what is the use of MemoryFile in android

If you want some cross-process use with MemoryFile you can use the following fugly method:

import android.os.MemoryFile;
import android.os.ParcelFileDescriptor;

import java.io.FileDescriptor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class MemoryFileUtil {
    private static final Method sMethodGetParcelFileDescriptor;
    private static final Method sMethodGetFileDescriptor;
    static {
        sMethodGetParcelFileDescriptor = get("getParcelFileDescriptor");
        sMethodGetFileDescriptor = get("getFileDescriptor");
    }

    public static ParcelFileDescriptor getParcelFileDescriptor(MemoryFile file) {
        try {
            return (ParcelFileDescriptor) sMethodGetParcelFileDescriptor.invoke(file);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }

    public static FileDescriptor getFileDescriptor(MemoryFile file) {
        try {
            return (FileDescriptor) sMethodGetFileDescriptor.invoke(file);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        } catch (InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }

    private static Method get(String name) {
        try {
            return MemoryFile.class.getDeclaredMethod(name);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException(e);
        }
    }
}

What you should be looking at is the #getParcelFileDescriptor(MemoryFile) method which you can return from an implementation of ContentProvider#openFile(Uri, String).