new Thread(new Runnable() {
            @Override
            public void run() {
                Toast.makeText(MainActivity.this, "haha", Toast.LENGTH_SHORT).show();
            }
        }).start();

 new Thread(new Runnable() {
            @Override
            public void run() {
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(MainActivity.this, "haha", Toast.LENGTH_SHORT).show();
                    }
                });
            }
        }).start();

1. 创建一个handler 对象

2. 使用handler对象 post一个Runable对象到主线程执行

   
   Handler mHandler = new Handler();
   
   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
   
       new Thread(new Runnable() {
           @Override
           public void run() {
              mHandler.post(new Runnable() {
                  @Override
                  public void run() {
                      Toast.makeText(MainActivity.this, "haah", Toast.LENGTH_SHORT).show();
                  }
              });
           }
       }).start();
   
   }

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

        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {
                Intent intent = new Intent(MainActivity.this,SecondActivity.class);
                startActivity(intent);
            }
        },1000);