Java string to date conversion ?

To convert a string to a date in Java, you can use the SimpleDateFormat class (legacy API) or the modern java.time API (Java 8+). Below is a detailed guide with examples for both approaches.

1. Using SimpleDateFormat (Legacy API)

The SimpleDateFormat class allows parsing strings into java.util.Date objects.
Note: This API is outdated and not thread-safe. Use the modern java.time API (recommended for new code).

Example 1: Basic Conversion

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class StringToDate {
    public static void main(String[] args) {
        String dateString = "2023-10-25";
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");

        try {
            Date date = formatter.parse(dateString);
            System.out.println(date); // Output: Wed Oct 25 00:00:00 GMT 2023
        } catch (ParseException e) {
            e.printStackTrace();
        }
    }
}

Example 2: Custom Date Format

Parse a date-time string with a specific pattern:

String dateTimeString = "25-Oct-2023 14:30:45";
SimpleDateFormat formatter = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss");

try {
    Date date = formatter.parse(dateTimeString);
    System.out.println(date); // Output: Wed Oct 25 14:30:45 GMT 2023
} catch (ParseException e) {
    e.printStackTrace();
}

Common Patterns:

Pattern	Description	Example
yyyy	4-digit year	2023
MM	2-digit month	10 (October)
dd	2-digit day	25
HH	24-hour hour	14
mm	Minutes	30
ss	Seconds	45
MMM	Abbreviated month	Oct

2. Using java.time API (Modern, Recommended)

Java 8 introduced the java.time package with classes like LocalDate, LocalDateTime, and DateTimeFormatter.
Advantages: Immutable, thread-safe, and more intuitive.

Example 3: String to LocalDate

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;

public class StringToDate {
    public static void main(String[] args) {
        String dateString = "2023-10-25";
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");

        LocalDate date = LocalDate.parse(dateString, formatter);
        System.out.println(date); // Output: 2023-10-25
    }
}

Example 4: String to LocalDateTime

Parse a date-time string with time components:

String dateTimeString = "2023-10-25T14:30:45";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss");

LocalDateTime dateTime = LocalDateTime.parse(dateTimeString, formatter);
System.out.println(dateTime); // Output: 2023-10-25T14:30:45

Example 5: Custom Pattern with Time Zone

Parse a string with a time zone:

String zonedDateTimeString = "2023-10-25 14:30:45 GMT";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss z");

ZonedDateTime zonedDateTime = ZonedDateTime.parse(zonedDateTimeString, formatter);
System.out.println(zonedDateTime); // Output: 2023-10-25T14:30:45Z[GMT]

3. Handling Different Date Formats

Example 6: Parse “dd/MM/yyyy”

String dateString = "25/10/2023";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd/MM/yyyy");
LocalDate date = LocalDate.parse(dateString, formatter);

Example 7: Parse “MMM dd, yyyy”

String dateString = "Oct 25, 2023";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MMM dd, yyyy");
LocalDate date = LocalDate.parse(dateString, formatter);

Key Notes

1. Exception Handling:

• SimpleDateFormat.parse() throws ParseException.
• java.time methods throw DateTimeParseException (unchecked).

1. Time Zones:

• Use ZonedDateTime or OffsetDateTime for time zone-aware parsing.

1. Locale Sensitivity:

• For month names (e.g., Oct), ensure the Locale matches the input string:
  java DateTimeFormatter formatter = DateTimeFormatter.ofPattern("MMM dd, yyyy", Locale.US);

Common Errors & Fixes

• Pattern Mismatch:
  Ensure the pattern matches the input string exactly (e.g., HH for 24-hour format vs. hh for 12-hour).
• Case Sensitivity:
  MM = month (e.g., 10), mm = minutes.
• Unknown Pattern Symbols:
  Use valid symbols (e.g., yyyy for year, not YYYY).

Summary of Methods

API	Class	Use Case	Thread-Safe?
SimpleDateFormat	java.util.Date	Legacy code (Java ≤7)	❌
java.time	LocalDate, LocalDateTime	Modern applications (Java 8+)	✔️

Final Recommendation

Use the java.time API (LocalDate, LocalDateTime, etc.) for new projects. It avoids thread-safety issues and provides better functionality.