Question

我正在尝试使用as wallpaper选项创建一个简单的图库应用程序，但每次我在：myWallpaperManager.setBitmap（mBitmap）行中都会出现Null Pointer Exception。这是我的代码：

package com.pbsoft.galeria;
import java.io.IOException;
import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.ImageView;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.app.WallpaperManager;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;


public class MainActivity extends Activity {


Button button;
Button button2;
ImageView image;
MenuItem item;
int checkImg;
Bitmap mBitmap;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    checkImg=0;
    addListenerOnButton();
}

public void addListenerOnButton() {
    image = (ImageView) findViewById(R.id.imageView1);
    button = (Button) findViewById(R.id.btnChangeImage);
    button2= (Button) findViewById(R.id.btnPrevious);

    button.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {

            if(checkImg<3)
                ++checkImg;
            else
                checkImg=0;

            setImg();   
        }
    });

    button2.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View arg0) {
            if(checkImg>0)
                --checkImg;
            else
                checkImg=3;

            setImg();
        }
    });
}

protected void setImg() {
    switch(checkImg){
    case 0: image.setImageResource(R.drawable.wilku); break;
    case 1: image.setImageResource(R.drawable.kotek); break;
    case 2: image.setImageResource(R.drawable.piesek); break;
    case 3: image.setImageResource(R.drawable.chomik); break;
    }       
}

@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);
    item = (MenuItem) findViewById(R.id.action_settings);       
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item){
    mBitmap = BitmapFactory.decodeResource(getResources(),image.getId());

    switch (item.getItemId()) {
    case R.id.action_settings:
        WallpaperManager myWallpaperManager = WallpaperManager.getInstance(getApplicationContext());//<-this I think could be not created
        try{
            myWallpaperManager.setBitmap(mBitmap); // <- 91 line, here I get an exception
            finish();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return true;
    default:
        return super.onOptionsItemSelected(item);
    }   
}

}

对我来说，myWallpaperManager对象看起来有问题 - 它没有初始化，但为什么呢？我还在清单中添加了一个权限：“uses-permission android：name =”android.permission.SET_WALLPAPER“/”但它仍然无效。我是非常新的android，它可能是noob问题，但请帮助 - 感谢每一个答案！

这也是我的logcat：

04-12 12:20:31.180: E/AndroidRuntime(3012): FATAL EXCEPTION: main
04-12 12:20:31.180: E/AndroidRuntime(3012): java.lang.NullPointerException
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.app.WallpaperManager.setBitmap(WallpaperManager.java:494)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.pbsoft.galeria.MainActivity.onOptionsItemSelected(MainActivity.java:91)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.app.Activity.onMenuItemSelected(Activity.java:2205)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.policy.impl.PhoneWindow.onMenuItemSelected(PhoneWindow.java:777)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:143)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:855)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.view.menu.IconMenuView.invokeItem(IconMenuView.java:532)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.view.menu.IconMenuItemView.performClick(IconMenuItemView.java:122)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.view.View$PerformClick.run(View.java:9112)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.os.Handler.handleCallback(Handler.java:587)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.os.Handler.dispatchMessage(Handler.java:92)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.os.Looper.loop(Looper.java:130)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at android.app.ActivityThread.main(ActivityThread.java:3835)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at java.lang.reflect.Method.invokeNative(Native Method)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at java.lang.reflect.Method.invoke(Method.java:507)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:864)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:622)
04-12 12:20:31.180: E/AndroidRuntime(3012):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

mBitmap = BitmapFactory.decodeResource(getResources(),image.getId());

这里bitmap为null，这就是它抛出null指针异常的原因。尝试提供有效的reasource_id。 image.getId()代替此R.drawable.your_resource_id。

Android壁纸管理器不会创建对象（NPE）

1 个答案: