move the code to unit tests

java-dates-2/src/main/java/com/baeldung/convert/ConvertDateTime.java

@@ -1,45 +0,0 @@
-package com.baeldung.convert;
-
-import org.joda.time.Instant;
-
-import java.time.LocalDateTime;
-import java.time.ZoneId;
-import java.time.ZonedDateTime;
-import java.util.Calendar;
-import java.util.Date;
-
-public class ConvertDateTime {
-
-    public static long coreDate() {
-        Date date = new Date();
-        return date.getTime();
-    }
-
-    public static long calendar() {
-        Calendar calendar = Calendar.getInstance();
-        calendar.setTime(new Date());
-        return calendar.getTimeInMillis();
-    }
-
-    public static long jodaInstant() {
-        Instant jodaInstant = Instant.now();
-        return jodaInstant.getMillis();
-    }
-
-    public static long jodaDateTime() {
-        org.joda.time.DateTime jodaDateTime = new org.joda.time.DateTime();
-        return jodaDateTime.getMillis();
-    }
-
-    public static long java8Instant() {
-        return java.time.Instant.now().toEpochMilli();
-    }
-
-    public static long java8LocalDateTime() {
-
-        LocalDateTime localDateTime = LocalDateTime.now();
-        ZoneId id = ZoneId.systemDefault();
-        ZonedDateTime zdt = ZonedDateTime.of(localDateTime, id);
-        return zdt.toInstant().toEpochMilli();
-    }
-}

java-dates-2/src/test/java/com/baeldung/convert/ConvertDateTimeUnitTest.java

@@ -1,42 +1,59 @@
 package com.baeldung.convert;
 
+import org.joda.time.Instant;
+import org.junit.Assert;
 import org.junit.Test;
 
+import java.time.LocalDateTime;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
+import java.util.Calendar;
+import java.util.Date;
+
 public class ConvertDateTimeUnitTest {
 
+    public final long millis = 1556175797428L;
+
     @Test
-    public void givenLocalDateTime_WhenGetMillis() {
-        long millis = ConvertDateTime.java8LocalDateTime();
-        System.out.println("LocalDateTime in milliseconds : " + millis);
+    public void givenLocalDateTime_WhenToEpochMillis_ThenMillis() {
+        ZoneId id = ZoneId.systemDefault();
+
+        LocalDateTime localDateTime =
+                LocalDateTime.ofInstant(java.time.Instant.ofEpochMilli(millis), id);
+
+        ZonedDateTime zdt = ZonedDateTime.of(localDateTime, id);
+
+        Assert.assertEquals(millis, zdt.toInstant().toEpochMilli());
     }
 
     @Test
-    public void givenJava8Instant_WhenGetMillis() {
-        long millis = ConvertDateTime.java8Instant();
-        System.out.println("Java 8 Instant in milliseconds : " + millis);
+    public void givenJava8Instant_WhenGToEpochMillis_ThenMillis() {
+        java.time.Instant instant = java.time.Instant.ofEpochMilli(millis);
+        Assert.assertEquals(millis, instant.toEpochMilli());
     }
 
     @Test
-    public void givenDate_WhenGetMillis() {
-        long millis = ConvertDateTime.coreDate();
-        System.out.println("Core Date in milliseconds : " + millis);
+    public void givenDate_WhenGetTime_ThenMillis() {
+        Date date = new Date(millis);
+        Assert.assertEquals(millis, date.getTime());
     }
 
     @Test
-    public void givenCalendar_WhenGetMillis() {
-        long millis = ConvertDateTime.calendar();
-        System.out.println("Calendar in milliseconds : " + millis);
+    public void givenCalendar_WhenGetTimeInMillis_ThenMillis() {
+        Calendar calendar = Calendar.getInstance();
+        calendar.setTime(new Date(millis));
+        Assert.assertEquals(millis, calendar.getTimeInMillis());
     }
 
     @Test
-    public void givenJodaInstant_WhenGetMillis() {
-        long millis = ConvertDateTime.jodaInstant();
-        System.out.println("JODA Instant in milliseconds : " + millis);
+    public void givenJodaInstant_WhenGetMillis_ThenMillis() {
+        Instant jodaInstant = Instant.ofEpochMilli(millis);
+        Assert.assertEquals(millis, jodaInstant.getMillis());
     }
 
     @Test
-    public void givenJODADateTime_WhenGetMillis() {
-        long millis = ConvertDateTime.jodaDateTime();
-        System.out.println("JODA DateTime in milliseconds : " + millis);
+    public void givenJODADateTime_WhenGetMillis_ThenMillis() {
+        org.joda.time.DateTime jodaDateTime = new org.joda.time.DateTime(millis);
+        Assert.assertEquals(millis, jodaDateTime.getMillis());
     }
 }