設置自定義ListView圖像時出錯

Question

我想使用ArrayAdapter設置自定義ListView 。 每個ListItem都有一個ImageView和兩個TextViews 。

每個Item rowlayout.xml的布局：

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<ImageView
    android:id="@+id/item_icon"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentTop="true"
    android:adjustViewBounds="true"
    android:maxHeight="80dp"
    android:maxWidth="80dp"
    android:src="@drawable/logo" />

    <TextView
    android:id="@+id/item_title"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/item_subTitle"
    android:layout_toRightOf="@+id/item_icon"
    android:text="Titel"
    android:textAppearance="?android:attr/textAppearanceLarge" />

<TextView
    android:id="@+id/item_subTitle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignBottom="@+id/item_icon"
    android:layout_alignLeft="@+id/item_title"
    android:layout_marginBottom="16dp"
    android:text="Untertitel" />

</RelativeLayout>

這是我在徽章活動中設置ListView ：

public class Badges extends ActionBarActivity{

List<Badge> myBadges = new ArrayList<Badge>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.badges_layout);

    populateBadgeList();
    populateListView();

}

private void populateBadgeList() {
    myBadges.add(new Badge("Some Text", "Some further Text", R.drawable.badge1));
    //and so on
}

private void populateListView() {
    ArrayAdapter<Badge> adapter = new MyListAdapter();
    ListView list = (ListView) findViewById(R.id.listView);
    list.setAdapter(adapter);
}

private class MyListAdapter extends ArrayAdapter<Badge> {
    public MyListAdapter() {
        super(Badges.this, R.layout.rowlayout, myBadges);
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {

        View itemView = convertView;
        if(itemView == null) {
            itemView = getLayoutInflater().inflate(R.layout.rowlayout, parent, false);
        }

        Badge currentBadge = myBadges.get(position);

        ImageView itemIcon = (ImageView) findViewById(R.id.item_icon);
        //the error is on the following line:
        itemIcon.setImageResource(currentBadge.getIconID());

        TextView itemTitle = (TextView) findViewById(R.id.item_title);
        itemTitle.setText(currentBadge.getTitle());

        TextView itemSubtitle = (TextView) findViewById(R.id.item_subTitle);
        itemSubtitle.setText(currentBadge.getSubtitle());

        return itemView;
    }
}

這是徽章類：

public class Badge {
String title;
String subtitle;
int iconID;

public Badge(String title, String subtitle, int iconID) {
    this.title = title;
    this.subtitle = subtitle;
    this.iconID = iconID;
}

public String getTitle() {
    return title;
}

public String getSubtitle() {
    return subtitle;
}

public int getIconID() {
    return iconID;
}

}

我收到以下錯誤：

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ImageView.setImageResource(int)' on a null object reference

我在代碼中標記了發生錯誤的行。 這可能是什么原因？

Answer 1

而不是使用

ImageView itemIcon = (ImageView) findViewById(R.id.item_icon);
TextView itemTitle = (TextView) findViewById(R.id.item_title);
TextView itemSubtitle = (TextView) findViewById(R.id.item_subTitle);

采用

ImageView itemIcon = (ImageView) itemView.findViewById(R.id.item_icon);
TextView itemTitle = (TextView) itemView.findViewById(R.id.item_title);
TextView itemSubtitle = (TextView)itemView.findViewById(R.id.item_subTitle);

Answer 2

更換

ImageView itemIcon = (ImageView) findViewById(R.id.item_icon);

這個與

ImageView itemIcon = (ImageView) itemView.findViewById(R.id.item_icon);

同樣

TextView itemTitle = (TextView) itemView.findViewById(R.id.item_title);
TextView itemSubtitle = (TextView) itemView.findViewById(R.id.item_subTitle);

與

TextView itemTitle = (TextView) findViewById(R.id.item_title);
TextView itemSubtitle = (TextView) findViewById(R.id.item_subTitle);

Answer 3

您剛剛膨脹的視圖未添加到活動的布局中。 所以用活動法

ImageView itemIcon = (ImageView) findViewById(R.id.item_icon);
itemIcon.setImageResource(currentBadge.getIconID());

您找不到具有特定ID的視圖。 相反，您應該使用

ImageView itemIcon = (ImageView) itemView.findViewById(R.id.item_icon);
itemIcon.setImageResource(currentBadge.getIconID());

它會正常工作。 其他觀點相似。

Answer 4

我認為您的問題出在這行：ArrayAdapter adapter = new MyListAdapter（）;

更改為：MyListAdapter adapter = new MyListAdapter（）;

另外，將項目視圖放在findViewById方法的前面（ImageView itemIcon =（ImageView）itemview.findViewById（R.id.item_icon）;）

我實際上建議對適配器類進行一般化，以便您可以傳遞任何資源（行布局）並將其列出給它：）（但是，該應用程序不必工作。）

所以像這樣：

private class MyListAdapter extends ArrayAdapter<Badge> {

   int resource;
   public MyListAdapter(Context context, int resource, List<Badge> items) {
       super(context, resource, items);
       this.resource = resource;
   }

@Override
public View getView(int position, View convertView, ViewGroup parent) {

    View itemView = convertView;
    if(itemView == null) {
        itemView = getLayoutInflater().inflate(resource, parent, false);
    }

    Badge currentBadge = getItem(position);

    ImageView itemIcon = (ImageView) itemview.findViewById(R.id.item_icon);
    //the error is on the following line:
    itemIcon.setImageResource(currentBadge.getIconID());

    TextView itemTitle = (TextView) itemview.findViewById(R.id.item_title);
    itemTitle.setText(currentBadge.getTitle());

    TextView itemSubtitle = (TextView) itemview.findViewById(R.id.item_subTitle);
    itemSubtitle.setText(currentBadge.getSubtitle());

    return itemView;
   }
}