnable> to a Map<String, Supplier<Integer>>. The lambda function itself wouldn't need changing.

@assylias pointed out in a comment that you could also use Map<String, IntSupplier>. Using an IntSupplier avoids boxing your int as an Integer.

Answer 2

Use Callable instead of Runnable.

Answer 3

You will need to use Callable instead of Runnable and also, you would need to override call() method.

You can get a clue from following snippet:

private Map<String, Callable<Integer>> timeUnitsMap = new HashMap<String, Callable<Integer>>(){
        {timeUnitsMap.put("minutes", () -> call());}};

and you would need to override call() method as well,

@Override
    public Integer call() throws Exception {
        return 1;
}