BAEL-9472

core-java-modules/core-java-8-datetime-4/src/main/java/com/baeldung/dateinstant/DateInstantConverter.java

@@ -0,0 +1,24 @@
+package com.baeldung.dateinstant;
+
+import java.time.Instant;
+import java.util.Date;
+
+public final class DateInstantConverter {
+
+    private DateInstantConverter() {
+    }
+
+    public static Instant toInstant(Date date) {
+        if (date == null) {
+            return null;
+        }
+        return date.toInstant();
+    }
+
+    public static Date toDate(Instant instant) {
+        if (instant == null) {
+            return null;
+        }
+        return Date.from(instant);
+    }
+}

core-java-modules/core-java-8-datetime-4/src/test/java/com/baeldung/dateinstant/DateInstantConverterUnitTest.java

@@ -0,0 +1,65 @@
+package com.baeldung.dateinstant;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.util.Date;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class DateInstantConverterUnitTest {
+
+    @Test
+    void shouldConvertDateToInstant() {
+        Date date = new Date(1708752000000L);
+
+        Instant instant = DateInstantConverter.toInstant(date);
+
+        assertNotNull(instant);
+        assertEquals(date.getTime(), instant.toEpochMilli());
+    }
+
+    @Test
+    void shouldConvertInstantToDate() {
+        Instant instant = Instant.ofEpochMilli(1708752000000L);
+
+        Date date = DateInstantConverter.toDate(instant);
+
+        assertNotNull(date);
+        assertEquals(instant.toEpochMilli(), date.getTime());
+    }
+
+    @Test
+    void shouldReturnNullWhenDateIsNull() {
+        Instant instant = DateInstantConverter.toInstant(null);
+        assertNull(instant);
+    }
+
+    @Test
+    void shouldReturnNullWhenInstantIsNull() {
+        Date date = DateInstantConverter.toDate(null);
+        assertNull(date);
+    }
+
+    @Test
+    void shouldPreserveMillisecondPrecisionInRoundTrip() {
+        Instant originalInstant = Instant.now();
+
+        Date date = DateInstantConverter.toDate(originalInstant);
+        Instant convertedBack = DateInstantConverter.toInstant(date);
+
+        assertEquals(originalInstant.toEpochMilli(),
+                convertedBack.toEpochMilli());
+    }
+
+    @Test
+    void shouldTruncateNanosecondsWhenConvertingToDate() {
+        Instant instantWithNanos = Instant.ofEpochSecond(1000, 123456789);
+
+        Date date = DateInstantConverter.toDate(instantWithNanos);
+        Instant convertedBack = DateInstantConverter.toInstant(date);
+
+        assertEquals(instantWithNanos.toEpochMilli(),
+                convertedBack.toEpochMilli());
+    }
+}