protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        //强制转换为Button,必然放在setContentView之后

        btnSatrtAty1=(Button)findViewById(R.id.btnStartAty1);

        //给按钮设定事件监听器

        btnSatrtAty1.setOnClickListener(

            //实现OnClickListener接口的了匿名类

            new View.OnClickListener() {

                @Override

                public void onClick(View view) {

                    //事件中启动Aty1

                    //参数一：MainActivity 的实例

                    //参数二：要启动的Activity的类的定义为Class类型

                    //this 指实现OnClickListener的内部类

                    Intent i=new Intent(MainActivity.this,Aty1.class);

                    i.putExtra("txt","Hello Aty1");

                    startActivity(i);

                }

        });

        System.out.print("onCreate");

    }

tvOut=(TextView)findViewById(R.id.tvOut);

tvOut.setText(getIntent().getStringExtra("txt"));

Intent i=new Intent(MainActivity.this,Aty1.class);

Bundle data=new Bundle();

data.putString("","Hello Aty1");

i.putExtras(data);

startActivity(i);

tvOut=(TextView)findViewById(R.id.tvOut);

Bundle data=getIntent().getExtras();

String txt=data.getString("txt");

tvOut.setText(txt);

        btnClose=(Button)findViewById(R.id.btnStartAty1);

        btnClose.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View view) {

                Intent i=new Intent();

                i.putExtra("result","Hello MainActivity");

                setResult(0, i);

                finish();

            }

        });

    protected void onActivityResult(int requestCode, int resultCode, Intent data) {

        String result=data.getStringExtra("result");

        tvOut.setText(result);

        super.onActivityResult(requestCode, resultCode, data);

    }