在Eclipse中运行应用程序时出错

Question

I'm having a problem with running my application in Eclipse. When I run my application it crashes immediately and says Unfortunately (App name) has stopped.

The application is a simple 2 screen ticket price application. First page just has a textview, imageview, and button. Second page has a text view, spinner, edit text, button and another textview. It's pretty much combining chapter 2 and 3 from the Android Bootcamp book.

Here is the logcat info:

02-15 14:31:28.061: D/dalvikvm(536): Not late-enabling CheckJNI (already on)
02-15 14:31:28.851: D/dalvikvm(536): GC_FOR_ALLOC freed 67K, 3% free 9936K/10179K, paused 51ms
02-15 14:31:28.851: I/dalvikvm-heap(536): Grow heap (frag case) to 10.257MB for 494176-byte allocation
02-15 14:31:28.941: D/dalvikvm(536): GC_CONCURRENT freed 1K, 3% free 10417K/10695K, paused 7ms+3ms
02-15 14:31:28.971: D/AndroidRuntime(536): Shutting down VM
02-15 14:31:28.971: W/dalvikvm(536): threadid=1: thread exiting with uncaught exception (group=0x409961f8)
02-15 14:31:28.981: E/AndroidRuntime(536): FATAL EXCEPTION: main
02-15 14:31:28.981: E/AndroidRuntime(536): java.lang.RuntimeException: Unable to start activity ComponentInfo{net.androidbootcamp.racetickets/net.androidbootcamp.racetickets.Main}: java.lang.NullPointerException
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1955)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread.access$600(ActivityThread.java:122)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.os.Looper.loop(Looper.java:137)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread.main(ActivityThread.java:4340)
02-15 14:31:28.981: E/AndroidRuntime(536):  at java.lang.reflect.Method.invokeNative(Native Method)
02-15 14:31:28.981: E/AndroidRuntime(536):  at java.lang.reflect.Method.invoke(Method.java:511)
02-15 14:31:28.981: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
02-15 14:31:28.981: E/AndroidRuntime(536):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
02-15 14:31:28.981: E/AndroidRuntime(536):  at dalvik.system.NativeStart.main(Native Method)
02-15 14:31:28.981: E/AndroidRuntime(536): Caused by: java.lang.NullPointerException
02-15 14:31:28.981: E/AndroidRuntime(536):  at net.androidbootcamp.racetickets.Main.onCreate(Main.java:31)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.Activity.performCreate(Activity.java:4465)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049)
02-15 14:31:28.981: E/AndroidRuntime(536):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1919)
02-15 14:31:28.981: E/AndroidRuntime(536):  ... 11 more

main.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  tools:context=".Main" >

  <TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:text="Gateway Raceway"
    android:textSize="38sp" />

  <ImageView
    android:id="@+id/imageView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="68dp"
    android:src="@drawable/gateway" />

  <Button
    android:id="@+id/btnTickets"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/imageView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="72dp"
    android:text="Find Tickets"
    android:textSize="32sp" />

</RelativeLayout>

tickets.xml (2nd page)

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent" >

  <TextView
    android:id="@+id/textView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_centerHorizontal="true"
    android:text="Gateway Tickets"
    android:textSize="38sp" />

  <TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="44dp"
    android:text="Which Tickets do you want?"
    android:textSize="20sp" />

  <Spinner
    android:id="@+id/txtRacing"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_below="@+id/textView2"
    android:layout_marginTop="40dp"
    android:entries="@array/Type" />

  <Button
    android:id="@+id/btnBuy"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="59dp"
    android:text="Buy Tickets"
    android:textSize="25sp" />

  <TextView
    android:id="@+id/txtPrice"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/btnBuy"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="36dp"
    android:text="results" />

  <EditText
    android:id="@+id/txtNumber"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/txtRacing"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="23dp"
    android:ems="10"
    android:hint="How many tickets?"
    android:inputType="number" >

    <requestFocus />
  </EditText>

</RelativeLayout>

Main.java

package net.androidbootcamp.racetickets;

import java.text.DecimalFormat;

import android.os.Bundle;
import android.app.Activity;
import android.content.Intent;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;

public class Main extends Activity {

  double costPerTicket=15.00;
  int numberOfTickets;
  double totalCost;
  String groupChoice;

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    final EditText tickets=(EditText) findViewById(R.id.txtNumber);
    final Spinner group = (Spinner) findViewById(R.id.txtRacing);
    Button cost = (Button) findViewById(R.id.btnBuy);
    final TextView result = ((TextView) findViewById(R.id.txtPrice));
    cost.setOnClickListener(new OnClickListener () {
      @Override
      public void onClick(View v) {
        //TODO Auto-generated method stub
        numberOfTickets = Integer.parseInt(tickets.getText().toString());
        totalCost = costPerTicket + numberOfTickets;
        DecimalFormat currency = new DecimalFormat ("###,###.##");
        groupChoice = group.getSelectedItem().toString();
        result.setText("Total Cost for" + groupChoice + " is " + currency.format(totalCost));
      }
    });

    Button b=(Button)findViewById(R.id.btnTickets);
    b.setOnClickListener(new OnClickListener () {
      public void onClick(View v) {
        //TODO Auto-generated method stub
        startActivity(new Intent(Main.this, Tickets.class));
      }
    });
  }

  @Override
  public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
  }
}

Tickets.java

package net.androidbootcamp.racetickets;

import android.app.Activity;
import android.os.Bundle;


public class Tickets extends Activity {
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    // TODO Auto-generated method stub
    super.onCreate(savedInstanceState);
    setContentView(R.layout.tickets);
  }             
}

Any Ideas?

Answer 1

This line

Button cost = (Button) findViewById(R.id.btnBuy); 

which you are calling in the main.java class will not work. Because your button is declared in the tickets.xml

i think you must replace

Button cost = (Button) findViewById(R.id.btnBuy); 

with to get it works

Button cost = (Button) findViewById(R.id.btnTickets);