BaseExpandableListAdapter in Android with Example

package org.geeksforgeeks.demo;

import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseExpandableListAdapter;
import android.widget.TextView;
import java.util.ArrayList;

public class CustomizedAdapter extends BaseExpandableListAdapter {

    private Context context;
    private ArrayList<GroupInformation> mainSetName;

    public CustomizedAdapter(Context context, ArrayList<GroupInformation> deptList) {
        this.context = context;
        this.mainSetName = deptList;
    }

    @Override
    public Object getChild(int groupPosition, int childPosition) {
        ArrayList<ChildInfo> productList = mainSetName.get(groupPosition).getSubsetName();
        return productList.get(childPosition);
    }

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

    @Override
    public View getChildView(int groupPosition, int childPosition, boolean isLastChild,
                             View view, ViewGroup parent) {

        ChildInfo detailInfo = (ChildInfo) getChild(groupPosition, childPosition);
        if (view == null) {
            LayoutInflater infalInflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = infalInflater.inflate(R.layout.child_items, null);
        }
        TextView childItem = (TextView) view.findViewById(R.id.childItm);
        childItem.setText(detailInfo.getName().trim());

        return view;
    }

    @Override
    public int getChildrenCount(int groupPosition) {

        ArrayList<ChildInfo> productList = mainSetName.get(groupPosition).getSubsetName();
        return productList.size();

    }

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

    @Override
    public int getGroupCount() {
        return mainSetName.size();
    }

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

    @Override
    public View getGroupView(int groupPosition, boolean isLastChild, View view,
                             ViewGroup parent) {

        GroupInformation headerInfo = (GroupInformation) getGroup(groupPosition);
        if (view == null) {
            LayoutInflater inf = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
            view = inf.inflate(R.layout.group_items, null);
        }

        TextView heading = (TextView) view.findViewById(R.id.data);
        heading.setText(headerInfo.getName().trim());

        return view;
    }

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

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

}

package org.geeksforgeeks.demo;

public class ChildInfo {
    private String name = "";
    
    // Getter , setter methods
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

package org.geeksforgeeks.demo;

import java.util.ArrayList;

public class GroupInformation {

    private String mainSetName;
    private ArrayList<ChildInfo> list = new ArrayList<ChildInfo>();

    public String getName() {
        return mainSetName;
    }

    public void setName(String mainSetName) {
        this.mainSetName = mainSetName;
    }

    public ArrayList<ChildInfo> getSubsetName() {
        return list;
    }

    public void setSubsetName(ArrayList<ChildInfo> subSetName) {
        this.list = subSetName;
    }

}

package org.geeksforgeeks.demo;

import android.os.Bundle;
import android.view.View;
import android.widget.ExpandableListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import java.util.ArrayList;
import java.util.LinkedHashMap;

public class MainActivity extends AppCompatActivity {

    private LinkedHashMap<String, GroupInformation> mainSet = new LinkedHashMap<String, GroupInformation>();
    private ArrayList<GroupInformation> subSet = new ArrayList<GroupInformation>();

    private CustomizedAdapter listAdapter;
    private ExpandableListView simpleExpandableListView1;

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

        // add data for displaying in expandable list view
        loadData();

        // get reference of the ExpandableListView from activity_main
        simpleExpandableListView1 = (ExpandableListView) findViewById(R.id.simpleExpandableListView1);
        
        // create the adapter and by passing your ArrayList data
        listAdapter = new CustomizedAdapter(MainActivity.this, subSet);
        simpleExpandableListView1.setAdapter(listAdapter);

        // setOnChildClickListener listener for child row click, so that we can get the value
        simpleExpandableListView1.setOnChildClickListener(new ExpandableListView.OnChildClickListener() {
            @Override
            public boolean onChildClick(ExpandableListView parent, View v, int groupPosition, int childPosition, long id) {
                // get the group header
                GroupInformation headerInfo = subSet.get(groupPosition);
                // get the child info
                ChildInfo detailInfo = headerInfo.getSubsetName().get(childPosition);
                // display it or do something with it
                Toast.makeText(getBaseContext(), headerInfo.getName() + "/" + detailInfo.getName(), Toast.LENGTH_LONG).show();
                return false;
            }
        });
        
        // setOnGroupClickListener listener for group heading click
        simpleExpandableListView1.setOnGroupClickListener(new ExpandableListView.OnGroupClickListener() {
            @Override
            public boolean onGroupClick(ExpandableListView parent, View v, int groupPosition, long id) {
                // get the group header
                GroupInformation headerInfo = subSet.get(groupPosition);
                // display it or do something with it
                Toast.makeText(getBaseContext(), headerInfo.getName(), Toast.LENGTH_LONG).show();
                return false;
            }
        });
    }

    // load some initial data into out list
    private void loadData() {

        addDetails("Programming_Languages", "Python");
        addDetails("Programming_Languages", "Java");
        addDetails("Programming_Languages", "Kotlin");
        addDetails("Programming_Languages", "NodeJS");
        addDetails("Programming_Languages", "GO");

        addDetails("Relational_Database", "Oracle");
        addDetails("Relational_Database", "SQLServer");
        addDetails("Relational_Database", "MySQL");

        addDetails("NoSQL_Database", "MongoDB");
        addDetails("NoSQL_Database", "Cassandra");
        addDetails("NoSQL_Database", "CouchDB");

    }
    
    // here we maintain main set like Programming languages and subsets like Python
    private int addDetails(String mainSet, String subSet) {

        int groupPosition = 0;

        // check the hash map if the group already exists
        GroupInformation headerInfo = this.mainSet.get(mainSet);
        
        // add the group if doesn't exists
        if (headerInfo == null) {
            headerInfo = new GroupInformation();
            headerInfo.setName(mainSet);
            this.mainSet.put(mainSet, headerInfo);
            this.subSet.add(headerInfo);
        }

        // get the children for the group
        ArrayList<ChildInfo> subList = headerInfo.getSubsetName();
        
        // size of the children list
        int listSize = subList.size();
        
        // add to the counter
        listSize++;

        // create a new child and add that to the group
        ChildInfo detailInfo = new ChildInfo();
        detailInfo.setName(subSet);
        subList.add(detailInfo);
        headerInfo.setSubsetName(subList);

        // find the group position inside the list
        groupPosition = this.subSet.indexOf(headerInfo);
        return groupPosition;
    }
}