在我尝试包含菜单，对话框和弹出框后，我开始在启动时收到NullPointerException。我试图注释掉部分代码，只有当我在onCreate方法下面注释类变量和所有内容时，progam才有效。我很感激帮助找到造成异常的原因。
package com.emailandcontactmanager;
import android.app.Activity;
import android.app.AlertDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.PopupWindow;
public class MainActivity extends Activity{
EditText title = (EditText) findViewById(R.id.txtTitle);
EditText body = (EditText) findViewById(R.id.txtBody);
View layout = (View) findViewById(R.layout.main);
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
Button btnSendMenu = (Button) findViewById(R.id.btnSendMenu);
btnSendMenu.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent sendMenu = new Intent(v.getContext(), SendMenu.class);
startActivity(sendMenu);
}
});
Button btnContactsMenu = (Button) findViewById(R.id.btnContactsMenu);
btnContactsMenu.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent contactsMenu = new Intent(v.getContext(), ContactsMenu.class);
startActivity(contactsMenu);
}
});
Button btnTemplatesMenu = (Button) findViewById(R.id.btnTemplatesMenu);
btnTemplatesMenu.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent templatesMenu = new Intent(v.getContext(), TemplatesMenu.class);
startActivity(templatesMenu);
}
});
Button btnTagsMenu = (Button) findViewById(R.id.btnTagsMenu);
btnTagsMenu.setOnClickListener(new View.OnClickListener() {
public void onClick(View v) {
Intent tagsMenu = new Intent(v.getContext(), TagsMenu.class);
startActivity(tagsMenu);
}
});
}//end onCreate
DialogInterface.OnClickListener dialogClickListener = new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int which) {
switch (which){
case DialogInterface.BUTTON_POSITIVE:
finish();
break;
case DialogInterface.BUTTON_NEGATIVE:
//No button clicked
break;
}
}
};
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.menu, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()) {
case R.id.help: showPopup(this);
break;
case R.id.back: AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setMessage("Are you sure?").setPositiveButton("Yes", dialogClickListener)
.setNegativeButton("No", dialogClickListener).show();
break;
}
return true;
}
private void showPopup(final Activity context){
int popupWidth = 200;
int popupHeight = 150;
// change title and body based on button
onHelpClick(layout);
LinearLayout viewGroup = (LinearLayout) this.findViewById(R.id.popup);
LayoutInflater layoutInfalter = (LayoutInflater) this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View layout = layoutInfalter.inflate(R.layout.popup, viewGroup);
final PopupWindow popup = new PopupWindow();
popup.setContentView(layout);
popup.setWidth(popupWidth);
popup.setHeight(popupHeight);
popup.setFocusable(true);
Button close = (Button) layout.findViewById(R.id.close);
close.setOnClickListener(new OnClickListener() {
public void onClick(View arg0) {
// TODO Auto-generated method stub
popup.dismiss();
}
});
}
private void onHelpClick(View v) {
switch(v.getId())
{
case R.id.btnSendMenu:
title.setText("Send Menu");
body.setText("Opens the send menu where you can send an email.");
break;
case R.id.btnContactsMenu:
title.setText("Contacts Menu");
body.setText("Opens the contact menu where you can add a contact or select from a list of" +
"recored contacts to edit or view.");
break;
case R.id.btnTemplatesMenu:
title.setText("Templates Menu");
body.setText("Opens the templates menu where you can add an email template or select from a list of" +
" recorded templates to edit or view.");
break;
case R.id.btnTagsMenu:
title.setText("Tags Menu");
body.setText("Opens the tags menu where you can add a tag used for sorting contacs, or select from a list of" +
" recorded tags to edit or view.");
break;
default:
throw new RuntimeException("Unknown button ID");
}
}
}//end Class
logcat消息是：
09-28 15:07:49.205: D/AndroidRuntime(1403): Shutting down VM
09-28 15:07:49.245: W/dalvikvm(1403): threadid=1: thread exiting with uncaught exception (group=0x409961f8)
09-28 15:07:49.365: E/AndroidRuntime(1403): FATAL EXCEPTION: main
09-28 15:07:49.365: E/AndroidRuntime(1403): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.emailandcontactmanager/com.emailandcontactmanager.MainActivity}: java.lang.NullPointerException
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1879)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1980)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread.access$600(ActivityThread.java:122)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1146)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.os.Handler.dispatchMessage(Handler.java:99)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.os.Looper.loop(Looper.java:137)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread.main(ActivityThread.java:4340)
09-28 15:07:49.365: E/AndroidRuntime(1403): at java.lang.reflect.Method.invokeNative(Native Method)
09-28 15:07:49.365: E/AndroidRuntime(1403): at java.lang.reflect.Method.invoke(Method.java:511)
09-28 15:07:49.365: E/AndroidRuntime(1403): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
09-28 15:07:49.365: E/AndroidRuntime(1403): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
09-28 15:07:49.365: E/AndroidRuntime(1403): at dalvik.system.NativeStart.main(Native Method)
09-28 15:07:49.365: E/AndroidRuntime(1403): Caused by: java.lang.NullPointerException
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.Activity.findViewById(Activity.java:1794)
09-28 15:07:49.365: E/AndroidRuntime(1403): at com.emailandcontactmanager.MainActivity.<init>(MainActivity.java:24)
09-28 15:07:49.365: E/AndroidRuntime(1403): at java.lang.Class.newInstanceImpl(Native Method)
09-28 15:07:49.365: E/AndroidRuntime(1403): at java.lang.Class.newInstance(Class.java:1319)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
09-28 15:07:49.365: E/AndroidRuntime(1403): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1870)
09-28 15:07:49.365: E/AndroidRuntime(1403): ... 11 more
告诉我你是否还需要像xml文件代码那样的东西。我感谢任何帮助。
答案 0 :(得分：6)
问题是您在Activity的布局被夸大之前尝试使用findViewById查找视图。在Android开发者文档中了解有关Activities的活动生命周期的更多信息。
请改为：
public class MainActivity extends Activity{
private EditText title;
private EditText body;
private View layout;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
title = (EditText) findViewById(R.id.txtTitle);
body = (EditText) findViewById(R.id.txtBody);
// also , the line below is incorrect, the findViewById method requires
// that you use an id(R.id.somthing) and not a layout resource like you do
layout = (View) findViewById(R.layout.main);
// Rest of your code.