Imagine you are building a notification system for a messaging app. Whenever a new message arrives, you want to process it—perhaps by displaying a notification, logging it, or triggering some other action. Instead of hard-coding the response, you decide to use a lambda expression to handle the event, making your system flexible and easy to extend. In this scenario, a lambda can be passed as an event handler to process each new message as it arrives.

package com.example;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.DisplayName;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Consumer;

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

public class MainTest {

    @Test
    @DisplayName("handleNewMessage applies lambda to message")
    void testHandleNewMessageLambdaCalled() {
        AtomicReference<String> result = new AtomicReference<>(null);
        String testMessage = "Test message!";
        Consumer<String> consumer = result::set;
        com.example.Main.handleNewMessage(testMessage, consumer);
        assertEquals(testMessage, result.get(), "Lambda should receive the exact message passed to handleNewMessage");
    }

    @Test
    @DisplayName("handleNewMessage works with different messages")
    void testHandleNewMessageDifferentInput() {
        AtomicReference<String> result = new AtomicReference<>(null);
        String testMessage = "Another message";
        Consumer<String> consumer = result::set;
        com.example.Main.handleNewMessage(testMessage, consumer);
        assertEquals(testMessage, result.get(), "Lambda should receive the exact message passed to handleNewMessage");
    }
}

MainTest.java

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Challenge: Event Handler with Lambda

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