Android Null Pointer Exception on OnClickListener line

I have a simple test application with a GridView containing an array of buttons, I'm trying to perform an o开发者_Go百科peration (any operation) touching one of these buttons.

The main code is this:

package com.example.convert;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.GridView;

public class convert extends Activity {
    //private ListView List;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        String [] elenco = {
                "ciao","questo","è","un esempio","Configurazione"
                };

        ArrayAdapter<String> arrayAdapter = new ArrayAdapter<String>(this,R.layout.oggetto,R.id.testogg,elenco);
        GridView gridview = (GridView) findViewById(R.id.gridview);
        gridview.setAdapter(arrayAdapter);
        Button btn = (Button) (findViewById(R.id.testogg));


        btn.setOnClickListener(new OnClickListener(){
              @Override
              public void onClick(View v) {
                  setContentView(R.layout.oggetto);
              }

            });

    }

}

The main.xml is the following:

<?xml version="1.0" encoding="utf-8"?>

<GridView xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/gridview"
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent"
    android:numColumns="auto_fit"
    android:verticalSpacing="10dp"
    android:horizontalSpacing="10dp"
    android:columnWidth="90dp"
    android:stretchMode="columnWidth"
    android:gravity="center"
/>

And oggetto.xml contains the following lines:

<?xml version="1.0" encoding="utf-8"?>

<Button
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:text="@+id/TextView01"
    android:layout_width="80dp"
    android:layout_height="80dp"
    android:id="@+id/testogg">
</Button>

But when I try to run the application it return me a NullPointerException on the btn.setOnClickListener(new OnClickListener()) line.

May be that happen because the buttons have the same id? If this is the case, how can I obtain repeated buttons with different ids?

Thanks for your help!

May be that happen cause the buttons have the same id

That could be. But it also shows something is wrong if that's even the case. Each button specified in XML is only created once. You can't create multiple buttons just by calling findViewById() multiple times or by passing in the resource ID.

But the problem comes because you call setContentView(R.layout.main);

findViewById() will ONLY return non-null views for views contained in main.xml. Your context knows nothing about your other XML layouts because they're not inflated.

You should probably start with simpler programs and go through the tutorials to get a feeling for how android views work.

What are you trying to accomplish? You're originally setting a layout that only contains a GridView, then you're trying to find a Button that doesn't even exist (at the faulting line, your content view is your main.xml document, which does not contain any buttons, let alone one with the id of testogg). This means that btn is null, as the button does not exist. That is the cause of your NullPointerException.

If you want to intercept a click event for an item in your GridView, you need to be using the OnItemClickListener, and attaching that to your GridView. See the link for the onItemClick method definition.