BAEL-5178:An introduction to InstantSource (#11646)

* BAEL-5178:An introduction to InstantSource

* BAEL-5178:PR fixes

Co-authored-by: Pablo Ubal <pablo.ubal@flexit.es>

core-java-modules/core-java-17/src/main/java/com/baeldung/instantsource/InstantExample.java

@@ -0,0 +1,21 @@
+package com.baeldung.instantsource;
+
+import java.time.Instant;
+import java.time.InstantSource;
+
+public class InstantExample {
+    InstantWrapper instantWrapper;
+    InstantSource instantSource;
+
+    public InstantExample(InstantWrapper instantWrapper, InstantSource instantSource) {
+        this.instantWrapper = instantWrapper;
+        this.instantSource = instantSource;
+    }
+
+    public Instant getCurrentInstantFromWrapper() {
+        return instantWrapper.instant();
+    }
+    public Instant getCurrentInstantFromInstantSource() {
+        return instantSource.instant();
+    }
+}

core-java-modules/core-java-17/src/main/java/com/baeldung/instantsource/InstantWrapper.java

@@ -0,0 +1,39 @@
+package com.baeldung.instantsource;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+import java.time.ZonedDateTime;
+
+public class InstantWrapper {
+    Clock clock;
+
+    private InstantWrapper() {
+        this.clock = Clock.systemDefaultZone();
+    }
+
+    private InstantWrapper(ZonedDateTime zonedDateTime) {
+        this.clock = Clock.fixed(zonedDateTime.toInstant(), zonedDateTime.getZone());
+    }
+
+    private InstantWrapper(LocalDateTime localDateTime) {
+        this.clock = Clock.fixed(localDateTime.toInstant(ZoneOffset.UTC), ZoneOffset.UTC);
+    }
+
+    public static InstantWrapper of() {
+        return new InstantWrapper();
+    }
+
+    public static InstantWrapper of(ZonedDateTime zonedDateTime) {
+        return new InstantWrapper(zonedDateTime);
+    }
+
+    public static InstantWrapper of(LocalDateTime localDateTime) {
+        return new InstantWrapper(localDateTime);
+    }
+
+    public Instant instant() {
+        return clock.instant();
+    }
+}

core-java-modules/core-java-17/src/test/java/com/baeldung/instantsource/InstantExampleUnitTest.java

@@ -0,0 +1,87 @@
+package com.baeldung.instantsource;
+
+import org.junit.jupiter.api.Test;
+
+import java.time.Clock;
+import java.time.Instant;
+import java.time.InstantSource;
+import java.time.LocalDateTime;
+import java.time.ZoneOffset;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+class InstantExampleUnitTest {
+
+    @Test
+    void getCurrentInstantFromWrapper_shouldReturnCurrentInstantBasedOnSystemClock() {
+        //given
+        InstantExample tested = new InstantExample(InstantWrapper.of(), null);
+        Instant currentInstant = Clock.systemDefaultZone().instant();
+        //when
+        Instant returnedInstant = tested.getCurrentInstantFromWrapper();
+
+        //then
+        assertTrue(returnedInstant.isAfter(currentInstant));
+    }
+
+    @Test
+    void getCurrentInstantFromWrapper_shouldReturnFixedInstant() {
+        //given
+        LocalDateTime now = LocalDateTime.now();
+        InstantExample tested = new InstantExample(InstantWrapper.of(now), null);
+        Instant currentInstant = now.toInstant(ZoneOffset.UTC);
+        //when
+        Instant returnedInstant = tested.getCurrentInstantFromWrapper();
+
+        //then
+        assertEquals(currentInstant, returnedInstant);
+    }
+
+    @Test
+    void getCurrentInstantFromInstantSource_shouldReturnCurrentInstantBasedOnSystemClock() {
+        //given
+        InstantSource instantSource = InstantSource.system();
+        InstantExample tested = new InstantExample(null, instantSource);
+        Instant currentInstant = instantSource.instant();
+
+        //when
+        Instant returnedInstant = tested.getCurrentInstantFromInstantSource();
+
+        //then
+        assertTrue(returnedInstant.isAfter(currentInstant));
+    }
+
+    @Test
+    void getCurrentInstantFromInstantSource_shouldReturnFixedInstant() {
+        //given
+        LocalDateTime now = LocalDateTime.now();
+        InstantSource instantSource = InstantSource.fixed(now.toInstant(ZoneOffset.UTC));
+        InstantExample tested = new InstantExample(null, instantSource);
+        Instant currentInstant = instantSource.instant();
+
+        LocalDateTime fixed = LocalDateTime.of(2022, 01, 01, 00, 00);
+        Instant i = InstantSource.fixed(fixed.toInstant(ZoneOffset.UTC)).instant();
+        System.out.println(i);
+
+        //when
+        Instant returnedInstant = tested.getCurrentInstantFromInstantSource();
+
+        //then
+        assertEquals(currentInstant, returnedInstant);
+    }
+
+    @Test
+    void getCurrentInstantFromInstantSource_shouldMatchGivenTimezone() {
+        //given
+        LocalDateTime now = LocalDateTime.now();
+        InstantSource instantSource = InstantSource.fixed(now.toInstant(ZoneOffset.of("-4")));
+        InstantExample tested = new InstantExample(null, instantSource);
+
+        //when
+        Instant returnedInstant = tested.getCurrentInstantFromInstantSource();
+
+        //then
+        assertEquals(now.atOffset(ZoneOffset.of("-4")).toInstant(), returnedInstant);
+    }
+}