public interface OnLoginCallback{
        /**
         * Called before username and password checking process is started.
         */
        void onBeforeLoginCallback();

        /**
         * Called after username and password checking process is finished.
         */
        void onAfterLoginCallback();

        /**
         * Called when username and password does not match.
         */
        void onLoginFailedCallback(String username,String password);

        /**
         * Called when username and password matches.
         */
        void onLoginSuccessCallback(String username,String password);
    }

public class Login{

        private String mUsername;

        private String mPassword;

        private OnLoginCallback mOnLoginCallback;

        /**
         *
         * @param username
         * @param password
         * @param onLoginCallback
         */
        public Login(String username, String password, OnLoginCallback onLoginCallback){
            mUsername = username;
            mPassword = password;
            mOnLoginCallback = onLoginCallback;
        }

        /**
         * Asynchronously checks if username and password matches in database. If not {@code onLoginFailedCallback} is called,
         * otherwise {@code onLoginSuccessCallback} is called.
         */
        public void login() {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() {mOnLoginCallback.onBeforeLoginCallback();
                        }
                    });
                    boolean ok = //implement logic to check is login ok from database
                    if(ok)
                        runOnUiThread(new Runnable() {
                        @Override
                        public void run() { mOnLoginCallback.onLoginSuccessCallback(mUsername, mPassword);}
                    });
                    else
                        runOnUiThread(new Runnable() {
                        @Override
                        public void run() { mOnLoginCallback.onLoginFailedCallback(mUsername, mPassword); }
                    });
                    runOnUiThread(new Runnable() {
                        @Override
                        public void run() { mOnLoginCallback.onAfterLoginCallback();
                        }
                    });
                }
            }).start();
        }
    }

new Login(etUsername.getText().toString(), etPassword.getText().toString(),
                new OnLoginCallback() {
            @Override
            public void onBeforeLoginCallback() {

            }

            @Override
            public void onAfterLoginCallback() {

            }

            @Override
            public void onLoginFailedCallback(String username, String password) {

            }

            @Override
            public void onLoginSuccessCallback(String username, String password) {

            }
        }).login();