is "toppings" in the method orderPizza a array or a string?

Pretty straightforward

Do not confuse with String and Array, they do not come under the same category. Array is a data structure, where as String is a data type. So you could have an Array which of the type String, Integer, Char, Float etc.

What you probably are looking to question is when and how to spot an array

When you pass the String data type (String is a class in Java and not a primitive type) itself -- the syntax would be as follows

public Pizza orderPizza(String toppings) 
{  //code 
    }

However when its an array it would, well have an array in the parameter list preceded by the dat type (denoted by two square brackets) like--

Note -- in java the default syntax is --- data_type variable_name;
The data type could not be a simple primitive or an array

public Pizza orderPizza(String [] toppings) 
{ //code 
   }

Hope this helps