Intent intent=new Intent("android.intent.action.SecondActivity");
startActivity(intent);

不知道大家有没有思考过这三个事情： 
1、当A应用 启动 B应用的SecondActivity，那么B应用的MainActivity会不会被启动呢？正常情况下，我们点击应用B，进到的是MainActivity这个活动，那么现在我们是通过跨应用启动，会不会要经过B的MainActivity呢？答案是不会。

2、当我们在SecondActivity中点击Back回退键时，回到的是A应用的mainActivity界面，这里时候大家有没有想过。 
SecondActivity和appA的mainActivity是不是同处于一个栈中呢？这时候就要去打印栈的ID了。

3、由上面的两件事，不知道大家想起：Android对于Activity的管理，也就是framework层的ActivityManager。也就是说，你手机上的N多应用，当你打开某一个应用是，这个应用的Activity都是由ActivityManager这娃来创建和管理的。应用本身并没有创建Activity的能力。当然这其中又涉及到了Ibinder的通讯。这里暂时不讲。

@Override
 public void onClick(View v) {
 Intent intent = new Intent(Intent.ACTION_VIEW);
 String packageName = "com.example.mylife.anotherapp";
 String className = "com.example.mylife.anotherapp.MainActivity";
 intent.setClassName(packageName, className);
 //second method
 //intent.setComponent(new ComponentName("com.example.mylife.anotherapp","com.example.mylife.anotherapp.MainActivity"));
 Bundle bundle = new Bundle();
 bundle.putString("msg", "this message is from project B ");
 intent.putExtras(bundle);
 intent.putExtra("pid", android.os.Process.myPid());
 startActivityForResult(intent, 1);
 //startActivity(intent);
 }

 @Override
 protected void onActivityResult(int requestCode, int resultCode, Intent data) {
 super.onActivityResult(requestCode, resultCode, data);
 switch (requestCode) {
 case 1:
 if(resultCode == RESULT_OK) {
 textView.setText(data.getStringExtra("result"));
 }
 break;
 }
 }

 @Override
 protected void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.activity_main);
 textView = (TextView)findViewById(R.id.text);

 Intent intent = getIntent();
 if(intent != null) {
 textView.setText(intent.getStringExtra("msg"));
 }
 }

 public void OnClick(View view) {
 Intent intent = new Intent();
 intent.putExtra("result","OK! from project a.");
 this.setResult(RESULT_OK,intent);
 this.finish();//要清楚这里为什么要用finish()。
 }

@Override
public void onBackPressed() {
 super.onBackPressed();
 Intent intent = new Intent();
 intent.putExtra("result","OK! from project a.");
 this.setResult(RESULT_OK,intent);
 this.finish();//要清楚这里为什么要用finish()。
}

public class MyService extends Service {

 private static final String TAG = "MyService";
 public MyService() {
 }

 @Override
 public IBinder onBind(Intent intent) {
 // TODO: Return the communication channel to the service.
 throw new UnsupportedOperationException("Not yet implemented");
 }

 @Override
 public void onCreate() {
 super.onCreate();
 Log.d(TAG, "onCreate: ");
 }

 @Override
 public int onStartCommand(Intent intent,int flags, int startId) {
 Log.d(TAG, "onStartCommand: ");
 if(intent != null) {
 Log.d(TAG, "onStartCommand: "+intent.getStringExtra("msg"));
 }
 return super.onStartCommand(intent, flags, startId);

 }

 @Override
 public void onDestroy() {
 super.onDestroy();
 Log.d(TAG, "onDestroy: ");
 }
 }

@Override
 public void onClick(View v) {
 Intent intent = new Intent(Intent.ACTION_VIEW);
 String packageName = "com.example.mylife.anotherapp";
 String className = "com.example.mylife.anotherapp.MyService";
 intent.setClassName(packageName, className);

 switch (v.getId()) {
 case R.id.btn_start:
 Bundle bundle = new Bundle();
 bundle.putString("msg", "this message is from project B ");
 intent.putExtras(bundle);
 intent.putExtra("pid", android.os.Process.myPid());
 startService(intent);
 break;
 case R.id.btn_stop:
 stopService(intent);
 break;
 }
 }