chore: Add tests for static helper methods

Author: lqiu96

google-cloud-bigquerystorage/src/main/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessage.java

@@ -1048,7 +1048,7 @@ static Instant fromEpochMicros(long micros) {
 
   /** Best effort to try and convert a timestamp to an ISO8601 string */
   @VisibleForTesting
-  String getTimestampAsString(Object val) {
+  static String getTimestampAsString(Object val) {
     if (val instanceof String) {
       // Validate the ISO8601 values before sending it to the server
       String value = (String) val;

google-cloud-bigquerystorage/src/test/java/com/google/cloud/bigquery/storage/v1/JsonToProtoMessageTest.java

@@ -16,6 +16,7 @@
 package com.google.cloud.bigquery.storage.v1;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 import com.google.cloud.bigquery.storage.test.JsonTest.*;
@@ -28,6 +29,7 @@
 import com.google.protobuf.DynamicMessage;
 import com.google.protobuf.Message;
 import java.math.BigDecimal;
+import java.time.Instant;
 import java.time.LocalTime;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -36,6 +38,8 @@
 import java.util.List;
 import java.util.Map;
 import java.util.logging.Logger;
+
+import com.google.protobuf.Timestamp;
 import org.json.JSONArray;
 import org.json.JSONObject;
 import org.junit.Assert;
@@ -1834,4 +1838,33 @@ public void testDoubleAndFloatToRepeatedBigNumericConversion() {
         JsonToProtoMessage.INSTANCE.convertToProtoMessage(TestBignumeric.getDescriptor(), ts, json);
     assertEquals(expectedProto, protoMsg);
   }
+
+  @Test
+  public void testGetTimestampAsString() {
+    assertEquals("2025-10-01 12:34:56.123456+00:00", JsonToProtoMessage.getTimestampAsString("2025-10-01 12:34:56.123456+00:00"));
+    assertEquals("2025-10-01 12:34:56.123456789123+00:00", JsonToProtoMessage.getTimestampAsString("2025-10-01 12:34:56.123456789123+00:00"));
+
+    assertEquals("1970-01-01T00:00:00.000001Z", JsonToProtoMessage.getTimestampAsString(1L));
+    assertEquals("1969-12-31T23:59:59.999999Z", JsonToProtoMessage.getTimestampAsString(-1L));
+
+    assertEquals("1970-01-02T10:17:36.000123456Z", JsonToProtoMessage.getTimestampAsString(Timestamp.newBuilder().setSeconds(123456).setNanos(123456).build()));
+    assertEquals("1969-12-30T13:42:23.999876544Z", JsonToProtoMessage.getTimestampAsString(Timestamp.newBuilder().setSeconds(-123456).setNanos(-123456).build()));
+
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString("2025-10-01"));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString("1234"));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString("abc"));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString(10.4));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString(-1000.4));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString(Timestamp.newBuilder()));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString(new Object()));
+    assertThrows(IllegalArgumentException.class, () -> JsonToProtoMessage.getTimestampAsString(null));
+  }
+
+  @Test
+  public void testFromEpochMicros() {
+    // The `+` is added if there are more than 4 digits for years
+    assertEquals("+294247-01-10T04:00:54.775807Z", JsonToProtoMessage.fromEpochMicros(Long.MAX_VALUE).toString());
+    assertEquals("-290308-12-21T19:59:05.224192Z", JsonToProtoMessage.fromEpochMicros(Long.MIN_VALUE).toString());
+    assertEquals(Instant.EPOCH.toString(), JsonToProtoMessage.fromEpochMicros(0L).toString());
+  }
 }