`Digits`

is #1 from the from the 2017 AP Computer Science A Free Response problems.

https://secure-media.collegeboard.org/ap/pdf/ap-computer-science-a-frq-2017.pdf

## Part (a) `Digits`

constructor

```
public Digits(int num)
{
digitList = new ArrayList<Integer>();
digitList.add(0, new Integer(num % 10));
int numRemaining = num / 10;
while(numRemaining > 0)
{
digitList.add(0, new Integer(numRemaining % 10));
numRemaining /= 10;
}
}
```

It is very common for students to miss the first line of this constructor. Adding to a list (or setting elements in an array) is not the same thing as initializing the variable.

Adding to the beginning of an `ArrayList`

isn’t my favorite approach, since all of the elements have to shift. Given the simplicity though, I think it’s a reasonable tradeoff.

## Part (b) `isStrictlyIncreasing`

method

```
public boolean isStrictlyIncreasing()
{
for(int i = 1; i < digitList.size(); i++)
if(digitList.get(i - 1).compareTo(digitList.get(i)) >= 0)
return false;
return true;
}
```

The use of `compareTo`

here is optional. Java will automatically unbox the `Integer`

objects into primitive types if >= is used.

For more on `compareTo`

see compareTo on the AP CS A Exam.

## 2017 AP CS Exam Free Response Solutions

## Help & comments

Get help from AP CS Tutor Brandon Horn