BAEL-5666 - Create Date from Unix timestamp in Java (#12650)

* moving SpringBootPersistenceApplication class to its own package

* from com.baeldung to com.baeldung.logging to prevent it from loading
contexts from other applications.

* moving SpringBootPersistenceApplication class to its own package

* from com.baeldung to com.baeldung.logging to prevent it from loading
contexts from other applications.

* Spring Data MongoDB - Configure Connection

Ready for revision.

* fixed tests to reflect article changes

* BAEL-5657

* reverting BAEL-5657

* reverting BAEL-5657

* ready for review

* removing bael-5366

* removing bael-5366

* removing extra blank space

* bael-5666

editor review

* editor review

* long literals
* junit 5
* private constructor for utils

* junit5 assertThat

core-java-modules/core-java-date-operations-3/src/main/java/com/baeldung/unixtime/UnixTimeUtils.java

@@ -0,0 +1,60 @@
+package com.baeldung.unixtime;
+
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.time.format.DateTimeFormatter;
+import java.util.Calendar;
+import java.util.Date;
+
+public class UnixTimeUtils {
+
+    private UnixTimeUtils() {
+    }
+
+    public static Date dateFrom(long timestamp) {
+        return new Date(timestamp);
+    }
+
+    public static Calendar calendarFrom(long timestamp) {
+        Calendar calendar = Calendar.getInstance();
+        calendar.setTimeInMillis(timestamp);
+        return calendar;
+    }
+
+    public static Instant fromNanos(long timestamp) {
+        long seconds = timestamp / 1_000_000_000;
+        long nanos = timestamp % 1_000_000_000;
+
+        return Instant.ofEpochSecond(seconds, nanos);
+    }
+
+    public static Instant fromTimestamp(long timestamp) {
+        return Instant.ofEpochMilli(millis(timestamp));
+    }
+
+    public static String format(Instant instant) {
+        LocalDateTime time = localTimeUtc(instant);
+        return time.format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
+    }
+
+    public static LocalDateTime localTimeUtc(Instant instant) {
+        return LocalDateTime.ofInstant(instant, ZoneOffset.UTC);
+    }
+
+    private static long millis(long timestamp) {
+        if (timestamp >= 1E16 || timestamp <= -1E16) {
+            return timestamp / 1_000_000;
+        }
+
+        if (timestamp >= 1E14 || timestamp <= -1E14) {
+            return timestamp / 1_000;
+        }
+
+        if (timestamp >= 1E11 || timestamp <= -3E10) {
+            return timestamp;
+        }
+
+        return timestamp * 1_000;
+    }
+}

core-java-modules/core-java-date-operations-3/src/test/java/com/baeldung/unixtime/UnixTimeUtilsUnitTest.java

@@ -0,0 +1,104 @@
+package com.baeldung.unixtime;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.temporal.ChronoField;
+import java.util.Calendar;
+import java.util.Date;
+
+import org.junit.jupiter.api.Test;
+
+class UnixTimeUtilsUnitTest {
+    private static final String AUG_16_2022_15h25m32_Z_FORMATTED = "2022-08-16T15:25:32";
+    private static final long AUG_16_2022_15h25m32_Z_NANOS = 1660663532747420283L;
+    private static final long AUGUST = 8;
+
+    private void assertInstantFieldsMatch(LocalDateTime time) {
+        assertEquals(AUGUST, time.get(ChronoField.MONTH_OF_YEAR));
+        assertEquals(16, time.get(ChronoField.DAY_OF_MONTH));
+        assertEquals(2022, time.get(ChronoField.YEAR));
+        assertEquals(15, time.get(ChronoField.HOUR_OF_DAY));
+        assertEquals(25, time.get(ChronoField.MINUTE_OF_HOUR));
+        assertEquals(32, time.get(ChronoField.SECOND_OF_MINUTE));
+    }
+
+    @Test
+    void givenMillis_whenDateFrom_thenLocalTimeMatches() {
+        long millis = AUG_16_2022_15h25m32_Z_NANOS / 1000 / 1000;
+
+        Date date = UnixTimeUtils.dateFrom(millis);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(date.toInstant());
+        assertInstantFieldsMatch(time);
+    }
+
+    @Test
+    void givenMillis_whenCalendarFrom_thenLocalTimeMatches() {
+        long millis = AUG_16_2022_15h25m32_Z_NANOS / 1000 / 1000;
+
+        Calendar calendar = UnixTimeUtils.calendarFrom(millis);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(calendar.toInstant());
+        assertInstantFieldsMatch(time);
+    }
+
+    @Test
+    void whenInstantFromNanos_thenLocalTimeMatches() {
+        Instant instant = UnixTimeUtils.fromNanos(AUG_16_2022_15h25m32_Z_NANOS);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertThat(time.toString()).startsWith(AUG_16_2022_15h25m32_Z_FORMATTED);
+    }
+
+    @Test
+    void givenWrongPrecision_whenInstantFromNanos_thenUnexpectedTime() {
+        long microseconds = AUG_16_2022_15h25m32_Z_NANOS / 1000;
+
+        Instant instant = UnixTimeUtils.fromNanos(microseconds);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertThat(time.toString()).doesNotStartWith(AUG_16_2022_15h25m32_Z_FORMATTED);
+        assertEquals("1970-01-20T05:17:43.532747420", time.toString());
+    }
+
+    @Test
+    void givenNanos_whenInstantFromTimestamp_thenLocalTimeMatches() {
+        Instant instant = UnixTimeUtils.fromTimestamp(AUG_16_2022_15h25m32_Z_NANOS);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertInstantFieldsMatch(time);
+    }
+
+    @Test
+    void givenMicroseconds_whenInstantFromTimestamp_thenLocalTimeMatches() {
+        long microseconds = AUG_16_2022_15h25m32_Z_NANOS / 1000;
+
+        Instant instant = UnixTimeUtils.fromTimestamp(microseconds);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertInstantFieldsMatch(time);
+    }
+
+    @Test
+    void givenMillis_whenInstantFromTimestamp_thenLocalTimeMatches() {
+        long millis = AUG_16_2022_15h25m32_Z_NANOS / 1000 / 1000;
+
+        Instant instant = UnixTimeUtils.fromTimestamp(millis);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertInstantFieldsMatch(time);
+    }
+
+    @Test
+    void givenSeconds_whenInstantFromTimestamp_thenLocalTimeMatches() {
+        long seconds = AUG_16_2022_15h25m32_Z_NANOS / 1000 / 1000 / 1000;
+
+        Instant instant = UnixTimeUtils.fromTimestamp(seconds);
+
+        LocalDateTime time = UnixTimeUtils.localTimeUtc(instant);
+        assertInstantFieldsMatch(time);
+    }
+}