Android开发中遇到的一个问题 明明存在该类却报错classnotfound?报错

如题，下面贴报错地方的代码

public void setAdapter(PagerAdapter adapter) {
		if (mAdapter != null) {
			VerticalViewPagerCompat.setDataSetObserver(mAdapter, null);
		}

		mAdapter = adapter;

		if (mAdapter != null) {
			if (mObserver == null) {
				mObserver = new DataSetObserver();
			}
			VerticalViewPagerCompat.setDataSetObserver(mAdapter, mObserver);
			mPopulatePending = false;
			if (mRestoredCurItem >= 0) {
				mAdapter.restoreState(mRestoredAdapterState,
						mRestoredClassLoader);
				setCurrentItemInternal(mRestoredCurItem, false, true);
				mRestoredCurItem = -1;
				mRestoredAdapterState = null;
				mRestoredClassLoader = null;
			} else {
				populate();
			}
		}
	}

当中 

mObserver = new DataSetObserver();

会提示 ClassNotFound，找不到这个DataSetObserver类，但是我在同一个文件中，自己新建了个这个类，代码如下

private class DataSetObserver implements
			VerticalViewPagerCompat.DataSetObserver {
		@Override
		public void onDataSetChanged() {
			dataSetChanged();
		}
	}

很疑惑，求解答