The following code snippet will show you how to convert the old java.util.TimeZone to java.time.ZoneId introduced in Java 8. In the first line of our main() method we get the default timezone using the TimeZone.getDefault() and convert it to ZoneId by calling the toZoneId() method. In the second example we create the TimeZone object by calling the getTimeZone() and pass the string of timezone id. To convert it to ZoneId we call the toZoneId() method.

package org.kodejava.datetime;

import java.time.ZoneId;
import java.util.TimeZone;

public class TimeZoneToZoneId {
    public static void main(String[] args) {
        ZoneId zoneId = TimeZone.getDefault().toZoneId();
        System.out.println("zoneId = " + zoneId);

        TimeZone timeZoneUsPacific = TimeZone.getTimeZone("US/Pacific");
        ZoneId zoneIdUsPacific = timeZoneUsPacific.toZoneId();
        System.out.println("zoneIdUsPacific = " + zoneIdUsPacific);
    }
}

This snippet prints the following output:

zoneId = Asia/Shanghai
zoneIdUsPacific = US/Pacific

To convert the other way around you can do it like the following code snippet. Below we convert the ZoneId to TimeZone by using the TimeZone.getTimeZone() method and pass the ZoneId.systemDefault() which return the system default timezone. Or we can create ZoneId using the ZoneId.of() method and specify the timezone id and then pass it to the getTimeZone() method of the TimeZone class.

package org.kodejava.datetime;

import java.time.ZoneId;
import java.util.TimeZone;

public class ZoneIdToTimeZone {
    public static void main(String[] args) {
        TimeZone timeZone = TimeZone.getTimeZone(ZoneId.systemDefault());
        System.out.println("timeZone = " + timeZone.getDisplayName());

        TimeZone timeZoneUsPacific = TimeZone.getTimeZone(ZoneId.of("US/Pacific"));
        System.out.println("timeZoneUsPacific = " + timeZoneUsPacific.getDisplayName());
    }
}

And here are the output of the code snippet above:

timeZone = China Standard Time
timeZoneUsPacific = Pacific Standard Time