RecyclerView with sections.

MainActivity.java

package com.example.recyclertest;

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.Menu;
import android.view.MenuItem;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private RecyclerView recyclerView;
    private GridLayoutManager gridLayoutManager;
    private SectionAdapter sectionAdapter;
    private SampleSubAdapter subAdapter;

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

        recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
        recyclerView.setHasFixedSize(true);

        gridLayoutManager = new GridLayoutManager(this, 2);
        recyclerView.setLayoutManager(gridLayoutManager);

        List<SampleSubAdapter.Section> data = new ArrayList<>();
        for (int i = 0; i < 5; i++) {
            List<String> items = new ArrayList<>();
            for (int j = 0; j < 10; j++) {
                items.add("Item " + (i + 1) + "-" + (j + 1));
            }
            data.add(new SampleSubAdapter.Section("Section #" + (i + 1), items));
        }

        subAdapter = new SampleSubAdapter(this, data, 2);
        sectionAdapter = new SectionAdapter(subAdapter);
        recyclerView.setAdapter(sectionAdapter);
        gridLayoutManager.setSpanSizeLookup(sectionAdapter.new SpanSizeLookup());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        menu.add(Menu.NONE, 1, Menu.NONE, "1 column");
        menu.add(Menu.NONE, 2, Menu.NONE, "2 columns");
        menu.add(Menu.NONE, 3, Menu.NONE, "3 columns");
        menu.add(Menu.NONE, 4, Menu.NONE, "4 columns");
        menu.add(Menu.NONE, 5, Menu.NONE, "5 columns");
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id >= 1 && id <= 5) {
            gridLayoutManager.setSpanCount(id);
            subAdapter.setSpanCount(id);
            recyclerView.requestLayout();
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

SampleSubAdapter.java

package com.example.recyclertest;

import android.content.Context;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import java.util.List;

public class SampleSubAdapter extends SectionAdapter.SubAdapter {

    private final LayoutInflater inflater;
    private List<Section> data;
    private int spanCount;

    public SampleSubAdapter(Context context, List<Section> data, int spanCount) {
        inflater = LayoutInflater.from(context);
        this.data = data;
        this.spanCount = spanCount;
    }

    @Override
    public int getSectionCount() {
        return data.size();
    }

    @Override
    public int getItemCountForSection(int section) {
        return data.get(section).items.size();
    }

    @Override
    public int getSpanCount() {
        return spanCount;
    }

    public void setSpanCount(int spanCount) {
        this.spanCount = spanCount;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        if (viewType == VIEW_TYPE_HEADER) {
            return new HeaderViewHolder(inflater.inflate(HeaderViewHolder.LAYOUT_ID, parent, false));
        } else {
            return new ItemViewHolder(inflater.inflate(ItemViewHolder.LAYOUT_ID, parent, false));
        }
    }

    @Override
    public void onBindSectionHeaderViewHolder(RecyclerView.ViewHolder holder, int section) {
        HeaderViewHolder h = (HeaderViewHolder) holder;
        h.textView.setText(data.get(section).title);
    }

    @Override
    public void onBindItemViewHolder(RecyclerView.ViewHolder holder, int section, int position) {
        ItemViewHolder h = (ItemViewHolder) holder;
        h.textView.setText(data.get(section).items.get(position));
    }

    public static class Section {
        public String title;
        public List<String> items;

        public Section(String title, List<String> items) {
            this.title = title;
            this.items = items;
        }
    }

    public static class HeaderViewHolder extends RecyclerView.ViewHolder {
        static final int LAYOUT_ID = android.R.layout.simple_list_item_1;

        public final TextView textView;

        public HeaderViewHolder(View itemView) {
            super(itemView);
            textView = (TextView) itemView.findViewById(android.R.id.text1);
            textView.setTextSize(20);
        }
    }

    public static class ItemViewHolder extends RecyclerView.ViewHolder {
        static final int LAYOUT_ID = android.R.layout.simple_list_item_1;

        public final TextView textView;

        public ItemViewHolder(View itemView) {
            super(itemView);
            textView = (TextView) itemView.findViewById(android.R.id.text1);
        }
    }
}

SectionAdapter.java

package com.example.recyclertest;

import android.support.v7.widget.GridLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.view.ViewGroup;

public class SectionAdapter extends RecyclerView.Adapter<RecyclerView.ViewHolder> {

    private SubAdapter subAdapter;

    public SectionAdapter(SubAdapter subAdapter) {
        this.subAdapter = subAdapter;
    }

    @Override
    public int getItemCount() {
        int count = 0;
        for (int section = 0; section < subAdapter.getSectionCount(); section++) {
            if (subAdapter.hasSectionHeader(section)) {
                count++;
            }
            count += subAdapter.getItemCountForSection(section);
        }
        return count;
    }

    @Override
    public int getItemViewType(int position) {
        IndexPath indexPath = indexPathOfPosition(position);
        if (indexPath.position == IndexPath.POSITION_HEADER) {
            return subAdapter.getSectionHeaderViewType(indexPath.section);
        } else {
            return subAdapter.getItemViewType(indexPath.section, indexPath.position);
        }
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        return subAdapter.onCreateViewHolder(parent, viewType);
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
        IndexPath indexPath = indexPathOfPosition(position);
        if (indexPath.position == IndexPath.POSITION_HEADER) {
            subAdapter.onBindSectionHeaderViewHolder(holder, indexPath.section);
        } else {
            subAdapter.onBindItemViewHolder(holder, indexPath.section, indexPath.position);
        }
    }

    public IndexPath indexPathOfPosition(int position) {
        if (position < 0) {
            return null;
        }
        for (int section = 0; section < subAdapter.getSectionCount(); section++) {
            if (subAdapter.hasSectionHeader(section)) {
                if (position == 0) {
                    return new IndexPath(section, IndexPath.POSITION_HEADER);
                }
                position--;
            }
            if (position < subAdapter.getItemCountForSection(section)) {
                return new IndexPath(section, position);
            }
            position -= subAdapter.getItemCountForSection(section);
        }
        return null;
    }

    public static class IndexPath {
        public static final int POSITION_HEADER = -1;

        public final int section;
        public final int position;

        public IndexPath(int section, int position) {
            this.section = section;
            this.position = position;
        }
    }

    public class SpanSizeLookup extends GridLayoutManager.SpanSizeLookup {
        @Override
        public int getSpanSize(int position) {
            IndexPath indexPath = indexPathOfPosition(position);
            if (indexPath.position == IndexPath.POSITION_HEADER) {
                return subAdapter.getSpanCount();
            } else {
                return subAdapter.getItemSpanSize(indexPath.section, indexPath.position);
            }
        }
    }

    public static abstract class SubAdapter {
        public static final int VIEW_TYPE_HEADER = 1;
        public static final int VIEW_TYPE_ITEM = 2;

        public abstract int getSectionCount();
        public boolean hasSectionHeader(int section) { return true; }
        public abstract int getItemCountForSection(int section);

        public int getSpanCount() { return 1; }
        public int getItemSpanSize(int section, int position) { return 1; }

        public int getSectionHeaderViewType(int section) { return VIEW_TYPE_HEADER; }
        public int getItemViewType(int section, int position) { return VIEW_TYPE_ITEM; }

        public abstract RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType);
        public abstract void onBindSectionHeaderViewHolder(RecyclerView.ViewHolder holder, int section);
        public abstract void onBindItemViewHolder(RecyclerView.ViewHolder holder, int section, int position);
    }
}