Action Class Code

public class DropDown extends ActionSupport{
    private List arrUserDetails = new ArrayList<Users>();

    public DropDown()
    {
        Users objUsers = new Users();
        objUsers.setUserId("101");
        objUsers.setUserName("User1");

        arrUserDetails.add(objUsers);

        objUsers.setUserId("102");
        objUsers.setUserName("User2");

        arrUserDetails.add(objUsers);

        objUsers.setUserId("103");
        objUsers.setUserName("User3");

        arrUserDetails.add(objUsers);
    }

    public String execute(){
        return SUCCESS;
    }

    public List getArrUserDetails() {
        return arrUserDetails;
    }


    public void setArrUserDetails(List arrUserDetails) {
        this.arrUserDetails = arrUserDetails;
    }
}

Users Bean Class

 public class Users
{
    private String userId;
    private String userName;

    public String getUserId() {
        return userId;
    }
    public void setUserId(String userId) {
        this.userId = userId;
    }
    public String getUserName() {
        return userName;
    }
    public void setUserName(String userName) {
        this.userName = userName;
    }
}

JSP Page Struts2 Tag

 <s:select label="Departments" list="departmentList" listKey="deptId" listValue="deptName" name="department"/>