public static int[] doublearray(int [] foo) {
    // declare an array twice as big as foo
    int [] returnval = new int[foo.length * 2];
    for (int a = 0; a < foo.length; a++) {
       // copy element a of foo into element a * 2 of returnval
       returnval[a*2] = foo[a];
       // copy element a of foo into element a * 2 + 1 of returnval
       returnval[a*2+1] = foo[a];
    }
    // return returnval
    return returnval;
}