Map<String,MockUser> users = new HashMap<>();
 try (Stream<String> stream = Files.lines(Paths.get(f.getAbsolutePath()))) {
            stream.forEach(s -> {
                String[] fields = s.split(",");
                MockUser user = new MockUser(fields[0], fields[1], fields[2]);
                users.put(user.getUsername(), user);
            });
        } catch (IOException e) {
            e.printStackTrace();
        }


private class MockUser {
        String username;
        String password;
        String role;

        MockUser(String username, String password, String role) {
            this.username = username;
            this.password = password;
            this.role = role;
        }

        public String getUsername() {
            return username;
        }

        public void setUsername(String username) {
            this.username = username;
        }

        public String getPassword() {
            return password;
        }

        public void setPassword(String password) {
            this.password = password;
        }

        public String getRole() {
            return role;
        }

        public void setRole(String role) {
            this.role = role;
        }
    }

Map<String, MockUser> users = Files.lines(Paths.get(f.getAbsolutePath()))
                               .map(line -> line.split(","))
                               .map(fields -> new MockUser(fields[0], fields[1], fields[2]))
                               .collect(Collectors.groupingBy(MockUser::getUserName))

users = Files.lines(Paths.get(file.getAbsolutePath()))
                    .map(line -> line.split(","))
                    .map(fields -> new MockUser(fields[0], fields[1], fields[2]))
                    .collect(Collectors.toMap(
                            MockUser::getUsername,
                            user -> user));