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

不知道大家有沒有思考過這三個事情： 
1、當(dāng)A應(yīng)用 啟動 B應(yīng)用的SecondActivity，那么B應(yīng)用的MainActivity會不會被啟動呢？正常情況下，我們點擊應(yīng)用B，進到的是MainActivity這個活動，那么現(xiàn)在我們是通過跨應(yīng)用啟動，會不會要經(jīng)過B的MainActivity呢？答案是不會。

2、當(dāng)我們在SecondActivity中點擊Back回退鍵時，回到的是A應(yīng)用的mainActivity界面，這里時候大家有沒有想過。 
SecondActivity和appA的mainActivity是不是同處于一個棧中呢？這時候就要去打印棧的ID了。

3、由上面的兩件事，不知道大家想起：Android對于Activity的管理，也就是framework層的ActivityManager。也就是說，你手機上的N多應(yīng)用，當(dāng)你打開某一個應(yīng)用是，這個應(yīng)用的Activity都是由ActivityManager這娃來創(chuàng)建和管理的。應(yīng)用本身并沒有創(chuàng)建Activity的能力。當(dāng)然這其中又涉及到了Ibinder的通訊。這里暫時不講。

@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();//要清楚這里為什么要用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();//要清楚這里為什么要用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;
 }
 }