Yes , we can use Java method in an Activity step.

Please find the below help topic for more details.

https://community.pega.com/sites/default/files/help_v72/methods/java/java.htm

Can you give an example of what Java command you want to execute?

Yes, you can, but we don't recommend it until it is absolutely necessary.

Pega recommends using the OOTB provided rules and the methods in case of the activity.

If there is no OOTB feature available for your requirement, then only go for Java Step.

Using Java Step in Activity rule results in guardrail warnings which you need to justify.

There are two ways,

1. Using the Java Step in activity

2. Using the Connect Java rules

Have a look on the below help links for more information:

https://community.pega.com/sites/default/files/help_v72/rule-/rule-connect-/rule-connect-java/main.htm#:~:text=Access,create%20a%20Connect%20Java%20rule.

https://community.pega.com/sites/default/files/help_v72/methods/java/java.htm