Predicates

Predicates.java

package uk.co.optimisticpanda.functionutils.predicates;

import java.util.function.Function;
import java.util.function.Predicate;

public class Predicates {

	public static <T, U> Predicate<T> has(Function<T, U> f, Predicate<U> p) {
		return t -> p.test(f.apply(t));
	}

}

PredicatesTest.java

package uk.co.optimisticpanda.functionutils.predicates;

import static java.util.Arrays.asList;
import static java.util.function.Predicate.isEqual;
import static java.util.stream.Collectors.toList;
import static org.assertj.core.api.Assertions.assertThat;
import static uk.co.optimisticpanda.functionutils.predicates.Predicates.has;

import java.util.List;
import java.util.function.Predicate;

import org.assertj.core.api.AbstractListAssert;
import org.assertj.core.api.ObjectAssert;
import org.junit.Test;

public class PredicatesTest {

	private final List<Customer> customers = asList(
			new Customer("sally", 1),
			new Customer("bob", 2),
			new Customer("mark", 0));
	
	@Test
	public void checkHas(){
		namesOfCustomersThat(has(Customer::getItemsInBasket, isEqual(1)))
			.containsExactly("sally");

		namesOfCustomersThat(has(Customer::getItemsInBasket, i -> i > 0))
			.containsExactly("sally", "bob");
	}
	
	private AbstractListAssert<?, List<? extends String>, String, ObjectAssert<String>> namesOfCustomersThat(Predicate<Customer> p) {
		return assertThat(customers.stream().filter(p).map(Customer::getName).collect(toList()));
	}
	
	private static class Customer {
		
		private final String name;
		private final int itemsInBasket;

		private Customer(String name, int itemsInBasket) {
			this.name = name;
			this.itemsInBasket = itemsInBasket;
		}
		
		private String getName() {
			return name;
		}
		
		private int getItemsInBasket() {
			return itemsInBasket;
		}
	}
}