public class MainActivity extends Activity { 
//必须重写的方法 
@Override 
protected void onCreate(Bundle savedInstanceState) { 
super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main);//activity的布局 
} 
}

<activity 
android:name="com.zy.demo.activity.MainActivity" 
android:label="@string/app_name" > 
<intent-filter> 
<action android:name="android.intent.action.MAIN" /> 
<category android:name="android.intent.category.LAUNCHER" /> 
</intent-filter> 
</activity>

//定义一个intent，指名要启动的activity：ToStartActivity 
Intent intent = new Intent(MainActivity.this,ToStartActivity.class); 
//使用startActivity(),启动activity 
startActivity(intent); 

//跨应用从google界面搜索 
Intent intent = new Intent(Intent.ACTION_WEB_SEARCH); 
intent.putExtra(SearchManager.QUERY, "zy"); 
startActivity(intent); 

Intent intent = new Intent(); 
//指定要启动组建完整的包名，对象名 
ComponentName cn = new ComponentName("com.android.settings", 
"com.android.settings.RunningServices"); 
intent.setComponent(cn); 
// 使用context.startActivity()时需要新启一个任务 
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
startActivity(intent);

Intent intent = new Intent(Intent.ACTION_PICK, 
Contacts.People.CONTENT_URI); 
//启动一个带有选择联系人返回结果的activity 
startActivityForResult(intent, PICK_CONTACT_REQUEST); 
这里的PICK_CONTACT_REQUEST为自定义的int型请求反馈结果代码。
//重新onActivityResult()用来处理接收到的返回结果 
@Override 
protected void onActivityResult(int requestCode, int resultCode, Intent data) { 
// 如果请求requestCode成功，且请求返回的结果resultCode是我们要的PICK_CONTACT_REQUEST 
if (resultCode == Activity.RESULT_OK 
&& requestCode == PICK_CONTACT_REQUEST) { 
// 处理Intent返回的数据，在联系人数据库中查找联系人的名字 
Cursor cursor = getContentResolver().query(data.getData(), 
new String[] { Contacts.People.NAME }, null, null, null); 
if (cursor.moveToFirst()) { // 如果cursor不为空，就查找出联系人的名字 
int columnIndex = cursor.getColumnIndex(Contacts.People.NAME); 
String name = cursor.getString(columnIndex); 
//添加其他功能 
} 
} 
}

//此方法用在关闭使用startActivityForResult(requestCode)启用的Activity 
this.finishActivity(requestCode); 

package mm.shandong.com.testusea;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
public class TestUseAActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_test_use_a);
}
//启动第一个activity
public void startFirstActivity(View view) {
Intent intent = new Intent(this, TestUseAActivity2.class);
startActivity(intent);
}
//启动第二个activity
public void startSecondActivity(View view) {
Intent intent = new Intent(this, TestUseAActivity3.class);
startActivity(intent);
}
//启动第三个activity，这个activity 4秒钟后被关闭
public void startThirdActivity(View view) {
Intent intent = new Intent(this, TestUseAActivity4.class);
startActivityForResult(intent, 1);
new Thread() {
@Override
public void run() {
try {
Thread.sleep(4000);
finishActivity(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}.start();
}
}