...
		
        rcySample = findViewById<RecyclerView>(R.id.rcySample)
        
		val _deco = object : RecyclerView.ItemDecoration() {  // ItemDecorationの実装
            override fun onDraw(
                    c: Canvas, parent: RecyclerView,
                    state: RecyclerView.State) {
                // Canvasへ装飾を描画
				// アイテムが描画される前に描画されるため、アイテムの下に表示
            }

            override fun onDrawOver(
                    c: Canvas, parent: RecyclerView,
                    state: RecyclerView.State) {
                // Canvasへ装飾を描画
				// アイテムが描画された後に描画されるため、アイテムの上に表示
            }

            override fun getItemOffsets(
                    outRect: Rect, view: View, parent: RecyclerView,
                    state: RecyclerView.State) {
                // アイテムの上下左右へ空間を設ける
				// outRect.set(leftの空間, topの空間, rightの空間, bottomの空間)
            }
        }

        rcySample.addItemDecoration(_deco)	// ItemDecorationの登録
		
		...

		...
		rcySample = findViewById<RecyclerView>(R.id.rcySample)

        ...
        val _deco0 = object : RecyclerView.ItemDecoration() {
            override fun onDraw(
                    c: Canvas, parent: RecyclerView,
                    state: RecyclerView.State) {
                Log.i("ItemDecoration", "onDraw   (0)")
            }
        }
        val _deco1 = object : RecyclerView.ItemDecoration() {...}
        val _deco2 = object : RecyclerView.ItemDecoration() {...}
        val _decoX = object : RecyclerView.ItemDecoration() {...}

        rcySample.addItemDecoration(_deco0)
        rcySample.addItemDecoration(_deco1)
        rcySample.addItemDecoration(_deco2)
        rcySample.addItemDecoration(_decoX, 1)  // 第2引数はインデックス番号
		...

    ...
    final ArrayList<ItemDecoration> mItemDecorations = new ArrayList<>();

    ...
    public void addItemDecoration(@NonNull ItemDecoration decor, int index) {
        ...
        if (index < 0) {
            mItemDecorations.add(decor);
        } else {
            mItemDecorations.add(index, decor);
        }
        markItemDecorInsetsDirty();
        requestLayout();
    }

    ...
    public void addItemDecoration(@NonNull ItemDecoration decor) {
        addItemDecoration(decor, -1);
    }
	
	...

		...
		rcySample = findViewById<RecyclerView>(R.id.rcySample)

        ...
        val _deco = DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
        rcySample.addItemDecoration(_deco)
		...

    ...
    private static final int[] ATTRS = new int[]{ android.R.attr.listDivider };

    private Drawable mDivider;
	
    ...
    public DividerItemDecoration(Context context, int orientation) {
        final TypedArray a = context.obtainStyledAttributes(ATTRS);
        mDivider = a.getDrawable(0);
        if (mDivider == null) {
            Log.w(TAG, "@android:attr/listDivider was not set in the theme used for this "
                    + "DividerItemDecoration. Please set that attribute all call setDrawable()");
        }
        a.recycle();
        setOrientation(orientation);
    }
    ...

<?xml version="1.0" encoding="utf-8"?>
...

<shape xmlns:android="http://schemas.android.com/apk/res/android"
       android:tint="?attr/colorListDivider">
    <solid android:color="?attr/opacityListDivider" />
    <size
        android:height="1dp"
        android:width="1dp" />
</shape>

    ...
    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent,
            RecyclerView.State state) {
        if (mDivider == null) {
            outRect.set(0, 0, 0, 0);
            return;
        }
        if (mOrientation == VERTICAL) {
            outRect.set(0, 0, 0, mDivider.getIntrinsicHeight());
        } else {
            outRect.set(0, 0, mDivider.getIntrinsicWidth(), 0);
        }
    }
    ...

    ...
	private final Rect mBounds = new Rect();

    ...
    @Override
    public void onDraw(Canvas c, RecyclerView parent, RecyclerView.State state) {
        if (parent.getLayoutManager() == null || mDivider == null) {
            return;
        }
        if (mOrientation == VERTICAL) {
            drawVertical(c, parent);
        } else {
            drawHorizontal(c, parent);
        }
    }

    private void drawVertical(Canvas canvas, RecyclerView parent) {
        canvas.save();
        final int left;
        final int right;
        //noinspection AndroidLintNewApi - NewApi lint fails to handle overrides.
        if (parent.getClipToPadding()) {				// (1)
            left = parent.getPaddingLeft();
            right = parent.getWidth() - parent.getPaddingRight();
            canvas.clipRect(left, parent.getPaddingTop(), right,
                    parent.getHeight() - parent.getPaddingBottom());
        } else {
            left = 0;
            right = parent.getWidth();
        }

        final int childCount = parent.getChildCount(); 
        for (int i = 0; i < childCount; i++) {          // (2)
            final View child = parent.getChildAt(i);    // アイテムのViewを取り出し
            parent.getDecoratedBoundsWithMargins(child, mBounds);
            final int bottom = mBounds.bottom + Math.round(child.getTranslationY());
            final int top = bottom - mDivider.getIntrinsicHeight();
            mDivider.setBounds(left, top, right, bottom);   // 区切り線を描画
            mDivider.draw(canvas);                          // 同上
        }
        canvas.restore();
    }
    ...

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rcySample"
        android:layout_width="200dp"
        android:layout_height="240dp"
        android:layout_marginTop="40dp"
        android:background="@android:color/darker_gray"
        android:paddingLeft="10dp"
        android:paddingTop="20dp"
        android:paddingRight="10dp"
        android:paddingBottom="20dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.0" />

        ...
        final int childCount = parent.getChildCount(); 
        for (int i = 0; i < childCount; i++) {
            final View child = parent.getChildAt(i);
            parent.getDecoratedBoundsWithMargins(child, mBounds);
            final int bottom = mBounds.bottom + Math.round(child.getTranslationY());
            final int top = bottom - mDivider.getIntrinsicHeight();
            mDivider.setBounds(left, top, right, bottom);
            mDivider.draw(canvas);
        }
        ...