我是android开发的新手，我需要从Acticity到Listactivity的帮助。
我的mainActivity中有一个按钮，下面是代码：
package com.mz47.kazeroon;
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;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Button btn = (Button) findViewById(R.id.button1);
btn.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, DescriptionListActivity.class);
//intent.putExtra("thetext", edt.getText().toString());
startActivity(intent);
}
});
}
@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;
}
}
如果单击该按钮，则其意图将是：
package com.mz47.kazeroon;
import java.util.List;
import android.app.ListActivity;
import android.content.Context;
import android.os.Bundle;
import android.util.EventLogTags.Description;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.TextView;
public class DescriptionListActivity extends ListActivity {
//Database Helper
MyDatabaseHelper db = new MyDatabaseHelper(getApplicationContext());;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.desc_list_layout);
setListAdapter(new DescriptionAdapter(this,
android.R.layout.simple_list_item_1,
R.id.textView1, db.getAllDescriptionNames()));
db.close();
}
private class DescriptionAdapter extends ArrayAdapter<String>{
public DescriptionAdapter(Context context, int resource,
int textViewResourceId, String[] objects) {
super(context, resource, textViewResourceId, objects);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View row = inflater.inflate(R.layout.list_items, parent, false);
MyDescription[] descriptions = (MyDescription[])db.getAllDescriptions().toArray();
ImageView imgv = (ImageView) row.findViewById(R.id.imageView1);
TextView txtv = (TextView) row.findViewById(R.id.textView1);
txtv.setText(descriptions[position].getDescriptionName());
imgv.setImageBitmap(db.getImage(descriptions[position].getDescriptionID()).getImageData());
return row;
}
}
}
我的问题是为什么我的应用程序意外停止？
在清单中，我将DescriptionListActivity类注册为Activity
这是我的清单文件：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.mz47.kazeroon"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="10"
android:targetSdkVersion="10" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="com.mz47.kazeroon.MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name="com.mz47.kazeroon.DescriptionListActivity"></activity>
</application>
</manifest>
Logcat：
03-11 16:10:26.887: D/dalvikvm(3186): GC_EXPLICIT freed 5K, 58% free 2569K/6023K, external 716K/1228K, paused 61ms
03-11 16:10:53.627: D/dalvikvm(3186): Debugger has detached; object registry had 1 entries
03-11 16:18:32.777: D/AndroidRuntime(16464): Shutting down VM
03-11 16:18:32.777: W/dalvikvm(16464): threadid=1: thread exiting with uncaught exception (group=0x40015560)
03-11 16:18:32.797: E/AndroidRuntime(16464): FATAL EXCEPTION: main
03-11 16:18:32.797: E/AndroidRuntime(16464): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.mz47.kazeroon/com.mz47.kazeroon.DescriptionListActivity}: java.lang.NullPointerException
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1569)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread.access$1500(ActivityThread.java:117)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.os.Handler.dispatchMessage(Handler.java:99)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.os.Looper.loop(Looper.java:123)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread.main(ActivityThread.java:3683)
03-11 16:18:32.797: E/AndroidRuntime(16464): at java.lang.reflect.Method.invokeNative(Native Method)
03-11 16:18:32.797: E/AndroidRuntime(16464): at java.lang.reflect.Method.invoke(Method.java:507)
03-11 16:18:32.797: E/AndroidRuntime(16464): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
03-11 16:18:32.797: E/AndroidRuntime(16464): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
03-11 16:18:32.797: E/AndroidRuntime(16464): at dalvik.system.NativeStart.main(Native Method)
03-11 16:18:32.797: E/AndroidRuntime(16464): Caused by: java.lang.NullPointerException
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:100)
03-11 16:18:32.797: E/AndroidRuntime(16464): at com.mz47.kazeroon.DescriptionListActivity.<init>(DescriptionListActivity.java:18)
03-11 16:18:32.797: E/AndroidRuntime(16464): at java.lang.Class.newInstanceImpl(Native Method)
03-11 16:18:32.797: E/AndroidRuntime(16464): at java.lang.Class.newInstance(Class.java:1409)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
03-11 16:18:32.797: E/AndroidRuntime(16464): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1561)
03-11 16:18:32.797: E/AndroidRuntime(16464): ... 11 more
其他xml文件：
desc_list_layout.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ListView
android:id="@android:id/list"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:padding="16dp" >
</ListView>
</LinearLayout>
list_items.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="horizontal" >
<ImageView
android:id="@+id/imageView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:maxHeight="48dp"
android:maxWidth="48dp"
android:minHeight="48dp"
android:minWidth="48dp"
android:src="@drawable/ic_launcher" />
<TextView
android:id="@+id/textView1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Large Text"
android:textAppearance="?android:attr/textAppearanceLarge" />
</LinearLayout>
最佳答案
您已将MyDatabaseHelper数据库初始化为尚不存在的上下文
MyDatabaseHelper db = new MyDatabaseHelper(getApplicationContext());;
应该这样完成：
MyDatabaseHelper db;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
this.db = new MyDatabaseHelper(getApplicationContext());
setContentView(R.layout.desc_list_layout);
setListAdapter(new DescriptionAdapter(this,
android.R.layout.simple_list_item_1,
R.id.textView1, db.getAllDescriptionNames()));
db.close();
}
关于java - 在Android上从Activity转到ListActivity，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/22596681/