public class MainActivity extends AppCompatActivity {

    Button btn_start, btn_stop;
    Thread thread;
    boolean isThread = false;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);


        // 스레드 시작
        btn_start = (Button)findViewById(R.id.btn_start);
        btn_start.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                isThread = true;
                thread = new Thread() {
                    public void run() {
                        while(isThread) {
                            try {
                                sleep(5000);
                            } catch (InterruptedException e) {
                                e.printStackTrace();
                            }
                            handler.sendEmptyMessage(0);
                        }
                    }
                };
                thread.start();

            }
        });

        // 스레드 종료
        btn_stop = (Button)findViewById(R.id.btn_stop);
        btn_stop.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                isThread = false;
            }
        });

    }


    private Handler handler = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            Toast.makeText(getApplicationContext(), "안녕하세요", Toast.LENGTH_SHORT).show();
        }
    };
}

안드로이드 앱 만들기 #15 (Thread & Handler 사용법) – 홍드로이드
https://www.youtube.com/watch?v=aYjdfpit6lc

홍드로이드 github
https://github.com/hongdroid94/15_ThreadHandler