Why does not putting this code into a function cause a null pointer exception?

Question

I'm doing Google's android courses on Udacity to learn Android and Java. Currently, the project I'm working on is building an app which represents ordering coffee. There's a quantity TextView, along with a + and - button which on click either increment or decrement the quantity, as well as a submit order button which calls a method that prints out an order summary.

Unfortunately, I lost the original source code for the app, so I tried rebuilding it myself. I got it to work, but not in the way I originally designed it. The issue I'm having is that if I reference the TextView which stores the quantity of coffees ordered outside of its own method, the app immediately crashes due to a NullPointerException. However, if I reference that TextView by creating a method called display(int number), the app runs fine. What is causing this NullPointerException? I've provided links below to the two source codes, one of which works fine and the other of which crashes immediately.

NullPointerException: https://gist.github.com/CalebF98/eeb7d4144b831b247264ad6a3de0cb76 The part that is crashing the app in this code is at line 18 in the MainActivity.java file

Works fine: https://gist.github.com/CalebF98/906967d4ebe8ae4d1293a0a13b01f477 In this one, on lines 37-39, I create a method that holds the same code to reference the TextView as in the example above, which is later called by the methods for the + and - buttons.

Thank you!

Please put the relevant code in the question itself, don't just link to it off-site. — EJoshuaS - Stand with Ukraine, Feb 21 '17 at 20:23
Sorry, I'm still getting used to posting code, I wasn't sure what was relevant or not. My apologies. — Caleb Fringer, Feb 21 '17 at 21:16

score 1 · Accepted Answer · answered Feb 21 '17 at 20:27

findViewById requires that the Activity has inflated the XML in onCreate(Bundle)

In the first code sample, you are initializing the TextView with the id "quantity" outside the onCreate method, so it will get initialized when the Activity object is created. At that point, there is no root View, so the findViewById method is trying to search the view hierarchy for the desired TextView but the root view is null.

In the second code sample, findViewById is only called in onClick methods, so it cannot be called until after the view hierarchy is inflated, so everything is fine.

score 0 · Answer 2 · answered Feb 21 '17 at 20:21

 <TextView
    android:text="Order Summary"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="16dp"
    android:textAllCaps="true"/>

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="16dp"
    android:id="@+id/order_summary"
    android:text="$0"/>

You forgot to put the id for the first text view. Hope adding the id to the first text view fixes the problem

Why does not putting this code into a function cause a null pointer exception?

2 Answers2