Why is savedInstanceState always null?

Question

Java

I have two activities. After entering data on the first, the application switches to the second activity. When I click back, I want to see the data entered earlier. But this does not happen - all fields are empty.
public void onSaveInstanceState(Bundle savedInstanceState) is always called when switching between activities. However, onCreate savedInstanceState is always null .
Why is that? And how to fix it?
The code:

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        ButterKnife.bind(this);

        if (savedInstanceState != null) {
            firstNameEdit.setText(savedInstanceState.getString(STATE_FIRST_NAME_EDITOR));
            secondNameEdit.setText(savedInstanceState.getString(STATE_SECOND_NAME_EDITOR));
            birthDate.setText(savedInstanceState.getString(STATE_BIRTH_DATE_EDITOR));
        }
    }

 @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
        savedInstanceState.putString(STATE_FIRST_NAME_EDITOR, firstNameEdit.getText().toString());
        savedInstanceState.putString(STATE_SECOND_NAME_EDITOR, secondNameEdit.getText().toString());
        savedInstanceState.putString(STATE_BIRTH_DATE_EDITOR, birthDate.getText().toString());

        super.onSaveInstanceState(savedInstanceState);
    }

Answer 1

Decision.
I set the property for the activity in the manifest: android:launchMode="singleTop"

Answer 2

When you open a new activity, do you happen to pull finish() somewhere on the old one? In this case, the data may not be saved.
If you just open a new activity, then the old one will go into the background and when you press up, you will simply return to the old task with all the data. An exception is the situation when the system itself kills your activity (flip, little memory), in this case the system will save the state and upon return a new activity will be created, where this state must be gutted.
Check launchmode in manifest.