如何每秒更新一次 Date()？

Question

I am trying to make a program that updates currentTime every second so that in the console it will go 1s, 2s, 3s, 4s and so on.

public static void main(String[] args) throws InterruptedException{
    OSpanel runner = new OSpanel();
    runner.currentTime();
}

public static void currentTime() throws InterruptedException{
    if(true) {
        Date currentTime = new Date();
        while(true) {
            Thread.sleep(1000);
            System.out.println(currentTime);
            Thread.sleep(1000);
            System.out.println(currentTime);
        }
    }
}

Answer 1

java.time

The java.util Date-Time API is outdated and error-prone. It is recommended to stop using it completely and switch to the modern Date-Time API ^* .

You can use Instant#now to get the current instant of time. In order to get it every second, you can use ScheduledExecutorService#scheduleWithFixedDelay eg

import java.time.Instant;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class Main {
    public static void main(String[] args) {
        Executors.newScheduledThreadPool(1)
            .scheduleWithFixedDelay(
                () -> System.out.println(Instant.now()), 
                0, 
                1,
                TimeUnit.SECONDS
            );
    }
}

Output from a sample run:

2021-10-03T13:53:42.462768Z
2021-10-03T13:53:43.469758Z
2021-10-03T13:53:44.470316Z
...

ONLINE DEMO

An Instant represents an instantaneous point on the timeline, normally represented in UTC time. The Z in the output is the timezone designator for a zero-timezone offset. It stands for Zulu and specifies the Etc/UTC timezone (which has the timezone offset of +00:00 hours).

Note: If you want to print just the running second, replace the print statement with the following:

System.out.println(ZonedDateTime.now(ZoneOffset.UTC).getSecond() + "s")

Learn more about the modern Date-Time API from Trail: Date Time .

---

^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project . Android 8.0 Oreo already provides support for java.time .}

Answer 2

You are updating currentTime outside of the while loop - you are outputting the date to the console every second but you are not updating the time.

Try this:

Main.java

public static void main(String[] args) throws InterruptedException{
    OSpanel runner = new OSpanel();
    runner.currentTime();
}

OSpanel.java

public void currentTime() throws InterruptedException{
        int counter = 1;
        while (true) {
            System.out.println(counter + "s");
            Thread.sleep(1000);
            counter++;
        }
    }
}

Answer 3

This will create a thread pool with one thread which will execute the lambda printing the number of seconds since the program started once every second.

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class SOAnswer {
    public static void main(String[] args) {
        ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
        long start = System.currentTimeMillis();
        executorService.scheduleAtFixedRate(() -> System.out.println(String.format("%ds", (System.currentTimeMillis() - start) / 1000)), 0, 1, TimeUnit.SECONDS);
    }
}

Answer 4

Duration

Adding to the good Answer by Avinash , let me show the use of Duration to track elapsed time.

Record a starting moment.

Instant start = Instant.now() ;  // Capture the current moment as seen in UTC.

At any other moment, calculate elapsed time on a scale of hours-minutes-seconds using the java.time.Duration class.

Duration d = Duration.between( start , Instant.now() ) ;

Generate text representing that elapsed time in standard ISO 8601 format: PnYnMnDTnHnMnS .

String output = Duration.toString() ;

PT21s

To generate text in a custom format, write a method that accesses the various parts. Call toHoursPart , toMinutesPart , etc. Put those parts together in whatever way you desire.

Pulling this all together, in the code of that other Answer, change this line:

() -> System.out.println( Instant.now() ) ,

… to this line:

() -> System.out.println( Duration.between( start , Instant.now() ).toString() ) ,

… or call your custom formatting method.