java 8 - 在HashMap中存储方法并从map中的方法获取返回值

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

使用Callable而不是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;
}