Listview Can'；我不理解NullPointerException

我正在试验ListView和ArrayAdapter，我遇到了一个runTimeException问题，DDMS错误报告告诉我在包含“public void onItemClick”的行中有一个NullPointerException，我不明白为什么它会给我一个问题

 package com.example.worksheet2;

import android.os.Bundle;
import android.app.ListActivity;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class MainActivity extends ListActivity {

ListView listview;

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

    ListView listview = (ListView) findViewById(R.layout.activity_main);
    String[] values = { "A", "B", "C" };

    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
            R.id.menu_settings, values);

    // asign adapter to listview
    listview.setAdapter(adapter);

    listview.setOnItemClickListener(new OnItemClickListener() {
        public void onItemClick(AdapterView<?> parent, View v,
                int position, long id) {
            Toast.makeText(getApplicationContext(),
                    "Click ListItem Number " + position, Toast.LENGTH_LONG)
                    .show();

        }
    });

}

   }

---

试着给listview一个id，然后通过id找到它。

OK，那么哪一行是

MainActivity.java:31

？就像我说的包含listview.setOnItemClickListener（）的行{}发布完整的堆栈跟踪。不，你说的是“onItemClick”。每次我测试它时，它都会在该区域之间跳跃

   Caused by: java.lang.NullPointerException
at com.example.worksheet2.MainActivity.onCreate(MainActivity.java:31)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1836)