public static void main(String[] args) throws IOException {
    for (CatalogResource machine : machines) {
	    example.viewDetails(machine);
	    List<CatalogPrincipal> owners = machine.getOwners();

	    for( CatalogPrincipal owner : owners ){
		    String ownerEmail = example.getUserEmail(owner.getRef());
	      System.out.println("OWNER: " + owner.getValue()
			  	+ " (" + owner.getType()
			  	+ ") : Username" + owner.getRef()
			  	+ "OWNER: email is " + ownerEmail);
	    }
    }

    ...
  }
    //Pass the machine owners getRef() value in the userName param

    private String getUserEmail(String userName) {
        if (!isAuthenticated()) {
            throw new IllegalStateException("You must call login() first to authenticate");
        }

        String userEmail = "";
        RestClient rcService = consumerService
                .getDefaultRestClientForService("identity");
        IdentityStoreClientServiceImpl iscs = new IdentityStoreClientServiceImpl(rcService);

        OdataQuery orderByName = OdataQuery.query().addAscOrderBy("name");
        Pageable page = PageOdataRequest.page(1, 100, orderByName);

        Set<IdentityStore> identityStores = iscs.getIdentityStores(tenant, page);

        for (IdentityStore item : identityStores) {
            if( userEmail.equals("")){
                RestClient principalService = consumerService.getDefaultRestClientForService("identity");
                PrincipalService psvc = new PrincipalService(principalService);
                User wkgUser = psvc.getPrincipal(tenant,userName);
                userEmail=wkgUser.getEmailAddress();
            }

        }
        return userEmail;
    }