What does FragmentManager and FragmentTransaction exactly do?

I have simple code below

FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
fragmentTransaction.replace(R.id.fragment_container, mFeedFragment);
fragmentTransaction.addToBackStack(null);
fragmentTransaction.commit();

What do these lines of code 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 https://stackoverflow.com/questions/22984950/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.html http://developer.android.com/reference/android/app/FragmentManager.html http://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.

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> 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