CODEWITHSUNDEEP
javalambdajava-8functional-interface
Apollocy
Apollocy

Reputation: 77

Mapping Lambda Expressions that return Booleans

I'm trying to map a String to Lambda Expressions. The expressions take a variable(map) and either return false or run a getOrDefault on the given map variable. But for some reason, I am getting errors.

Map<String, Runnable> order_function = new HashMap<>();
order_function.put("AlternativesValues", (Map x) -> { return false; });
order_function.put("AlternativesConstituent", (Map x) -> { x.getOrDefault("orderCloseCons", false); });

Upvotes: 0

Views: 1517

Answers (3)

the_tech_maddy
the_tech_maddy

Reputation: 597

If they mean to be constants, you can use enum like below.

enum OrderFunctions {
    ALTERNATIVES_VALUES("AlternativesValues", map -> false),
    ALTERNATIVES_CONSTITUENT("AlternativesConstituent", map -> map.getOrDefault("orderCloseCons", false));

    private final String name;
    private final Function<Map<String, Boolean>, Boolean> orderFunction;

    OrderFunctions(String name, Function<Map<String, Boolean>, Boolean> orderFunction) {
        this.name = name;
        this.orderFunction = orderFunction;
    }

    public String getName() {
        return name;
    }

    public Function<Map<String, Boolean>, Boolean> getOrderFunction() {
        return orderFunction;
    }

}

and you can use it like below.

OrderFunctions.ALTERNATIVES_CONSTITUENT.getOrderFunction().apply(someMap);

Upvotes: 0

Nikolas
Nikolas

Reputation: 44466

The Runnable doesn't return anything. If you expand the lambda to the anonymous class implementation, you will see the void is the return type:

Runnable runnable = new Runnable() {
    @Override
    public void run() {
        // implementation
    }
};

This is in conflict with your lambda> (Map x) -> { return false; }.

Since you need an expression which takes a Map and returns a boolean, then you need Predicate<Map<? ,Boolean>> (keep the wildcard ? or replace it with whatever is needed):

Map<String, Predicate<Map<? ,Boolean>>> order_function = new HashMap<>();
order_function.put("AlternativesValues", x -> false);
order_function.put("AlternativesConstituent", map -> map.getOrDefault("orderCloseCons", false));

Upvotes: 2

Adrian
Adrian

Reputation: 3134

perhaps you wanted to use Function instead of Runnable:

Map<String, Function<Map<String, Boolean>, Boolean>> order_function = new HashMap<>();
order_function.put("AlternativesValues", x -> false);
order_function.put("AlternativesConstituent", map -> map.getOrDefault("orderCloseCons", false));

//...
boolean alternativesValues = order_function.get("AlternativesValues").apply(someMap);

Upvotes: 0

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