I did

Use a spoiler flag please, comments are visible to everyone.

It's not boolean but Boolean ... so some items might be null. When you do if(arrayOfSheeps[i]) Java evaluates it as if(arrayOfSheeps[i].booleanValue() but because arrayOfSheeps[i] is null you get the null pointer

Nice demonstration that interfaces, despite a popular belief to the contrary, can have methods with code (i.e. not just abstract methods) :)

Sorry but is a great example of how to complicate things unnecessarily:

• input.split("") is not necessary; you can already iterate through any string using String.chatAt
• the else is also unnecessary;