What does FragmentManager and FragmentTransaction exactly do?

getFragmentManager()

Return the FragmentManager for interacting with fragments associated with this activity.

FragmentManager which is used to create transactions for adding, removing or replacing fragments.

fragmentManager.beginTransaction();

Start a series of edit operations on the Fragments associated with this FragmentManager.

The FragmentTransaction object which will be used.

fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);

Replaces the current fragment with the mFeedFragment on the layout with the id: R.id.fragment_container

fragmentTransaction.addToBackStack(null);

Add this transaction to the back stack. This means that the transaction will be remembered after it is committed, and will reverse its operation when later popped off the stack.

Useful for the return button usage so the transaction can be rolled back.The parameter name:

Is an optional name for this back stack state, or null.

See for information the other question What is the meaning of addToBackStack with null parameter?

The Last statement commits the transaction and executes all commands.

See the google documentation for more help:

http://developer.android.com/reference/android/support/v4/app/FragmentActivity.htmlhttp://developer.android.com/reference/android/app/FragmentManager.htmlhttp://developer.android.com/reference/android/app/FragmentTransaction.html

Android FragmentManager

A FragmentManager manages Fragments in Android, specifically it handles transactions between fragments. A transaction is a way to add, replace, or remove fragments.

Android FragmentTransaction

As said before a FragmentTransaction gives us methods to add, replace, or remove fragments in Android. It gives us an interface for interacting with fragments.

fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);

The method replace(int containerViewId, Fragment fragment) replaces an existing Fragment object from the container containerViewId and adds the the Fragment fragment

fragmentTransaction.addToBackStack(null);

This method, addToBackOfStack(String name), adds this transaction to the back stack, this can be used so that Fragments are remembered and can be used again by the Activity

fragmentTransaction.commit();

The method commit() schedules this transaction, this is not instantaneous; It is scheduled on the main thread to be done when the thread is ready.

Reference