Retrieve RelativeLayout inside included layout get null

Question

i get null pointer error in line :

rl.setBackgroundResource(R.drawable.headerbackground);

MainActivity code:

LayoutInflater inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);
View mainLayout = inflater.inflate(R.layout.main_layout, null, false);
View includedHeader = mainLayout.findViewById(R.id.header);
RelativeLayout rl = (RelativeLayout) includedHeader.findViewById(R.id.header_root);
rl.setBackgroundResource(R.drawable.headerbackground);
setContentView(mainLayout);

in main_layout.xml I include header_layout :

<include android:id="@+id/header" layout="@layout/header_layout"/>

and in header_layout.xml:

<RelativeLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" 
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:id="@+id/header_root" />

question: why rl is null and how access to it in my code?

EDITED: logcat logs(mainActivity.java:58 is rl.setBackground(R.drawable.headerbackround) in my code):

Caused by: java.lang.NullPointerException
        at com.npi.blureffect.MainActivity.onCreate(MainActivity.java:58)
        at android.app.Activity.performCreate(Activity.java:5243)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2140)

   

Answer 1

You are setting your ContentView in the wrong order. You have to first set the ContentView, then change the background color :

setContentView(R.layout.main_layout);
// the include tag is included dynamically in the root view
RelativeLayout rl = (RelativeLayout) this.findViewById(R.id.header_root);
rl.setBackgroundResource(R.drawable.headerbackground);

Answer 2

Instead of fetching it from includedHeader try to fetch it directly from mainLayout. You don't need to assign id to include tag and bind it in java. You can directly fetch views

Replace

RelativeLayout rl = (RelativeLayout) includedHeader.findViewById(R.id.header_root);

with

RelativeLayout rl = (RelativeLayout) mainLayout.findViewById(R.id.header_root);

Your code will be

LayoutInflater inflater = (LayoutInflater) getSystemService(LAYOUT_INFLATER_SERVICE);
View mainLayout = inflater.inflate(R.layout.main_layout, null, false);
RelativeLayout rl = (RelativeLayout) mainLayout.findViewById(R.id.header_root);
rl.setBackgroundResource(R.drawable.headerbackground);
setContentView(mainLayout);

Answer 3

Some code like your works very well for me in Android 4.2.2 until nowadays.

But, I don't know why, after Android 4.4 a (RelativeLayout) included dynamically inside another (RelativeLayout) returns null after a rootLayout.findViewById( ... ).

In this cases, you can safely find by views using root layout directly!

So, to correct work this code you must include only one line of code:

LayoutInflater inflater = (LayoutInflater) 
getSystemService(LAYOUT_INFLATER_SERVICE);
View mainLayout = inflater.inflate(R.layout.main_layout, null, false);
View includedHeader = mainLayout.findViewById(R.id.header);
RelativeLayout rl = (RelativeLayout) includedHeader.findViewById(R.id.header_root);

if ( rl == null ) { rl = includedHeader; } // include this line

rl.setBackgroundResource(R.drawable.headerbackground);
setContentView(mainLayout);

This code works succesfully for all Androids before and after 4.4

Berst regards!

Answer 4

You should try debugging and see at line rl.setBackgroundResource(R.drawable.headerbackground); if the variable rl is null. In case it's not then maybe the drawable is null.

EDIT:

rl is null because includedHeader must be null. Try removing the line View includedHeader = mainLayout.findViewById(R.id.header); and replace with RelativeLayout rl = (RelativeLayout) mainLayout.findViewById(R.id.header_root); Afterwards check to see if mainLayout or rl are null (while debugging)

EDIT 2:

When you are creating reusable layouts (ones that will be later included using include) it's always best to make the root a FrameLayout

Also check this page out and maybe you can use the <merge> tag instead.