如何在默认情况下打开展开的可扩展Listview组

// 导入相关依赖库
import android.app.Activity;
import android.os.Bundle;
import import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.ExpandableListView;
import android.widget.TextView;

public class MainActivity extends Activity {
    private ExpandableListView expandableListView;
    private MyExpandableListAdapter adapter;

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

        // 初始化ExpandableListView
        expandableListView = findViewById(R.id.expandableListView);

        // 创建适配器
        adapter = new MyExpandableListAdapter();

        // 设置适配器
        expandableListView.setAdapter(adapter);
    }

    // 自定义适配器
    class MyExpandableListAdapter extends BaseExpandableListAdapter {
        // 定义数据源
        private String[] groups = {"Group 1", "Group 2", "Group 3"};
        private String[][] children = {
                {"Child 1-1", "Child 1-2", "Child 1-3"},
                {"Child 2-1", "Child 2-2", "Child 2-3"},
                {"Child 3-1", "Child 3-2", "Child 3-3"}
        };

        @Override
        public int getGroupCount() {
            return groups.length;
        }

        @Override
        public int getChildrenCount(int groupPosition) {
            return children[groupPosition].length;
        }

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

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

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

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

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

        @Override
        public View getGroupView(int groupPosition, boolean isExpanded, View convertView, ViewGroup parent) {
            if (convertView == null) {
                convertView = getLayoutInflater().inflate(R.layout.list_group, null);
            }
            TextView textView = convertView.findViewById(R.id.group_text);
            textView.setText(groups[groupPosition]);
            return convertView;
        }

        @Override
        public View getChildView(int groupPosition, int childPosition, boolean isLastChild, View convertView, ViewGroup parent) {
            if (convertView == null) {
                convertView = getLayoutInflater().inflate(R.layout.list_item, null);
            }
            TextView textView = convertView.findViewById(R.id.item_text);
            textView.setText(children[groupPosition][childPosition]);
            return convertView;
        }

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