BAEL-5386: Format Instant to String in Java (#11970)

* first commit

* remove extra spaces

* add the requested changes

core-java-modules/core-java-datetime-string/src/test/java/com/baeldung/formatinstant/FormatInstantUnitTest.java

@@ -0,0 +1,54 @@
+package com.baeldung.formatinstant;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.time.Instant;
+import java.time.ZoneId;
+import java.time.format.DateTimeFormatter;
+import java.time.temporal.UnsupportedTemporalTypeException;
+
+import org.joda.time.format.DateTimeFormat;
+import org.junit.Test;
+
+public class FormatInstantUnitTest {
+    
+    private static final String PATTERN_FORMAT = "dd.MM.yyyy";
+
+    @Test
+    public void givenInstant_whenUsingDateTimeFormatter_thenFormat() {
+        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(PATTERN_FORMAT)
+            .withZone(ZoneId.systemDefault());
+
+        Instant instant = Instant.parse("2022-02-15T18:35:24.00Z");
+        String formattedInstant = formatter.format(instant);
+
+        assertThat(formattedInstant).isEqualTo("15.02.2022");
+    }
+
+    @Test(expected = UnsupportedTemporalTypeException.class)
+    public void givenInstant_whenNotSpecifyingTimeZone_thenThrowException() {
+        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(PATTERN_FORMAT);
+
+        Instant instant = Instant.now();
+        formatter.format(instant);
+    }
+
+    @Test
+    public void givenInstant_whenUsingToString_thenFormat() {
+        Instant instant = Instant.ofEpochMilli(1641828224000L);
+        String formattedInstant = instant.toString();
+
+        assertThat(formattedInstant).isEqualTo("2022-01-10T15:23:44Z");
+    }
+    
+    @Test
+    public void givenInstant_whenUsingJodaTime_thenFormat() {
+        org.joda.time.Instant instant = new org.joda.time.Instant("2022-03-20T10:11:12");
+        
+        String formattedInstant = DateTimeFormat.forPattern(PATTERN_FORMAT)
+            .print(instant);
+
+        assertThat(formattedInstant).isEqualTo("20.03.2022");
+    }
+
+}