Question

I’m trying to move a paren balancer I wrote in C++ to Java.

I’m trying to implement the stack with an ArrayDeque class from the Deque interface by declaring an ArrayDeque of characters like so:

Deque<char> parens = new ArrayDeque<char>();

and the compiler chokes on it claiming
expected: reference
found: char

What am I missing?

Answer 1

You can’t use primitive types as generic parameters. You need the corresponding Object wrappers:

Deque<Character> parens = new ArrayDeque<Character>();

Let’s update our Box class to use generics. We’ll first create a
generic type declaration by changing the code public class Box to
public class Box<T>; this introduces one type variable, named T,
that can be used anywhere inside the class. This same technique can be
applied to interfaces as well. There’s nothing particularly complex
about this concept. In fact, it’s quite similar to what you already
know about variables in general. Just think of T as a special kind of
variable, whose “value” will be whatever type you pass in; this can be
any class type, any interface type, or even another type variable. It
just can’t be any of the primitive data types. In this context, we
also say that T is a formal type parameter of the Box class.

[Source: Java Tutorial : Generics : Generic Types]

See:

• Java Tutorial: Generics
• JLS: 4.4 Type Variables
• JLS: 4.5 Parameterized Types