[BAEL-3488] Timer Class (#8769)

* [BAEL-3488] Moving code to core-java where the original article code was located

* [BAEL-3488] Moved original code under timer package

core-java-modules/core-java/src/main/java/com/baeldung/timer/DatabaseMigrationTask.java

@@ -0,0 +1,19 @@
+package com.baeldung.timer;
+
+import java.util.List;
+import java.util.TimerTask;
+
+public class DatabaseMigrationTask extends TimerTask {
+    private List<String> oldDatabase;
+    private List<String> newDatabase;
+
+    public DatabaseMigrationTask(List<String> oldDatabase, List<String> newDatabase) {
+        this.oldDatabase = oldDatabase;
+        this.newDatabase = newDatabase;
+    }
+
+    @Override
+    public void run() {
+        newDatabase.addAll(oldDatabase);
+    }
+}

core-java-modules/core-java/src/main/java/com/baeldung/timer/NewsletterTask.java

@@ -0,0 +1,14 @@
+package com.baeldung.timer;
+
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.util.TimerTask;
+
+public class NewsletterTask extends TimerTask {
+    @Override
+    public void run() {
+        System.out.println("Email sent at: "
+          + LocalDateTime.ofInstant(Instant.ofEpochMilli(scheduledExecutionTime()), ZoneId.systemDefault()));
+    }
+}

core-java-modules/core-java/src/test/java/com/baeldung/timer/DatabaseMigrationTaskUnitTest.java

@@ -0,0 +1,44 @@
+package com.baeldung.timer;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.util.*;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DatabaseMigrationTaskUnitTest {
+    @Test
+    void givenDatabaseMigrationTask_whenTimerScheduledForNowPlusTwoSeconds_thenDataMigratedAfterTwoSeconds() throws Exception {
+        List<String> oldDatabase = Arrays.asList("Harrison Ford", "Carrie Fisher", "Mark Hamill");
+        List<String> newDatabase = new ArrayList<>();
+
+        LocalDateTime twoSecondsLater = LocalDateTime.now().plusSeconds(2);
+        Date twoSecondsLaterAsDate = Date.from(twoSecondsLater.atZone(ZoneId.systemDefault()).toInstant());
+
+        new Timer().schedule(new DatabaseMigrationTask(oldDatabase, newDatabase), twoSecondsLaterAsDate);
+
+        while (LocalDateTime.now().isBefore(twoSecondsLater)) {
+            assertThat(newDatabase).isEmpty();
+            Thread.sleep(500);
+        }
+        assertThat(newDatabase).containsExactlyElementsOf(oldDatabase);
+    }
+
+    @Test
+    void givenDatabaseMigrationTask_whenTimerScheduledInTwoSeconds_thenDataMigratedAfterTwoSeconds() throws Exception {
+        List<String> oldDatabase = Arrays.asList("Harrison Ford", "Carrie Fisher", "Mark Hamill");
+        List<String> newDatabase = new ArrayList<>();
+
+        new Timer().schedule(new DatabaseMigrationTask(oldDatabase, newDatabase), 2000);
+
+        LocalDateTime twoSecondsLater = LocalDateTime.now().plusSeconds(2);
+
+        while (LocalDateTime.now().isBefore(twoSecondsLater)) {
+            assertThat(newDatabase).isEmpty();
+            Thread.sleep(500);
+        }
+        assertThat(newDatabase).containsExactlyElementsOf(oldDatabase);
+    }
+}

core-java-modules/core-java/src/test/java/com/baeldung/timer/JavaTimerLongRunningUnitTest.java

@@ -1,4 +1,4 @@
-package com.baeldung;
+package com.baeldung.timer;
 
 import org.junit.Test;
 import org.slf4j.Logger;

core-java-modules/core-java/src/test/java/com/baeldung/timer/NewsletterTaskUnitTest.java

@@ -0,0 +1,33 @@
+package com.baeldung.timer;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import java.util.Timer;
+
+class NewsletterTaskUnitTest {
+    private final Timer timer = new Timer();
+
+    @AfterEach
+    void afterEach() {
+        timer.cancel();
+    }
+
+    @Test
+    void givenNewsletterTask_whenTimerScheduledEachSecondFixedDelay_thenNewsletterSentEachSecond() throws Exception {
+        timer.schedule(new NewsletterTask(), 0, 1000);
+
+        for (int i = 0; i < 3; i++) {
+            Thread.sleep(1000);
+        }
+    }
+
+    @Test
+    void givenNewsletterTask_whenTimerScheduledEachSecondFixedRate_thenNewsletterSentEachSecond() throws Exception {
+        timer.scheduleAtFixedRate(new NewsletterTask(), 0, 1000);
+
+        for (int i = 0; i < 3; i++) {
+            Thread.sleep(1000);
+        }
+    }
+}