SUCCESS, ERROR, INPUT not recognized in execute() (Webwork action class)

[Sandra Priedīte]

Hello,
I am complete beginner in writing plugins and working through some tutorials.
I have created webwork1 plugin module using atlas-create-jira-plugin-module command, which automatically created java class for the action with method to be overridden:
@Override
public String execute() throws Exception {

        return super.execute(); //returns SUCCESS

}

When I look for more examples, everywhere code contains as return statement:
return SUCCESS;
return INPUT;
return ERROR;
with no specific imports or definitions.

When I attempt to use these constants SUCCESS, INPUT and ERROR - they are not recognized.

When I place code to be executed in this execute() method - success scenario executes. But I need to add code that would execute Error scenario.

For me it looks like all of the examples are based on the deprecated JiraActionSupport class, so I don’t understand where to look further.

Thanks in advance for any resource, hint and other help provided!!