Develop Reference

How to leave the current app and launch an activity of another app?

I have a simple Android app that needs to launch another app under certain condition, and I will need to check the condition upon the app launch. That is, either continue to launch my app or just launch another app:
if (A == true) {
launch another activity of another app
leave the current app without creating the main activity
} else {
launch the main activity of the current app
}
Could anyone please let me know how to deal with A == true case? I am able to launch another app's activity but I have trouble leaving the current app without even opening the main activity.
Any help will be greatly appreciated!

You can launch other application using following intent
Intent LaunchIntent = getPackageManager().getLaunchIntentForPackage("com.example.abc");//pass the packagename of app you want to open
startActivity( LaunchIntent );
If you don't know the package name of application that you wanted to launch then try your hand on
PackageManager pm;
pm = getPackageManager();
// get a list of installed apps.
packages = pm.getInstalledApplications(0);
Pass the packagename of app you want to open
You can use this if A == true
else You can launch the MainActivity as
startActivity(new Intent(CurrentActivity.this,MainActivity.class));

If you want to start another activity of another app without the normal IntentFilter then the easiest solutions is:
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.setComponent(new ComponentName("com.anotherapp.package","com.anotherapp.package.MainActivity"));
startActivity(intent);
Of course, you would need to know the package name and the Activity name you want to start
As for finishing your application call
finishAndRemoveTask();

How to resume an existing task in its current state

My application has two activities A and B. A is the root of the task, and is the one that is launched from the launch icon. B can be started from A.
As well as starting A from the launch icon, it is possible to launch A by clicking on a file in another application, e.g. clicking on an email attachment or a file in Drive. I have done this by adding actions and categories to the intent filter in the manifest file.
I want to make it so that when A is launched from another application, instead of creating a new task I want the existing task to resume in the same state it was in before. This could be activity A or B, wherever the user happened to be before they pressed home.
I have tried all kinds of launch modes and intent flags but nothing seems to work.

Broadcast an intent that's identical to the launcher intent in your manifest:
Intent intent = new Intent(context, MainActivity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
intent.setAction(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_LAUNCHER);
I don't know why, but this raises the existing task instead of starting a new one. By contrast, a launcher intent obtained the "official" way will actually start a new task:
Intent intent = context.getPackageManager()
.getLaunchIntentForPackage(context.getPackageName());

change launchmode to singletask. and listening onNewIntent()

How to change main activity in Android programmatically

Is it possible to create two entry points to an application in Android, I mean can I switch the main activity programmatically?

Every exported activity is a potential entry point into your app; a foreign app can start any of them with an intent. (An intent-filter comes with an implicit android:export.) You can however only have one entry point that the launcher will respect. To simulate a second launch-point, either
Provide a completely separate app with the purpose of starting one of your exported activities, or
Give your 'launch' activity the sole purpose of immediately starting one or another activity based on some logic (a saved preference, a phase-of-moon calculation, anything).

check this one below
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_LAUNCHER);
intent.setComponent(new ComponentName(packageName,mainActivity));
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
ctx.startActivity(intent);

I think you are talking about launching activity decision based on some events, then you need to add a broadcast receiver, like by clicking on app icon on launcher if you want to start Activity1. then add intent filters to this activity Action_MAIN and ACTION_LAUNCHER, if you want to start Activity2 on phone boot up, then add filter to this activity, BOOT_COMPLETED.

If you are talking about to launch other apps from your apps then this can be the code:
Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_LAUNCHER);
intent.setComponent(new ComponentName(packageName,mainActivity));
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
ctx.startActivity(intent);

Launching activity from another application not displaying the activity, but it is running

I've done a lot of searching for the last day and haven't found anything that seems to match the problem I'm having.
(For reasons I won't go into) the app is divided into two separate apks. Each have activities. Only the "core" apk has a MAIN activity that is launched from the Android Launcher. The "plugin" apk has activities that only exist to be called from the first apk's activities and does not have a Launcher icon.
The issue I'm having is that when I create an intent to launch an activity from the "plugin" apk, it does the "launching new activity" sliding animation but immediately bounces back. But it is actually launching the activity because I'm seeing log statements in logcat coming from the new activity. I'm not getting any exceptions and it seems to be working other than the fact I'm not seeing the activity on the screen.
I've tried creating the intent in both of the following ways:
Intent myIntent = new Intent(Intent.ACTION_VIEW);
myIntent.setClassName("com.test.plugin", "com.test.plugin.PluginActivity");
startActivity(myIntent);
and
Intent myIntent = new Intent();
myIntent.setComponent(new ComponentName("com.test.plugin", "com.test.plugin.PluginActivity"));
startActivity(myIntent);
But both result in the same thing happening as described above.

It bounces back means the plugin activity has some problem in launching.

Open my application from another in android

My boss asked me to prove that my application behaves properly when summoned by another application (dunno why he asked that).
So I have two apps here, one launches a second one. How I launch the specific app I want? Using Intent launch seemly any generic app that reaches a certain goal, not the app I really want.

Give this a try.
Intent secondIntent = new Intent();
secondIntent.setAction(Intent.ACTION_MAIN);
secondIntent.setClassName("com.example", "com.example.YourSecondApp");
startActivity(secondIntent);
I should point out that com.example should be the package of your second application (the one you want to call) and com.example.YourSecondapp is the class name where you have your onCreate() method.

Intent secondApp = new Intent("com.test.SecondApp");
startActivity(secondApp);
Check out for more examples
http://developer.android.com/resources/faq/commontasks.html#opennewscreen

Create one Intent using the following code
Explicit Intent
When you know the particular component(activity/service) to be loaded
Intent intent = new Intent();
intent.setClass("className/package name");
start<Activity/Service>(intent);
Imlicit Intent
When we do not have the idea which class to load and we know the Action to be perform by the launched application we can go with this intent.
Action needs to set, and the Android run time fallows the intent Resolution technique and list out(one or more components) the components to perform the action. from the list out components (if more than one), user will get the chance to launch his chosen application