Imagine you are building a product search feature for an online store. You want to help users quickly find products by filtering a list of product names. For instance, you might want to find all products that start with the letter "S". Instead of hardcoding the filtering logic, you decide to create a custom predicate interface that allows you to pass any filtering condition as a lambda expression. This makes your code flexible and reusable for different search criteria.

package com.example;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;

import java.util.List;
import java.util.Collections;

import static org.junit.jupiter.api.Assertions.*;

public class MainTest {

    @Test
    @DisplayName("Returns only strings that satisfy the predicate")
    void testFilterMatchingPredicate() {
        List<String> input = List.of("Apple", "Banana", "Avocado", "Berry");
        List<String> expected = List.of("Apple", "Avocado");
        List<String> result = Main.filterByPredicate(input, s -> s.startsWith("A"));
        assertEquals(expected, result, "Should return only items starting with 'A'");
    }

    @Test
    @DisplayName("Returns empty list if no strings match the predicate")
    void testFilterNoMatch() {
        List<String> input = List.of("Cat", "Dog", "Elephant");
        List<String> result = Main.filterByPredicate(input, s -> s.startsWith("Z"));
        assertTrue(result.isEmpty(), "Should return empty list when no items match");
    }

    @Test
    @DisplayName("Returns all items if predicate is always true")
    void testFilterAllMatch() {
        List<String> input = List.of("One", "Two", "Three");
        List<String> result = Main.filterByPredicate(input, s -> true);
        assertEquals(input, result, "Should return all items when predicate always true");
    }

    @Test
    @DisplayName("Returns empty list if input list is empty")
    void testFilterEmptyInput() {
        List<String> input = Collections.emptyList();
        List<String> result = Main.filterByPredicate(input, s -> s.length() > 0);
        assertTrue(result.isEmpty(), "Should return empty list for empty input");
    }
}


MainTest.java

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Challenge: Implement a Custom Predicate

Solution

Challenge: Implement a Custom Predicate

Solution