13. Practice A

Two practice problems covering the basic properties (and gotcas) of ArrayList.

ArrayList and Random

Make an ArrayList of Integer.
Add in a random number (10 through 20) of random integers in the range [-50,50].
Change 3 random locations in your arraylist to -500.
Remove the items at index 2 and 6 and print out their sum.
Print out the number of items in the ArrayList.
Modify the list by removing all of the negative numbers.
Print the list.

Null

String bad = null;
String empty = "";

Input an array of integers.
```
   public static void prob2(int[] xs)
```
Output an ArrayList of the same length.

Translate each of the numbers into a string:
```
  0 -> null
  1 -> "One"
  3 -> "Three"
  _ -> "."
```
Print the total # characters in all of the Strings in the ArrayList.
Now remove all of the “Three” strings in the ArrayList.
Print the remaining ArrayList.