Create a Calculator class that implements the provided StackCalculator interface

Feature	Signature	Requirement
Constructor	Calculator()	Precondition: none Postcondition: a new calculator with an empty number stack
Methods	public void enter(String entry)	Precondition: entry is either a double value a recognized operator [ +, -, *, /] Postcondition: if the entry is a double, the value is pushed onto the calculators number stack. If an operator, the top two values are popped from the stack, the operation performed, and the result pushed onto the stack. Note: the second operand is popped first.
	double peek()	Precondition: number stack is not empty, otherwise throws EmptyStackException Looks at the number at the top of this stack without removing it from the stack.
	double pop()	Precondition: number stack is not empty, otherwise throws EmptyStackException Removes the number at the top of this stack and returns that value.
	void clear()	Removes all numbers from this stack. The stack will be empty after this call returns
	boolean isEmpty	Tests If the number stack of this calculator is empty
	int size()	Returns the number of values in this calculators number stack.