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why viewPage keeps previous viewpage content after clicking back button

I have a problem when I click back button while using ViewPage. When I enter to ViewPage after going out and coming back, it shows me the content of previous ViewPage, not new ones.
I'm entering to ViewPage from a RecyclerView and I want to change ViewPage content by clicking on items of RecyclerView.
So my problem is that why ViewPage is not reset to new content?

before committing the fragment do this! it won't remember the last open fragment and please read about BackStack management in fragments here
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);
// Commit the transaction
transaction.commit();

If by saying ViewPage you mean 'ViewPager'. Then you must be displaying the content using a fragment. So when you open the ViewPager screen 'Replace' the fragment with new content.
Use this syntax:
getSupportFragmentManager().beginTransaction()
.replace(R.id.container, new Frag_four()).commit();
Or this one
getFragmentManager().beginTransaction()
.replace(R.id.container, new Frag_four()).commit();
You are facing this issue because may be you are not removing previous content from the container. So just give it a try.
EDIT:
In your fragment displaying activity place the following code in 'OnDestory' method.
getFragmentManager().beginTransaction().remove(getFragmentManager().findFragmentById(R.id.your_container)).commit();
This will empty the container. So next time it will load the new fragment.

Why does not `replace` replace a fragment?

I have the following code that adds fragments to the same container R.id.container. First I add one fragment, then after user interacts with it, I add another one with the same code
FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
fragmentTransaction.replace(R.id.container, fragment);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();
My question when I cell getSupportFragmentManager().getFragments() I can see two fragments. Why doesn't replace actually replace but add?

It keeps the two fragments because
addToBackStack(null);
means the user is able to revert back to the previous state.
If you add fragment1 (using replace), then fragment2, only fragment2 is shown, but if the user press back, the FragmentManager needs to show fragment1, so it keeps a reference to both fragment.

Navigate back to activity from fragment

I have class Demo and I am extends the demo by FragmentActivity class. Also I have another class Fragment1 extends Fragment. And the onclick of button i am navigate from activity Demo to the Fragment Fragment1. Now I want to come back on Demo from the Fragment1. So how can I back to Demo activity?
Thanks.

Your question is lacking a lot of detail, so I'm taking a stab in the dark here, but...
I presume in your onClick code, you use the Fragment Manager to create a new Fragment Transaction, then add the fragment to that transaction and commit it?
Your problem with the activity being closed when you hit the back button is likely because your fragment was not added to something called the "back stack". You can find to documentation here http://developer.android.com/reference/android/app/FragmentTransaction.html#addToBackStack%28java.lang.String%29, but crucially the main thing you need to do is modify your code to include the line below:
FragmentManager fragmentManager = getFragmentManager()
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.add(R.id.fragment_container, new Fragment1());
fragmentTransaction.addToBackStack("Transaction ID"); // <-- This is key!
fragmentTransaction.commit();
Once that's done, Android will remember the adding of the fragment as a navigation act, and should reverse the transaction when the back button is hit.
If this wasn't what you're looking for, or if it doesn't work, please provide some more detail and some code samples and I can take another look.

Try adding the fragment you want to go back to manually to the backstack.
FragmentTransaction transaction = getFragmentManager().beginTransaction();
YourFragmentName myFragment = new YourFragmentName();
transaction.replace(R.id.fragment_container, myFragment);
//if you with to add it to backStack, do this, otherwise skip the line below
transaction.addToBackStack(null);
transaction.commit();

Best practice for keeping Fragments for later display

I have an activity with a FrameLayout in it.
The activity should show four steps, and each step is a Fragment. When I want to go back-further, I don't want my fragments to be recreated. I would like to retain them and simply replace their view in my fragment.
I used to first create my Fragments and add them in the backstack like this:
Fragment step= new Frag1ActCompleteFragsCommTrack();
FragmentTransaction ft= getSupportFragmentManager().beginTransaction();
ft.add(step, ""+onStepNr);
ft.addToBackStack(null);
ft.commit();
notice that I don't show it, I simply create it and add to the backstack.
So, once I need one of my fragments to show, I add it (in this example I don't remove any fragment from the framelayout just because it's my first add):
FragmentTransaction ft= getSupportFragmentManager().beginTransaction();
ft.add(R.id.my_frameLayout, step);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.commit();
So: the problem is that I obtain a
Caused by: java.lang.IllegalStateException: Fragment already added: Frag1ActCompleteFragsCommTrack{410dcb20 #0 id=0x7f050041 -1}
But I think I can't add directly into my framelayout the first time, otherwise the next time I replace it, I could lose my fragment. Am I right? So.. what's the best practice for retaining fragments that could interchange each other in a framelayout?

Ladies and gentlemen, I did it!
If you add a Fragment, and you want it to be shown in a framelayout, remember to put it in the Fragment backstack. That's it! If you replace it in the framelayout with another one, no worries: you can put it back by finding it thanks to its tag.
It was easier than I thought actually
//step is an int describing the step associated to the fragment I wanna place
FragmentTransaction ft= getSupportFragmentManager().beginTransaction();
ft.replace(R.id.act_complete_track_frameLayout, f, ""+step);
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
if(firstAttach)
ft.addToBackStack(null);
ft.commit();
imagine a fragment with tag "1" replaced through the code above by a fragment with tag "2". If I want to go back to step1, I reuse that code by obtaining my old fragment with getSupportFragmentManager().findFragmentByTag("1")
To be short, I thought that FragmentTransaction.replace removed the fragment from the backstack as well. That seems not to be the case (luckily)

You can always do something like fragmentManager.putFragment(yourFragment);

If I understand correctly, you are trying to add all the fragments but not show them until you are ready. FragmentTransaction.add() doesn't exactly do that though. It will also be shown after its added. You should use hide() after adding each fragment, and then later you can use show() to make it visible, and hide() again to make other fragments invisible.
Like this:
Fragment step = new Frag1ActCompleteFragsCommTrack();
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.add(step, ""+onStepNr);
ft.hide(step);
ft.commit();
Then later:
Fragment step = getSupportFragmentManager().findFragmentByTag(""+onStepNr);
FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.show(step);
// may want to hide other fragments here
ft.setTransition(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
ft.commit();

Android Fragment Unable to go back

I have an app that uses fragments with tabs/viewpager
[Tab 1][Tab 2][Tab 3]
Tab2 has a ListView and in the onClick method of the ListView I am showing a detail view with the following code
FragmentTransaction transaction = getChildFragmentManager().beginTransaction();
NextFragment nextFragment = new NextFragment();
transaction.replace(R.id.container, nextFragment);
transaction.addToBackStack(null);
transaction.commit();
The issue is that in the NextFragment when I use the hardware back button the whole app closes?

I'm not sure why it doesn't let you return when back is pressed, but what I've done in my application is the following:
I have overriden the onBackPressed action and created a check that once it is pressed and the user is in the fragment that he wants to return from, just create another fragment transaction to the previous fragment.
I have done so because I needed to update the previous fragment, but I'm sure there is a better way to solve your problem.