错误:类型ActivityManager的方法xxx未定义

我得到以下错误:

http://developer.android.com/reference/android/app/ActivityManager.html#killBackgroundProcesses%28java.lang.String%29 .

这是密码,请帮我找出哪里出错了?

package com.robosoft.killswitch;

import java.util.List;

import android.app.ActivityManager;
import android.app.ListActivity;
import android.app.ActivityManager.RunningAppProcessInfo;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ListView;
import android.widget.AdapterView.OnItemClickListener;

public class KillSwitch extends ListActivity {
/** Called when the activity is first created. */
private RunningApplicationAdapter runningApplicationAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ActivityManager am = (ActivityManager)getSystemService(Context.ACTIVITY_SERVICE);
    final List<ActivityManager.RunningAppProcessInfo> RunningApp = am.getRunningAppProcesses();        
    runningApplicationAdapter = new RunningApplicationAdapter(this, RunningApp);
    //setContentView(R.layout.main);

    setListAdapter(runningApplicationAdapter);
    ListView lv = getListView();
    lv.setTextFilterEnabled(true);
    lv.setOnItemClickListener(new OnItemClickListener()
    {  
        public void onItemClick(AdapterView<?> parent, View v, int pos, long id) {
            RunningAppProcessInfo x = RunningApp.get(pos);
            String y = x.processName;
            am.killBackgroundProcesses(x.processName); //Error Here!
    }
    });
}
}