Create custom Predicate with Set<String> and String as parameter

问题

I have a String as "ishant" and a Set<String> as ["Ishant", "Gaurav", "sdnj"] . I need to write the Predicate for this. I have tried as below code, but it is not working

Predicate<Set<String>,String> checkIfCurrencyPresent = (currencyList,currency) -> currencyList.contains(currency);

How can I create a Predicate which will take Set<String> and String as a parameter and can give the result?

回答1:

A Predicate<T> which you're currently using represents a predicate (boolean-valued function) of one argument.

You're looking for a BiPredicate<T,U> which essentially represents a predicate (boolean-valued function) of two arguments.

BiPredicate<Set<String>,String>  checkIfCurrencyPresent = (set,currency) -> set.contains(currency);

or with method reference:

BiPredicate<Set<String>,String> checkIfCurrencyPresent = Set::contains;

回答2:

If you were to stick with using Predicate, use something similar as :

Set<String> currencies = Set.of("Ishant", "Gaurav", "sdnj");
String input = "ishant";
Predicate<String> predicate = currencies::contains;
System.out.print(predicate.test(input)); // prints false

The primary difference between the BiPredicate and Predicate would be their test method implementation. A Predicate would use

public boolean test(String o) {
    return currencies.contains(o);
}

and a BiPredicate would instead use

public boolean test(Set<String> set, String currency) {
    return set.contains(currency);
}

回答3:

Aomine's answer is complete. using of BiFunction<T, U, R> is another way:

BiFunction<Set<String>,String,Boolean> checkIfCurrencyPresent = Set::contains;

来源：https://stackoverflow.com/questions/54040486/create-custom-predicate-with-setstring-and-string-as-parameter