空指针异常

Question

嗨，我是 android 编程新手，在我开始获得 NPE 之前，一切都很顺利。我已经扫描了与此相关的问题和答案，但我没有看到任何精确的解决方案。大多数答案都是重复我在教科书中已有的内容。这个论坛上的任何好老师可以帮助我通过这段代码吗？我需要知道我做错了什么。

我的 MainActivity.java 看起来像这样： 包 com.westwall.helloworldlist;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ArrayAdapter;
import android.widget.ListView; 


public class MainActivity extends ActionBarActivity {

private String[] countArray = {"India", "Europe", "Africa"};

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

ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
R.layout.activity_listview, countArray);

ListView listView = (ListView)findViewById(R.id.label); 
listView.setAdapter(adapter); 

 }

@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);
return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

--logcat

05-02 20:38:13.443: E/AndroidRuntime(377): FATAL EXCEPTION: main
05-02 20:38:13.443: E/AndroidRuntime(377): java.lang.IllegalStateException: ArrayAdapter requires the resource ID to be a TextView
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:347)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ArrayAdapter.getView(ArrayAdapter.java:323)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.AbsListView.obtainView(AbsListView.java:1315)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ListView.makeAndAddView(ListView.java:1727)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ListView.fillDown(ListView.java:652)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ListView.fillFromTop(ListView.java:709)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ListView.layoutChildren(ListView.java:1580)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.AbsListView.onLayout(AbsListView.java:1147)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.RelativeLayout.onLayout(RelativeLayout.java:909)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.FrameLayout.onLayout(FrameLayout.java:333)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1249)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1125)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.LinearLayout.onLayout(LinearLayout.java:1042)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.FrameLayout.onLayout(FrameLayout.java:333)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.FrameLayout.onLayout(FrameLayout.java:333)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.View.layout(View.java:7035)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.ViewRoot.performTraversals(ViewRoot.java:1045)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.view.ViewRoot.handleMessage(ViewRoot.java:1727)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.os.Handler.dispatchMessage(Handler.java:99)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.os.Looper.loop(Looper.java:123)
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.app.ActivityThread.main(ActivityThread.java:4627)
05-02 20:38:13.443: E/AndroidRuntime(377):  at java.lang.reflect.Method.invokeNative(Native Method)
05-02 20:38:13.443: E/AndroidRuntime(377):  at java.lang.reflect.Method.invoke(Method.java:521)
05-02 20:38:13.443: E/AndroidRuntime(377):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
05-02 20:38:13.443: E/AndroidRuntime(377):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
05-02 20:38:13.443: E/AndroidRuntime(377):  at dalvik.system.NativeStart.main(Native Method)
05-02 20:38:13.443: E/AndroidRuntime(377): Caused by: java.lang.ClassCastException: android.widget.ListView
05-02 20:38:13.443: E/AndroidRuntime(377):  at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:340)
05-02 20:38:13.443: E/AndroidRuntime(377):  ... 30 more
05-02 21:30:31.781: E/ArrayAdapter(406): You must supply a resource ID for a TextView

Answer 1

你的问题来了，

ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
R.layout.activity_listview, countArray);

因为，ArrayAdapter 需要第三个参数作为资源 ID 为 TextView。由于您没有在此处指定第三个参数，即TextView id，因此您会收到错误消息。

如果你的 TextView id 是 textView in R.layout.activity_listview like，

<?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/textView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >
</TextView>
</LinearLayout>

应该是这样的，

 ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
    R.layout.activity_listview, R.id.textView, countArray);

检查第三个参数R.id.textView，您在代码中遗漏了该参数。

Answer 2

您必须指定您在R.layout.activity_listview 中使用的TextView ID

改变这个：

ArrayAdapter<String> adapt = new ArrayAdapter<String>(this, R.layout.activity_listview, countArray); 

到这里：

ArrayAdapter<String> adapt = new ArrayAdapter<String>(this, R.layout.activity_listview, R.id.YOUR_TEXT_VIEW, countArray);