android - 实例化convertView时的NPE

这是我为ListView创建自定义视图的代码。它使用名为SquareProgressBar的自定义视图：

package com.example.simpledownloader.adapter;

import net.yscs.android.square_progressbar.SquareProgressBar;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.TextView;

import com.example.simpledownloader.R;
import com.example.simpledownloader.sharable.Sharable;

public class TaskAdapter extends BaseAdapter {
    Context ctx = null;


    public TaskAdapter(Context ctx){
        this.ctx = ctx;
    }
//--------------------------------------------------------------------------------
    @Override
    public int getCount() {
        return Sharable.downloads.size();
    }
//--------------------------------------------------------------------------------
    @Override
    public Object getItem(int index) {
        return Sharable.downloads.get(index);
    }
//--------------------------------------------------------------------------------
    @Override
    public long getItemId(int id) {
        return id;
    }
//--------------------------------------------------------------------------------
    @Override
    public View getView(int index, View convertView, ViewGroup parent) {
        if(convertView == null){
            LayoutInflater inflater = LayoutInflater.from(ctx);
            convertView = inflater.inflate(R.layout.task, parent, false);
        }
        SquareProgressBar taskProgressBar = (SquareProgressBar) convertView.findViewById(R.id.taskProgressBar);
        TextView statusMsg = (TextView) convertView.findViewById(R.id.statusMsg);
        TextView name = (TextView) convertView.findViewById(R.id.taskName);

        taskProgressBar.setProgress((int)Sharable.downloads.get(index).getProgress());
        taskProgressBar.setImage(R.drawable.task_image);
        taskProgressBar.setColor("#C9C9C9");
        taskProgressBar.setWidth(8);
        name.setText(Sharable.downloads.get(index).toString());
        statusMsg.setText(Sharable.downloads.get(index).getStatus());

        return convertView;
    }
//--------------------------------------------------------------------------------
}

但是，我得到了一个NPE：

08-18 15:10:56.184: E/AndroidRuntime(5639): FATAL EXCEPTION: main
08-18 15:10:56.184: E/AndroidRuntime(5639): java.lang.NullPointerException
08-18 15:10:56.184: E/AndroidRuntime(5639):     at net.yscs.android.square_progressbar.SquareProgressBar.setOpacity(SquareProgressBar.java:184)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at net.yscs.android.square_progressbar.SquareProgressBar.setProgress(SquareProgressBar.java:105)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at com.example.simpledownloader.adapter.TaskAdapter.getView(TaskAdapter.java:47)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.AbsListView.obtainView(AbsListView.java:2159)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.ListView.measureHeightOfChildren(ListView.java:1246)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.ListView.onMeasure(ListView.java:1158)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.View.measure(View.java:15518)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1404)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:695)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.View.measure(View.java:15518)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.View.measure(View.java:15518)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.LinearLayout.measureVertical(LinearLayout.java:847)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.LinearLayout.onMeasure(LinearLayout.java:588)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.View.measure(View.java:15518)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:4825)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2176)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.View.measure(View.java:15518)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1874)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1089)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1265)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:989)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4351)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.Choreographer.doCallbacks(Choreographer.java:562)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.Choreographer.doFrame(Choreographer.java:532)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.os.Handler.handleCallback(Handler.java:725)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.os.Handler.dispatchMessage(Handler.java:92)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.os.Looper.loop(Looper.java:137)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at android.app.ActivityThread.main(ActivityThread.java:5041)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at java.lang.reflect.Method.invokeNative(Native Method)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at java.lang.reflect.Method.invoke(Method.java:511)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-18 15:10:56.184: E/AndroidRuntime(5639):     at dalvik.system.NativeStart.main(Native Method)

为什么？
我想是找不到方形进度条。
ctx是传递给这个类的应用程序上下文
sharable是一个自定义类，它包含在活动之间共享的所有单例对象。

最佳答案

而不是：

taskProgressBar.setProgress((int)Sharable.downloads.get(index).getProgress());
taskProgressBar.setImage(R.drawable.task_image);

使用：

taskProgressBar.setImage(R.drawable.task_image);
taskProgressBar.setProgress((int)Sharable.downloads.get(index).getProgress());

关于android - 实例化convertView时的NPE，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/18300576/