ArrayList<Character> answer = new ArrayList<Character>();
ArrayList<Button> buttons = new ArrayList<Button>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button btn1 = (Button) findViewById(R.id.button1);
    Button btn2 = (Button) findViewById(R.id.button2);
    Button btn3 = (Button) findViewById(R.id.button3);
    Button btn4 = (Button) findViewById(R.id.button4);
    Button btn5 = (Button) findViewById(R.id.button5);
    Button btn6 = (Button) findViewById(R.id.button6);
    Button btn7 = (Button) findViewById(R.id.button7);
    Button btn8 = (Button) findViewById(R.id.button8);
    Button btn9 = (Button) findViewById(R.id.button9);
    b.add(btn1);
    b.add(btn2);
    b.add(btn3);
    b.add(btn4);
    b.add(btn5);
    b.add(btn6);
    b.add(btn7);
    b.add(btn8);
    b.add(btn9);

    String testAnswer = "Answer";
    //convert to chars and add to arraylist for shuffling
    for(char a : answer.toCharArray()){
        al.add(a);
    }
    Collections.shuffle(al);


    //loop over buttons and letters and assign each button with a letter
    for(char a : al){
        for(Button bb : b){
            bb.setText(String.valueOf(a));
        }
    }

}