您可以使用以下代码示例来获取Android设备上一个空的MediaStore存储/卷的可用空间：

import android.content.Context;
import android.database.Cursor;
import android.net.Uri;
import android.os.Build;
import android.provider.MediaStore;
import android.util.Log;

import androidx.annotation.RequiresApi;
import androidx.documentfile.provider.DocumentFile;

public class StorageUtils {

    private static final String TAG = StorageUtils.class.getSimpleName();

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    public static long getAvailableSpace(Context context) {
        Uri externalUri = MediaStore.Files.getContentUri("external");

        // Get the volume name of the primary external storage
        String volumeName = getPrimaryVolumeName(context, externalUri);

        if (volumeName == null) {
            Log.e(TAG, "Failed to get primary volume name");
            return 0;
        }

        // Get the document tree Uri of the primary external storage
        Uri documentTreeUri = DocumentsContract.buildTreeDocumentUri(externalUri.getAuthority(), volumeName);

        if (documentTreeUri == null) {
            Log.e(TAG, "Failed to get document tree Uri");
            return 0;
        }

        // Get the available space of the primary external storage
        DocumentFile documentFile = DocumentFile.fromTreeUri(context, documentTreeUri);

        if (documentFile == null) {
            Log.e(TAG, "Failed to get document file");
            return 0;
        }

        return documentFile.getFreeSpace();
    }

    @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
    private static String getPrimaryVolumeName(Context context, Uri externalUri) {
        Cursor cursor = null;

        try {
            cursor = context.getContentResolver().query(externalUri, new String[]{MediaStore.VOLUME_NAME}, null, null, null);

            if (cursor != null && cursor.moveToFirst()) {
                return cursor.getString(cursor.getColumnIndex(MediaStore.VOLUME_NAME));
            }
        } catch (Exception e) {
            Log.e(TAG, "Failed to get primary volume name", e);
        } finally {
            if (cursor != null) {
                cursor.close();
            }
        }

        return null;
    }
}

要使用上面的代码示例，您需要在AndroidManifest.xml文件中添加以下权限：

然后，您可以在您的Activity或Fragment中调用以下方法来获取可用空间：

long availableSpace = StorageUtils.getAvailableSpace(this);

请注意，此示例仅适用于Android 5.0（API级别21）及更高版本。对于更旧的Android版本，您可能需要使用其他方法来获取可用空间。