In Java, which java.io.File method is used to obtain the last modified timestamp of a file so that you can determine when it was last changed?

Introduction / Context:
Many applications need to know when a file was last changed, for example to implement caching, synchronisation, or backup logic. The java.io.File class provides a method to retrieve the last modified time stored by the file system. This question asks you to identify the correct File method and understand the form in which the timestamp is returned.

Given Data / Assumptions:

• We are using java.io.File to represent a file path on disk.
• The underlying file system records a last modified time for files.
• Java maps this timestamp into a long value following the standard epoch time convention.
• The application can convert this long into higher-level date objects for display.

Concept / Approach:
The File class method long lastModified() returns the time the file was last modified, represented as the number of milliseconds since midnight, January 1, 1970 UTC (the Unix epoch). If the file does not exist or the information is not available, it may return 0. This is distinct from length(), which returns file size, and from getPath(), which returns the path string but not any timestamps. createNewFile() is a creation method and does not directly provide modification times. Therefore, lastModified() is the appropriate method to query a file modification timestamp.

Step-by-Step Solution:
Step 1: Construct a File object for the file you want to inspect, for example File f = new File("report.txt").Step 2: Optionally verify existence with f.exists() to avoid interpreting a 0 result incorrectly.Step 3: Call long t = f.lastModified() to fetch the last modified time as a millisecond count.Step 4: Convert t into a human-readable date using a date-time API, for example new java.util.Date(t) or newer java.time classes.Step 5: Conclude that lastModified() is the correct File method for retrieving a file's last changed time, as stated in option A.

Verification / Alternative check:
You can verify the behaviour by editing a file and then running a small Java program that prints the result of lastModified() both before and after the edit. Converting the long value to a Date object will show the updated timestamp. Comparing this value with the file properties displayed by the operating system will confirm that lastModified() returns the expected modification time.

Why Other Options Are Wrong:
Option B refers to length(), which returns the size of the file in bytes, not a timestamp. Option C suggests that the path includes date and time, but getPath() merely provides the textual path such as "C:/data/report.txt". Option D describes createNewFile(), which may change the file system but does not directly report the last modified time; it only returns a boolean indicating whether a file was created. Thus, only option A correctly identifies lastModified() as the method for retrieving modification timestamps.

Common Pitfalls:
A common pitfall is assuming that a 0 value from lastModified() always indicates the epoch; in practice it often signals that the file does not exist or the timestamp could not be read. Another issue is misunderstanding time zones; the long value is in UTC-based milliseconds, and you must apply appropriate time zone handling when formatting for users. Being aware of these details helps you build reliable file monitoring, synchronisation, and logging features in Java applications.

Final Answer:
Correct answer: lastModified(), which returns the time the file was last changed as a long value in milliseconds since the epoch