如何更改背景图片？

Question

I studied all possible answers and found something that should work:

int[] tabelazdjec={R.drawable.pic2,R.drawable.pic3};
LinearLayout mylay=(LinearLayout)findViewById(R.id.mylay);
@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.yess);
    Thread timer = new Thread(){
        public void run()
        {
            for(int i=0;i<tabelazdjec.length;i++)
            {
            try
            {

                mylay.setBackgroundResource(tabelazdjec[i]);
                sleep(500);
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }

        }}
    };
    timer.start();

It doesn't. The error appears in the second line where I declare the LinearLayout variable. If I move the declaration into try{}catch{} part, the app runs but try{} doesn't execute, so it is quite certainly this exact line. The xml file goes as follows:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:id="@+id/mylay"
android:layout_height="match_parent"
android:orientation="vertical"
android:background="@drawable/pic1" >


</LinearLayout>

Sorry if the question seems trivial, i've spent last few hours staring at that line and can't find anything wrong with it. Maybe I'm blind...

Answer 1

You can't initialize a View until you have inflated its layout . Move the LinearLayout initialization to below setContentView()

LinearLayout mylay;

@Override
protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.yess);
    mylay=(LinearLayout)findViewById(R.id.mylay);
    Thread timer = new Thread(){

Your Views will return null if you try to initialize them before inflating the layout that they belong to as with setContentView() .

Also, as E.Odebugg has pointed out in a comment, you are trying to update a UI element in a background Thread which will be your next problem. You need to use runOnUiThread() to update the UI .