StackQueue.java

import java.util.ArrayDeque;
import java.util.Deque;

public class StackQueue<T> {

    private Deque<T> in;
    private Deque<T> out;

    public StackQueue() {
        this.in = new ArrayDeque<>();
        this.out = new ArrayDeque<>();
    }

    public void enqueue(T item) {
        in.push(item);
    }

    public T dequeue() {
        if (out.isEmpty()) {
            while (!in.isEmpty()) {
                out.push(in.pop());
            }
        }

        return out.pop();
    }

    public boolean isEmpty() {
        return in.isEmpty() && out.isEmpty();
    }

    public int size() {
        return in.size() + out.size();
    }
}