Java Android Studio主活动不是封闭类

我正在尝试运行一个三级可扩展列表视图程序。我从这个网站上找到了代码：

代码如下：

package net.stf.threelevelexpandablelistview;

import android.content.Context;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.ExpandableListView;
import android.widget.TextView;

/**
 * New and improved. Feel free to contact me if you have any questions.
 */

public class ThreeLevelExpandableListView extends AppCompatActivity {

    public static final int FIRST_LEVEL_COUNT = 6;
    public static final int SECOND_LEVEL_COUNT = 4;
    public static final int THIRD_LEVEL_COUNT = 20;
    private ExpandableListView expandableListView;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        expandableListView = (ExpandableListView) findViewById(R.id.mainList);
        expandableListView.setAdapter(new ParentLevel(this));
    }

    public class ParentLevel extends BaseExpandableListAdapter {

        private Context context;

        public ParentLevel(Context context) {
            this.context = context;
        }

        @Override
        public Object getChild(int arg0, int arg1) {
            return arg1;
        }

        @Override
        public long getChildId(int groupPosition, int childPosition) {
            return childPosition;
        }

        @Override
        public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
            SecondLevelExpandableListView secondLevelELV = new SecondLevelExpandableListView(MainActivity.this);
            secondLevelELV.setAdapter(new SecondLevelAdapter(context));
            secondLevelELV.setGroupIndicator(null);
            return secondLevelELV;
        }

        @Override
        public int getChildrenCount(int groupPosition) {
            return SECOND_LEVEL_COUNT;
        }

        @Override
        public Object getGroup(int groupPosition) {
            return groupPosition;
        }

        @Override
        public int getGroupCount() {
            return FIRST_LEVEL_COUNT;
        }

        @Override
        public long getGroupId(int groupPosition) {
            return groupPosition;
        }

        @Override
        public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
            if (convertView == null) {
                LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                convertView = inflater.inflate(R.layout.row_first, null);
                TextView text = (TextView) convertView.findViewById(R.id.eventsListEventRowText);
                text.setText("FIRST LEVEL");
            }
            return convertView;
        }

        @Override
        public boolean hasStableIds() {
            return true;
        }

        @Override
        public boolean isChildSelectable(int groupPosition, int childPosition) {
            return true;
        }
    }

    public class SecondLevelExpandableListView extends ExpandableListView {

        public SecondLevelExpandableListView(Context context) {
            super(context);
        }

        protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
            //999999 is a size in pixels. ExpandableListView requires a maximum height in order to do measurement calculations. 
            heightMeasureSpec = MeasureSpec.makeMeasureSpec(999999, MeasureSpec.AT_MOST);
            super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        }
    }

    public class SecondLevelAdapter extends BaseExpandableListAdapter {

        private Context context;

        public SecondLevelAdapter(Context context) {
            this.context = context;
        }

        @Override
        public Object getGroup(int groupPosition) {
            return groupPosition;
        }

        @Override
        public int getGroupCount() {
            return 1;
        }

        @Override
        public long getGroupId(int groupPosition) {
            return groupPosition;
        }

        @Override
        public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
            if (convertView == null) {
                LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                convertView = inflater.inflate(R.layout.row_third, null);
                TextView text = (TextView) convertView.findViewById(R.id.eventsListEventRowText);
                text.setText("SECOND LEVEL");
            }
            return convertView;
        }

        @Override
        public Object getChild(int groupPosition, int childPosition) {
            return childPosition;
        }

        @Override
        public long getChildId(int groupPosition, int childPosition) {
            return childPosition;
        }

        @Override
        public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
            if (convertView == null) {
                LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
                convertView = inflater.inflate(R.layout.row_second, null);
                TextView text = (TextView) convertView.findViewById(R.id.eventsListEventRowText);
                text.setText("THIRD LEVEL");
            }
            return convertView;
        }

        @Override
        public int getChildrenCount(int groupPosition) {
            return THIRD_LEVEL_COUNT;
        }

        @Override
        public boolean hasStableIds() {
            return true;
        }

        @Override
        public boolean isChildSelectable(int groupPosition, int childPosition) {
            return true;
        }
    }
}

---

不幸的是，我收到一条错误消息，说MainActivity不是一个封闭类。我非常感谢你的帮助，谢谢你

用

ThreeLevelExpandableListView替换
MainActivity.this
。this

---

ParentLevel

是

ThreeLevelExpandableListView

的内部类，而不是

MainActivity

。阅读有关内部类和封闭类的内容。

从何处获得此错误？欢迎使用堆栈溢出！为了帮助人们回答您的问题，您需要更具体地说明错误。请在帖子中加入您从中获得的确切错误（最好使用复制+粘贴以避免抄写错误）。请共享您登录第55行：SecondLevelExpandableListView secondLevelELV=new SecondLevelExpandableListView（MainActivity.this）；天哪，太简单了，哈哈，太谢谢你了！