How to programmatically set a lock or pin for an app

String lastAppPN = "";
public void checkRunningApps() {
    ActivityManager mActivityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
    String activityOnTop;
    if (Build.VERSION.SDK_INT > 20) {
        activityOnTop = mActivityManager.getRunningAppProcesses().get(0).processName;
    } else {
        List<ActivityManager.RunningTaskInfo> RunningTask = mActivityManager.getRunningTasks(1);
        ActivityManager.RunningTaskInfo ar = RunningTask.get(0);
        activityOnTop = ar.topActivity.getPackageName();
    }
    //Log.e("activity on TOp", "" + activityOnTop);

    // Provide the packagename(s) of apps here, you want to show password activity
    if (activityOnTop.contains("whatsapp")  // you can make this check even better
            || activityOnTop.contains(CURRENT_PACKAGE_NAME)) {
        if (!(lastAppPN.equals(activityOnTop))) {
            lastAppPN = activityOnTop;
            Log.e("Whatsapp", "started");
        }
    } else {
        if (lastAppPN.contains("whatsapp")) {
            if (!(activityOnTop.equals(lastAppPN))) {
                Log.e("Whatsapp", "stoped");
                lastAppPN = "";
            }
        }
        // DO nothing
    }
}

I have created small demo project. Hope this might be useful to someone Link to project

Logic

• You have to make and start a service when you want to block apps,
• And In Service you have to check packagenames of the apps, so that you can decide which app to run and which to show a pin/password activity

Now Code Example:

• To Start a service, code like this,

  startService(new Intent(this, SaveMyAppsService.class));
  

• Now, Inside your service, check packages like this,

  public class SaveMyAppsService extends android.app.Service 
  {
  
      String CURRENT_PACKAGE_NAME = {your this app packagename};
      String lastAppPN = "";
      boolean noDelay = false;
      public static SaveMyAppsService instance;
  
      @Override
      public IBinder onBind(Intent intent) {
          // TODO Auto-generated method stub
          return null;
      }
  
      @Override
      public int onStartCommand(Intent intent, int flags, int startId) {
          // TODO Auto-generated method stub
  
          scheduleMethod();
          CURRENT_PACKAGE_NAME = getApplicationContext().getPackageName();
          Log.e("Current PN", "" + CURRENT_PACKAGE_NAME);
  
          instance = this;
  
          return START_STICKY;
      }
  
      private void scheduleMethod() {
          // TODO Auto-generated method stub
  
          ScheduledExecutorService scheduler = Executors
                  .newSingleThreadScheduledExecutor();
          scheduler.scheduleAtFixedRate(new Runnable() {
  
              @Override
              public void run() {
                  // TODO Auto-generated method stub
  
                  // This method will check for the Running apps after every 100ms
                  if(30 minutes spent){
                       stop();
                  }else{
                     checkRunningApps();
                 }
              }
          }, 0, 100, TimeUnit.MILLISECONDS);
      }
  
      public void checkRunningApps() {
          ActivityManager mActivityManager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
          List<ActivityManager.RunningTaskInfo> RunningTask = mActivityManager.getRunningTasks(1);
          ActivityManager.RunningTaskInfo ar = RunningTask.get(0);
          String activityOnTop = ar.topActivity.getPackageName();
          Log.e("activity on TOp", "" + activityOnTop);
  
          // Provide the packagename(s) of apps here, you want to show password activity
      if (activityOnTop.contains("whatsapp")  // you can make this check even better
              || activityOnTop.contains(CURRENT_PACKAGE_NAME)) {
              // Show Password Activity                
          } else {
              // DO nothing
          }
       }
  
      public static void stop() {
          if (instance != null) {
          instance.stopSelf();
          }
      }
  }   
  

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Edit: (Get Top Package Name for Lollipop)

A very good answer is here.