How do I check if the String is null or not empty in Java?

public ArrayList findEmployees(String str, int dep)
       throws ClassNotFoundException, SQLException{

    System.out.println("List IN");
    ArrayList list = new ArrayList();
    java.sql.Statement stmt;
    java.sql.ResultSet rs;
    Class.forName("com.mysql.jdbc.Driver");
    String url = "jdbc:mysql://localhost:3306/general";
    java.sql.Connection con = DriverManager.getConnection(url, "root", "1234");
    System.out.println("URL: " + url);
    System.out.println("Connection: " + con);
    stmt = con.createStatement();
    stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,
            ResultSet.CONCUR_READ_ONLY);
    String qry = "SELECT * FROM PERSON ";
    String werstr = "WHERE";
    if(str!= null && str != "**here i want to check the 'str' is empty or not." )
    {
        qry += werstr + " NAME LIKE '%"+str+"%'";
        System.out.println(qry);
        werstr = "AND";
    }
    if(dep != 0)
    {
        qry += werstr + "dept="+dep;
    }
    qry += ";";
    System.out.println(qry);
    rs = stmt.executeQuery(qry);
    while (rs.next()) {
        Employee employee = new Employee();
        String name = rs.getString(2);
        employee.setName(name);
        int id = rs.getInt(1);
        employee.setId(id);
        int dept = rs.getInt(4);
        employee.setDept(dept);
        int age = rs.getInt(3);
        employee.setAge(age);
        list.add(employee);
    }
    System.out.println("List Out");
    return list;
}