Question

我在列表视图中遇到空指针异常。当在activity_main.xml中创建列表视图时，它工作正常，但是当我在单独的文件中创建listview时发生错误。

package pk.edu.pucit.lab_02;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import android.app.Activity;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.SimpleAdapter;

public class MainActivity extends Activity {

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

        String[] username = { "hamad", "ali", "ahmed", "riaz", "hamad", "ali",
                "ahmed", "riaz", "hamad", "ali", "ahmed", "riaz", "hamad",
                "ali", "ahmed", "riaz", "hamad", "ali", "ahmed", "riaz",
                "hamad", "ali", "ahmed", "riaz", "hamad", "ali", "ahmed",
                "riaz" };

        ListView listView = (ListView) findViewById(R.id.listView);

        ArrayAdapter<String> adapter = new ArrayAdapter<String>(
                getApplicationContext(), R.layout.acticity_linear_layout,
                R.id.names, username);
        listView.setAdapter(adapter);
    }
}

这是我的activity_linear_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" >

    <TextView
        android:id="@+id/names"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:layout_marginTop="10dp">
    </TextView>

    <TextView
        android:id="@+id/scores"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:layout_marginTop="10dp">
    </TextView>
</LinearLayout>

这是我的activity_list_view.xml

<ListView xmlns:android="http://schemas.android.com/apk/res/android"

        android:id="@+id/listView"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        >
</ListView>

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="pk.edu.pucit.lab_02"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="19" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".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>
    </application>

</manifest>

Answer 1

你的问题在这里

setContentView(R.layout.activity_main);

但listView是activity_list_view的一部分，因此ListView listView = (ListView) findViewById(R.id.listView);将返回空指针。

通过在主要布局中移动listView来修复它。

Answer 2

R.id.listView已不再是R.layout.activity_main

的一部分

这里没有

setContentView(R.layout.activity_main);

但你在这里搜索

ListView listView = (ListView) findViewById(R.id.listView);

给你null。

Answer 3

请在主xml中试试这个。

<?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" >
    <TextView
        android:id="@+id/names"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:layout_marginTop="10dp">
    </TextView>
 <include
        android:id="@+id/container_header_lyt"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_above="..."
        android:layout_toLeftOf="..."
        layout="@layout/activity_list_view" />
    <TextView
        android:id="@+id/scores"
        android:layout_height="match_parent"
        android:layout_width="match_parent"
        android:layout_marginTop="10dp">
    </TextView>
</LinearLayout>

列表视图的空指针异常

3 个答案: