Android Studio (coding with Java) extract elements code

I am trying to extract some elements randomly from several different groups and make a new group with the extracted elements.

private int[] firstgroup = {R.drawable.element_1, R.drawable.element_2, R.drawable.element_3, R.drawable.element_4};

private int[] secondgroup = {R.drawable.element_5, R.drawable.element_6, R.drawable.element_7, R.drawable.element_8};

Like this

private int[] newgroup = {R.drawable.element_1, R.drawable.element_4, R.drawable.element_6, R.drawable.element_8};

How can I code to deal with this problem?