// since we know that the integer must be
// between 30 and 39999 inclusive, the length
// of the string must be between 2 and 5.
if((length < 2) || (length > 5)) return false;

if(testInteger.charAt(0) != '3') return false;

// no need to actaully calculate the int value
// since we know that it starts with 3 and has
// a length of between 2 and 5. Just need to
// check that all the characters are digits

for(int i = 1; i < length; i++)
if(!Character.isDigit(testInteger.charAt(i)))
return false;

// passed all the tests
return true;
}