下划线

public class DividerItemDecoration extends RecyclerView.ItemDecoration {
private Drawable mDivider;

public DividerItemDecoration(Context context) {
mDivider = ContextCompat.getDrawable(context, R.drawable.divider);
}

@Override
public void onDraw(Canvas canvas, RecyclerView parent, RecyclerView.State state) {
int left = parent.getPaddingLeft();
int right = parent.getWidth() - parent.getPaddingRight();

for (int i = 0; i < parent.getChildCount() - 1; i++) {
View child = parent.getChildAt(i);
RecyclerView.LayoutParams params = (RecyclerView.LayoutParams) child.getLayoutParams();

int top = child.getBottom() + params.bottomMargin;
int bottom = top + mDivider.getIntrinsicHeight();

mDivider.setBounds(left, top, right, bottom);
mDivider.draw(canvas);
}
}
}
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="rectangle">
<size android:height="1dp" />
<solid android:color="@color/color_grey" />
</shape>
<color name="color_grey">#f5f5f5</color>
val dividerItemDecoration = DividerItemDecoration(this)
rv.addItemDecoration(dividerItemDecoration)