如何更改回默认工具栏navigationicon

Question

使用

更改工具栏navigationicon后

toolbar.setNavigationIcon(R.drawable.abc_ic_ab_back_mtrl_am_alpha);

我无法继续工作切换回默认（汉堡）图标：

int[] attrs = {android.R.attr.navigationIcon};
TypedArray ta = myContext.obtainStyledAttributes(attrs);
Drawable drawableFromTheme = ta.getDrawable(0 /* index */);
toolbar.setNavigationIcon(drawableFromTheme);
ta.recycle();