用java实现android的垂直侧边栏，切换会出现指定文字，尽量简洁，不使用viewpager

您好，可以使用RecyclerView和LinearLayoutManager实现垂直侧边栏。以下是实现步骤：

1. 在布局文件中添加RecyclerView：

<androidx.recyclerview.widget.RecyclerView
    android:id="@+id/recyclerview"
    android:layout_width="wrap_content"
    android:layout_height="match_parent"
    android:scrollbars="vertical" />

2. 创建一个侧边栏的数据类，包含需要显示的文字信息：

public class SidebarItem {
    private String text;

    public SidebarItem(String text) {
        this.text = text;
    }

    public String getText() {
        return text;
    }
}

3. 创建一个RecyclerView的Adapter，用于显示侧边栏的数据项：

public class SidebarAdapter extends RecyclerView.Adapter<SidebarAdapter.ViewHolder> {
    private List<SidebarItem> mItems;

    public class ViewHolder extends RecyclerView.ViewHolder {
        public TextView textView;

        public ViewHolder(View itemView) {
            super(itemView);
            textView = itemView.findViewById(R.id.item_text);
        }
    }

    public SidebarAdapter(List<SidebarItem> items) {
        mItems = items;
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.sidebar_item, parent, false);
        return new ViewHolder(view);
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        final SidebarItem item = mItems.get(position);
        holder.textView.setText(item.getText());
    }

    @Override
    public int getItemCount() {
        return mItems.size();
    }
}

4. 在布局文件中为侧边栏的每个数据项创建一个TextView并设置样式：

<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/item_text"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:textSize="16sp"
    android:paddingStart="16dp"
    android:paddingEnd="16dp"
    android:paddingTop="8dp"
    android:paddingBottom="8dp"
    android:textColor="@color/black" />

5. 在Activity中初始化RecyclerView并设置LayoutManager和Adapter：

RecyclerView recyclerView = findViewById(R.id.recyclerview);
List<SidebarItem> items = new ArrayList<>();
// 添加侧边栏的数据项
items.add(new SidebarItem("Item 1"));
items.add(new SidebarItem("Item 2"));
items.add(new SidebarItem("Item 3"));
// ...
SidebarAdapter adapter = new SidebarAdapter(items);
recyclerView.setAdapter(adapter);
LinearLayoutManager layoutManager = new LinearLayoutManager(this);
recyclerView.setLayoutManager(layoutManager);

6. 在RecyclerView的ItemDecoration中实现侧边栏的指示文字，根据RecyclerView的滚动位置计算当前显示的数据项，设置指示文字的显示内容：

public class SidebarDecoration extends RecyclerView.ItemDecoration {
    private TextView mIndicatorView;

    public SidebarDecoration(Context context) {
        mIndicatorView = new TextView(context);
        mIndicatorView.setTextColor(ContextCompat.getColor(context, R.color.white));
        mIndicatorView.setTextSize(TypedValue.COMPLEX_UNIT_SP, 20);
        mIndicatorView.setGravity(Gravity.CENTER);
        mIndicatorView.setBackgroundColor(ContextCompat.getColor(context, R.color.colorAccent));
        mIndicatorView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT));
    }

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);
        int childCount = parent.getChildCount();
        if (childCount == 0) {
            return;
        }
        int position = ((LinearLayoutManager) parent.getLayoutManager()).findFirstVisibleItemPosition();
        if (position == RecyclerView.NO_POSITION) {
            return;
        }
        SidebarItem item = ((SidebarAdapter) parent.getAdapter()).getItem(position);
        if (item == null) {
            return;
        }
        String text = item.getText().substring(0, 1);
        mIndicatorView.setText(text);
        int viewWidth = parent.getWidth();
        int viewHeight = parent.getHeight();
        int indicatorWidth = mIndicatorView.getWidth();
        int indicatorHeight = mIndicatorView.getHeight();
        int x = viewWidth - indicatorWidth - 40;
        int y = (int) (position * 1.0f / parent.getAdapter().getItemCount() * viewHeight + (viewHeight * 1.0f / parent.getAdapter().getItemCount() - indicatorHeight) / 2);
        c.translate(0, 0);
        c.drawRect(x, y, x + indicatorWidth, y + indicatorHeight, new Paint());
        mIndicatorView.layout(x, y, x + indicatorWidth, y + indicatorHeight);
        mIndicatorView.draw(c);
    }
}

7. 在Activity中添加ItemDecoration并设置RecyclerView的滚动监听，实现侧边栏指示文字的更新：

SidebarDecoration decoration = new SidebarDecoration(this);
recyclerView.addItemDecoration(decoration);
recyclerView.addOnScrollListener(new RecyclerView.OnScrollListener() {
    @Override
    public void onScrolled(@NonNull RecyclerView recyclerView, int dx, int dy) {
        super.onScrolled(recyclerView, dx, dy);
        int position = ((LinearLayoutManager) recyclerView.getLayoutManager()).findFirstVisibleItemPosition();
        SidebarItem item = adapter.getItem(position);
        if (item == null) {
            return;
        }
        String text = item.getText().substring(0, 1);
        decoration.setText(text);
        recyclerView.invalidateItemDecorations();
    }
});

至此，垂直侧边栏的实现就完成了。