[HUDI-2129] StreamerUtil.medianInstantTime should return a valid date time string (#3221)

hudi-flink/src/test/java/org/apache/hudi/utils/TestStreamerUtil.java

@@ -24,25 +24,30 @@ import org.apache.hudi.configuration.FlinkOptions;
 import org.apache.hudi.util.StreamerUtil;
 
 import org.apache.flink.configuration.Configuration;
-
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.io.TempDir;
 
 import java.io.File;
 import java.io.IOException;
 
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
-import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
+/**
+ * Test cases for {@link StreamerUtil}.
+ */
 public class TestStreamerUtil {
 
   @TempDir
   File tempFile;
 
   @Test
-  public void testInitTableIfNotExists() throws IOException {
+  void testInitTableIfNotExists() throws IOException {
     Configuration conf = TestConfigurations.getDefaultConf(tempFile.getAbsolutePath());
 
     // Test for partitioned table.
@@ -70,5 +75,25 @@ public class TestStreamerUtil {
             .build();
     assertFalse(metaClient2.getTableConfig().getPartitionColumns().isPresent());
   }
+
+  @Test
+  void testMedianInstantTime() {
+    String higher = "20210705125921";
+    String lower = "20210705125806";
+    String median1 = StreamerUtil.medianInstantTime(higher, lower);
+    assertThat(median1, is("20210705125843"));
+    // test symmetry
+    assertThrows(IllegalArgumentException.class,
+        () -> StreamerUtil.medianInstantTime(lower, higher),
+        "The first argument should have newer instant time");
+  }
+
+  @Test
+  void testInstantTimeDiff() {
+    String higher = "20210705125921";
+    String lower = "20210705125806";
+    long diff = StreamerUtil.instantTimeDiffSeconds(higher, lower);
+    assertThat(diff, is(75L));
+  }
 }