Why Does My `java.time.DateTimeFormatterBuilder` Fail in JUnit Tests But Work During Runtime?

java.time.DateTimeFormatterBuilder Fails during Testing

During runtime, applying java.time.DateTimeFormatterBuilder to parse a specific pattern works flawlessly. However, when executing the same operation in a JUnit test using the same input string value, an error occurs.

Test Case Details

• Expected Result: A calculated time difference of "06:00" when provided with the input strings "25-May-2018 11:10" for both startDate and endDate.
• Actual Result: An error occurs when parsing the startDate using DateTimeFormatterBuilder with the provided pattern.

Code Snippet

@Test
public void testFormat() throws Exception {
    final String startDateFormatA = "25-May-2018 11:10";
    final String endDateFormatA = "25-May-2018 11:10";
    assertEquals("06:00", callDbController.getTimeDifference(startDateFormatA, endDateFormatA)[1]);
}

The parsing method uses the following pattern within a DateTimeFormatterBuilder:

new DateTimeFormatterBuilder().parseCaseInsensitive().appendPattern("dd-MMM-yyyy HH:mm").toFormatter();

Error Encountered

The test fails with the same input string value that works during runtime.

Resolution

The month name in the input string is in English. To ensure consistency, a specific locale (in this case, Locale.ENGLISH) must be set when creating the formatter.

new DateTimeFormatterBuilder().parseCaseInsensitive().appendPattern("dd-MMM-yyyy HH:mm").toFormatter(Locale.ENGLISH);

By setting the locale explicitly, the formatter can consistently parse the input string regardless of the JVM's default locale, resolving the discrepancy between runtime and test-time behavior.

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