//得到文件对象
File file = getFilesDir();

String inPath = file.getAbsolutePath();

Log.d("test-file","getFileDir:"+ inPath);

//在inPath中写入hello
FileOutputStream fileOutputStream = openFileOutput("hello.txt", MODE_PRIVATE);
fileOutputStream.write("hello".getBytes());

fileOutputStream.close();

//判断文件可用性
/* 可写 */
public boolean isExternalStorageWritable() {
    String state = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(state)) {
        return true;
    }
    return false;
}

/* 可读 */
public boolean isExternalStorageReadable() {
    String state = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(state) ||
        Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
        return true;
    }
    return false;
}

//得到外部存储私有对象(File),会随着APP的卸载而卸载
Environment.getExternalStorageDirectory();

//得到外部存储公共对象, 文件不会随着APP的卸载而卸载
//传入的参数代表文件类型 如Environment.DIRECTORY_MUSIC 代表音乐
//DIRECTORY_RINGTONES 代表铃音
Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_MUSIC));

implementation 'android.arch.lifecycle:extensions:1.1.1'
implementation 'android.arch.paging:runtime:1.0.0'
implementation 'com.android.support:recyclerview-v7:27.0.2'
//可选, 我习惯使用约束布局
implementation 'com.android.support.constraint:constraint-layout:1.1.3'

import android.arch.paging.PositionalDataSource;
import android.support.annotation.NonNull;
import android.util.Log;

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

public class PositionPageDataSource extends PositionalDataSource<Concert> {

    @Override
    public void loadInitial(@NonNull LoadInitialParams params, @NonNull LoadInitialCallback<Concert> callback) {
        int requestedLoadSize = params.requestedLoadSize;
        Log.d("ldo","PositionPageDataSource loadInitial "+requestedLoadSize);
        List<Concert> subList = getSubList(0, requestedLoadSize);
        callback.onResult(subList,0,40);//初始化加载从index == 0开始 40假设为数据总数
    }

    @Override
    public void loadRange(@NonNull LoadRangeParams params, @NonNull LoadRangeCallback<Concert> callback) {
        Log.d("ldo","PositionPageDataSource loadRange");
        callback.onResult(getSubList(params.startPosition,params.loadSize));
    }

    public List<Concert> getSubList(int start, int end) {
        Log.d("ldo", "getSubList");
        ArrayList<Concert> strings = new ArrayList<>();
        for (int i = 0; i < end; i++) {
            strings.add(new Concert(start, "" + i));
        }
        return strings;
    }
}

//Factory
import android.arch.paging.DataSource;

public class DataSourceFactorty extends DataSource.Factory<Integer,Concert> {

    @Override
    public DataSource<Integer,Concert> create() {
        return new PositionPageDataSource();
    }
}

import android.arch.paging.PagedListAdapter;
import android.support.annotation.NonNull;
import android.support.v7.util.DiffUtil;
import android.support.v7.widget.RecyclerView;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;


public class ConcertAdapter extends PagedListAdapter<Concert, ConcertAdapter.ViewHolder> {

    ConcertAdapter() {
        super(showDiffCallBack);
    }

    static DiffUtil.ItemCallback<Concert> showDiffCallBack = new DiffUtil.ItemCallback<Concert>() {

        @Override
        public boolean areItemsTheSame(@NonNull Concert oldItem, @NonNull Concert newItem) {
            return (oldItem).id == (newItem).id;
        }

        @Override
        public boolean areContentsTheSame(@NonNull Concert oldItem, @NonNull Concert newItem) {
            return oldItem.equals(newItem);

        }
    };

    @NonNull
    @Override
    public ConcertAdapter.ViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate(R.layout.item_concert_list, null));
    }

    @Override
    public void onBindViewHolder(@NonNull ViewHolder holder, int position) {
        Concert concert = getItem(position);
        holder.v.setText(concert.getId()+"");
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        TextView v;

        public ViewHolder(@NonNull View itemView) {
            super(itemView);
            this.v = itemView.findViewById(R.id.textView);
        }

    }
}

import android.arch.lifecycle.LiveData;
import android.arch.lifecycle.Observer;
import android.arch.paging.LivePagedListBuilder;
import android.arch.paging.PagedList;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import com.example.android.displayingbitmaps.R;


public class PagedListFragment extends Fragment {

 
    
    private RecyclerView recyclerView;
    public static final int INITIALLOADSIZE = 40;//初始加载数量
    public static final int RELOADSIZE = 20;//往下滑动加载数量
    private LiveData<PagedList<Concert>> data;
    private ConcertAdapter adapter;


    public static PagedListFragment newInstance() {

        return new PagedListFragment();
    }

    @Nullable
    @Override
    public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container,
                             @Nullable Bundle savedInstanceState) {

        View inflate = inflater.inflate(R.layout.paged_list_fragment, container, false);
       
        recyclerView = inflate.findViewById(R.id.recyclerView);
        return inflate;
    }
    @Override
    public void onActivityCreated(@Nullable Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
    
        
        PagedList.Config config = new PagedList.Config.Builder()
                .setPageSize(INITIALLOADSIZE / 2) // 分页加载的数量
                .setEnablePlaceholders(false) // 当item为null是否使用PlaceHolder展示
                .setInitialLoadSizeHint(INITIALLOADSIZE) // 预加载的数量, 与分页加载的数量成倍数关系
//                    .setPrefetchDistance(5)
                .build();
        data = new LivePagedListBuilder(new DataSourceFactorty(), config).build();//可以放在ViewModel层处理
        data.observe(this, new Observer<PagedList<Concert>>() {
            @Override
            public void onChanged(@Nullable PagedList<Concert> concerts) {
                Log.d("ldo","onChanged "+concerts.size());
                adapter.submitList(concerts);//调用PagedListAdapter中的方法绑定
            }
        });
        adapter = new ConcertAdapter();
        recyclerView.setLayoutManager(new LinearLayoutManager(getActivity()));
        recyclerView.setAdapter(adapter);
    }

}