Legacy Date and Time

Prior to Java 8, date and time representation relied on classes like java.util.Date, java.util.Calendar, and java.text.SimpleDateFormat. These classes suffered from severe design flaws, particularly thread-safety bugs.

Legacy API Design Flaws

• Mutability: java.util.Date is mutable. Calling setTime() modifies the value directly, making it dangerous to pass around without defensive copying.
• Non-thread-safe Formatting: SimpleDateFormat maintains internal state during parsing. Sharing a static formatter across threads causes overlapping state updates and silent date corruptions.
• Confusing Offsets: Month indices start at 0 (January is 0), but days start at 1. Years are offset by 1900.

Code Walkthrough

The following example demonstrates how a shared SimpleDateFormat can corrupt data under concurrent execution.

import java.text.SimpleDateFormat;
import java.util.Date;
public class SimpleDateFormatBugDemo {
    private static final SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
public static void main(String[] args) {
        Runnable task = () -> {
            try {
                // Multiple threads executing this concurrently will corrupt internal state!
                Date d = sdf.parse("2026-06-13");
                System.out.println("Parsed: " + sdf.format(d));
            } catch (Exception e) {
                // Frequently throws NumberFormatException under race conditions
            }
        };
for (int i = 0; i < 5; i++) {
            new Thread(task).start();
        }
    }
}

Interview-Relevant Information