CODEWITHSUNDEEP
javalambdajava-8
bencollier
bencollier

Reputation: 63

Correct way of declaring a lambda inside a HashMap

I'm trying to declare a HashMap which will take a string and return a Callable.

I have:

Map<String, Callable<String>> commands = new HashMap<>();

commands.put("get", (String item) -> get(item));

where get is a function which returns void.

But I'm getting

"Bad return type in Lambda expression: Void cannot be converted into a string."

Why is it expecting the function to return a string? I want to pass the lambda a String and get void back.

Upvotes: 6

Views: 10271

Answers (2)

Mena
Mena

Reputation: 48404

The correct syntax for a putting a lambda representation of a Callable<String> as value into your map would be:

Map<String, Callable<String>> commands = new HashMap<>();
commands.put("get", () -> "some value");

That is because the functional interface Callable<T> has a single method returning a T value.

Upvotes: 0

Eran
Eran

Reputation: 393801

A Callable<String> has a method that returns a String (V call() throws Exception), so you can't use a lambda with void return type. You can use a Consumer<String> instead.

Map<String, Consumer<String>> commands = new HashMap<>();

commands.put("get", (String item) -> get(item));

Upvotes: 8

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