Hello stackoverflow社区，
我最近开始学习和编写Android应用程序。对于我的第一个应用程序，我决定创建一个简单的应用程序，按下按钮（当你去洗手间时），计数在可编辑的文本字段中上升。还有一个重置按钮。在模拟器中启动时，它会打开到我可以看到应用程序标题的位置，但随后会意外崩溃。代码如下，请帮助：
MainActivity.java：
package com.CPTech.bathroomtracker;
import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends Activity {
private int TOTAL_TRIPS = 0;
private EditText tripsNumberEditText;
Button bathroomTripButton;
Button resetButton;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
tripsNumberEditText.setText(TOTAL_TRIPS);
bathroomTripButton = (Button) findViewById(R.id.bathroomTripButton);
bathroomTripButton.setOnClickListener(bathroomTripButtonListener);
resetButton = (Button) findViewById(R.id.resetButton);
resetButton.setOnClickListener(resetButtonListener);
tripsNumberEditText = (EditText) findViewById(R.id.tripsNumberEditText);
}
public OnClickListener bathroomTripButtonListener = new OnClickListener(){
@Override
public void onClick(View v) {
TOTAL_TRIPS++;
tripsNumberEditText.setText(TOTAL_TRIPS);
}
};
public OnClickListener resetButtonListener = new OnClickListener(){
@Override
public void onClick(View v) {
TOTAL_TRIPS = 0;
tripsNumberEditText.setText(TOTAL_TRIPS);
}
};
@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;
}
}
MainActivity.xml：
<TableLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/TableLayout1"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:background="@color/dark_blue"
tools:context=".MainActivity" >
<TableRow
android:id="@+id/tableRow1"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
<Button
android:id="@+id/bathroomTripButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:background="@color/ash_grey"
android:layout_marginTop="30dp"
android:text="@string/bathroom_trip_button"
/>
</TableRow>
<TableRow
android:id="@+id/tableRow2"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="30dp" >
<TextView
android:id="@+id/todayTripsTextView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginLeft="20dp"
android:text="@string/today_trips"
android:textAppearance="?android:attr/textAppearanceLarge"
android:textSize="16pt" />
</TableRow>
<TableRow
android:id="@+id/tableRow3"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="25dp" >
<EditText
android:id="@+id/tripsNumberEditText"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center_horizontal"
android:layout_marginLeft="20dp"
android:textColor="@color/white"
android:textSize="20pt"
android:background="@color/davy_grey"
android:ems="2" >
<requestFocus />
</EditText>
</TableRow>
<TableRow
android:id="@+id/tableRow4"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="75dp" >
<Button
android:id="@+id/resetButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_weight="1"
android:background="@color/ash_grey"
android:text="@string/reset_count" />
</TableRow>
</TableLayout>
logcat的：
10-23 10:43:13.230: D/AndroidRuntime(837): Shutting down VM
10-23 10:43:13.230: W/dalvikvm(837): threadid=1: thread exiting with uncaught exception (group=0x41465700)
10-23 10:43:13.340: E/AndroidRuntime(837): FATAL EXCEPTION: main
10-23 10:43:13.340: E/AndroidRuntime(837): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.CPTech.bathroomtracker/com.CPTech.bathroomtracker.MainActivity}: java.lang.NullPointerException
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread.access$600(ActivityThread.java:141)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.os.Handler.dispatchMessage(Handler.java:99)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.os.Looper.loop(Looper.java:137)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread.main(ActivityThread.java:5103)
10-23 10:43:13.340: E/AndroidRuntime(837): at java.lang.reflect.Method.invokeNative(Native Method)
10-23 10:43:13.340: E/AndroidRuntime(837): at java.lang.reflect.Method.invoke(Method.java:525)
10-23 10:43:13.340: E/AndroidRuntime(837): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
10-23 10:43:13.340: E/AndroidRuntime(837): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
10-23 10:43:13.340: E/AndroidRuntime(837): at dalvik.system.NativeStart.main(Native Method)
10-23 10:43:13.340: E/AndroidRuntime(837): Caused by: java.lang.NullPointerException
10-23 10:43:13.340: E/AndroidRuntime(837): at com.CPTech.bathroomtracker.MainActivity.onCreate(MainActivity.java:23)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.Activity.performCreate(Activity.java:5133)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
10-23 10:43:13.340: E/AndroidRuntime(837): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
10-23 10:43:13.340: E/AndroidRuntime(837): ... 11 more
答案 0 :(得分：2)
看起来像这一行
tripsNumberEditText.setText(TOTAL_TRIPS);
正在提供您的NPE，因为您尚未对其进行初始化。将此行移至初始化
tripsNumberEditText = (EditText) findViewById(R.id.tripsNumberEditText);
tripsNumberEditText.setText(TOTAL_TRIPS);
将是您的下一个错误
此外，您使用了错误的setText()方法。您正在给它一个int，它会告诉它查找resource id，它显然找不到它。您需要使用类似
String
tripsNumberEditText.setText("" + TOTAL_TRIPS);
此外，由于您在用户有机会提供任何输入之前在onCreate()设置文本，您还可以使用
android:text="0"
注意如果你使用像这样的硬编码字符串Eclipse会给你一个警告，但是最好在strings.xml中定义值。
答案 1 :(得分：0)
使用这样的方法，在实例化之前访问了tripsNumberEditText
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
bathroomTripButton = (Button) findViewById(R.id.bathroomTripButton);
bathroomTripButton.setOnClickListener(bathroomTripButtonListener);
resetButton = (Button) findViewById(R.id.resetButton);
resetButton.setOnClickListener(resetButtonListener);
tripsNumberEditText = (EditText) findViewById(R.id.tripsNumberEditText);
tripsNumberEditText.setText(TOTAL_TRIPS);
}
答案 2 :(得分：0)
运行此行：
tripsNumberEditText = (EditText) findViewById(R.id.tripsNumberEditText);
在这一行之前：
tripsNumberEditText.setText(TOTAL_TRIPS);
onCreate() 中的