Why is the new array null even though I populate it with values from other initialized arrays?

Question

//row 0 are questions and row 1 are answers

public String[][] correctAnswers = {
    {"What name was on the door the day he ran away?"},
    {"Mike Smith"}
};

public String[] wrongAnswers = new String[]{"Rory Mcilroy", "Bob Jones", "Arnold Palmer"};

protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    Log.d("new game","onCreate");

    setContentView(R.layout.game_layout);

    //Get three random indexes for the wrong answers here   
    //Get the three wrong answers for buttons

    String[] afb = new String[3];
    for(int i = 0;i<4;i++){
        Random rnd = new Random();
        int a = rnd.nextInt(2);
        System.arraycopy(wrongAnswers, a, afb, i, wrongAnswers.length);
    }
}

Answer 1

Look that you have i<4 in your loop. And you just have 3 positions in your array. You have to remember that arrays start on position 0 , not 1 .

If you look at Java documentation you can see what happens:

Copies an array from the specified source array

You are copying the wrongAnswers array.

beginning at the specified position

Your a variable could be 0 or 1.

to the specified position of the destination array

If your i variable it's 4 then it couldn't get any value from wrongAnswers array and it will crash with an ArrayIndexOutOfBoundsException .

What can you try it's to put your System.arraycopy without the for loop. Like this:

System.arraycopy(wrongAnswers, 0, afb, wrongAnswer.length - 1, wrongAnswers.length);

I really don't understand why do you need a random number to do this (look that with System.arraycopy you can copy all the array in one time).

Answer 2

If you want to get all the non-repeated indexes in random order , you may consider to copy the entire array and then shuffle it.

private static void shuffle(String[] ar){
  Random rnd = new Random();
  for (int i = ar.length - 1; i > 0; --i) {
    int index = rnd.nextInt(i + 1);
    String s = ar[index];
    ar[index] = ar[i];
    ar[i] = s;
  }
}

protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    Log.d("new game","onCreate");

    setContentView(R.layout.game_layout);

    //Get three random indexes for the wrong answers here   
    //Get the three wrong answers for buttons

    String[] afb = new String[3];       
    System.arraycopy(wrongAnswers, 0, afb, 0, afb.length);

    //prints the original array
    for(int i=0; i<afb.length; ++i)
        System.out.println(afb[i]);

    shuffle(afb);

    //prints the shuffled array
    for(int i=0; i<afb.length; ++i)
        System.out.println(afb[i]);
}

The above private static void shuffle(String[] ar) is an edit from the method found in this answer provided by the user PhiLho .