Question

this is the code used in tha java class.it doesnt show any errors in eclipse.the logcat shows null pointer exception and class caste exception

package com.myfirstapp;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.*;


public class MyfirstappActivity extends Activity {
/** Called when the activity is first created. */

int count ;
Button add,sub;
TextView txt;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    count = 0;
    add = (Button) findViewById(R.id.add_one);
    sub = (Button) findViewById(R.id.sub_one);
    txt = (Button) findViewById(R.id.display);
    setContentView(R.layout.main);
    add.setOnClickListener(new View.OnClickListener() { 
        public void onClick(View v) {
            // TODO Auto-generated method stub
            count = count + 1;
            txt.setText("Your total is " + count);
        }
    });

    sub.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            // TODO Auto-generated method stub
            count = count - 1;
            txt.setText("Your total is " + count);
        }
    }); 

}

this is the main.xml layout file created. i tried cleaning the project meanwhile.tried using the try catch mechanism.the app s running fine when i commented the button referncing part of the above code

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:weightSum="1">

    <TextView  
        android:layout_width="fill_parent" 
        android:layout_height="60dp" 
        android:text="@string/hello"
        android:id="@+id/display"
        android:textSize="40dp"
        android:gravity="center" />

   <Button
       android:text="@string/add1"
       android:layout_gravity="center" 
       android:layout_width="fill_parent"
       android:layout_height="60dp"
       android:id="@+id/add_one"
       android:textSize="40dp"
       android:gravity="center">
   </Button>

   <Button
       android:text="@string/sub1"
       android:layout_gravity="center" 
       android:layout_width="fill_parent"
       android:layout_height="60dp"
       android:id="@+id/sub_one"
       android:textSize="40dp"
       android:gravity="center">
   </Button>

</LinearLayout>

Answer 1

txt is TextView, you are Casting it to button. Use

txt = (TextView) findViewById(R.id.display);

Also you need set the Layout resource for this. So use,

super.onCreate();
setContentView(R.layout.main);

Edit- You code should be something like,

super.onCreate(savedInstanceState);
setContentView(R.layout.main);
count = 0;
add = (Button) findViewById(R.id.add_one);
sub = (Button) findViewById(R.id.sub_one);
txt = (TextView) findViewById(R.id.display);