刚刚接触Android和Java并且在干中学习,但我似乎在检测ListView中的项目点击时漏掉了一些基本的东西。当调用setOnItemClickListener时出现空指针异常
我有一个活动,它是一个ListView下面的几个按钮。每个ListView项目都有一个可以点击的图标,最终会转到另一个活动,并且有一个时间戳(当前仅表示点击通过Toast工作)。单击检测图标正在工作,但我试图在ListView中实现项目的选择,这是我遇到麻烦的地方。
我已审查的例子不胜枚举,如this,我认为我这样做的规定,但显然我失去了一些东西,因为我收到一个运行时异常“显示java.lang.NullPointerException:试图调用虚拟方法无效android.widget.ListView.setOnItemClickListener(android.widget.AdapterView $ OnItemClickListener)”上的空对象引用:”
下面是视图的布局:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.foo.magma.ViewPassagesActivity">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/linearLayout">
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Update"
android:onClick="update"
android:id="@+id/updateButton"
android:layout_alignParentTop="true"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_weight="1"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Delete"
android:onClick="delete"
android:id="@+id/deleteButton"
android:layout_alignParentTop="true"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_weight="1"/>
</LinearLayout>
<ListView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/passagesListView"
android:layout_alignParentBottom="true"
android:layout_centerHorizontal="true"
android:layout_below="@+id/linearLayout" />
</RelativeLayout>
下面是一个布局ListView中的项目。如前所述,它的左边有一个时间戳图标:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
<ImageView
android:id="@+id/icon"
android:layout_width="50dp"
android:layout_height="50dp"
android:layout_marginLeft="4dp"
android:layout_marginRight="10dp"
android:layout_marginTop="4dp"
android:src="@drawable/curve" >
</ImageView>
<TextView
android:id="@+id/label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Timestamp: "
android:textStyle="bold"
android:textSize="20dp"
android:layout_centerVertical="true"
android:layout_toRightOf="@+id/icon"
android:layout_toEndOf="@+id/icon">
android:layout_toRightOf="@+id/icon"
</TextView>
<TextView
android:id="@+id/time"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="layouttesttime"
android:textSize="20dp"
android:layout_below="@+id/date"
android:layout_alignRight="@+id/date"
android:layout_alignEnd="@+id/date"
android:layout_alignLeft="@+id/date"
android:layout_alignStart="@+id/date">
</TextView>
<TextView
android:id="@+id/date"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="10dp"
android:text="layouttestdate"
android:textSize="20dp"
android:layout_alignTop="@+id/icon"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_toRightOf="@+id/label"
android:layout_toEndOf="@+id/label">
</TextView>
</RelativeLayout>
这里是他的OnCreate()从我的活动,它集我的适配器:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_view_passages);
ListView passagesListView = (ListView) findViewById(R.id.passagesListView);
assert passagesListView != null;
buildPassageList();
PassagesViewAdapter adapter = new PassagesViewAdapter(this, R.layout.passages_row_layout, passages);
passagesListView.setAdapter(adapter);
}
而且这里是我遇到的问题。该图标的OnClickListener起作用。我的意图是检测选择一个ListView项目,以便它可以被删除。但是,当我添加setOnItemClickListener()的调用时,我收到一个运行时异常。
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import java.util.List;
public class PassagesViewAdapter extends ArrayAdapter<Passage>{
private int resource;
private List<Passage> passages;
public PassagesViewAdapter(Context context, int resource, List<Passage> passages) {
super(context, resource, passages);
this.resource = resource;
this.passages = passages;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View view = convertView;
if (view == null) {
LayoutInflater li = LayoutInflater.from(getContext());
view = li.inflate(resource, null);
}
Passage passage = this.passages.get(position);
TextView time = (TextView) view.findViewById(R.id.time);
TextView date = (TextView) view.findViewById(R.id.date);
ImageView icon = (ImageView) view.findViewById(R.id.icon);
icon.setTag(new Integer(position));
final String positionStr = icon.getTag().toString();
// This works great!
icon.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(getContext(), "You clicked on " + positionStr, Toast.LENGTH_SHORT).show();
}
});
ListView passagesListView = (ListView) view.findViewById(R.id.passagesListView);
assert passagesListView != null;
// ** This call results in a runtime error for null pointer reference.
passagesListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapter, View view, int position,long arg3) {
Toast.makeText(getContext(), "Foo", Toast.LENGTH_SHORT).show();
}
});
time.setText(passage.getPassageTime());
date.setText(passage.getPassageDate());
return view;
}
}
啊哈。我知道了。感谢您的及时回复。移动到OnCreate()工作。 – sje