Queue with Two Stacks

public class Solver
{
    public static void main(String args[])
    {

    }
}

class Queue
{
    Stack st = new Stack();

    public void push(int d)  // O(n)
    {
        Stack temp_st = new Stack();
        while (isEmpty() == false)
        {
            temp_st.push(st.pop().data);
        }
        temp_st.push(d);
        while (temp_st.isEmpty() == false)
        {
            st.push(temp_st.pop().data);
        }
    }

    public Node pop()   // O(1)
    {
        if (isEmpty())
        {
            System.out.println("Can't pop, The Queue is Empty");
            return null;
        }
        return st.pop();
    }

    public Node front() // O(1)
    {
        if (isEmpty())
        {
            System.out.println("The Queue is Empty");
            return null;
        }
        return st.top();
    }

    public void display()   // O(n)
    {
        if (isEmpty())
        {
            System.out.println("The Queue is Empty");
            return;
        }
        st.display();
    }

    public boolean isEmpty()
    {
        return st.isEmpty();
    }
}

class Node
{
    int data;
    Node next;

    public Node(int d)
    {
        data = d;
    }
}

class Stack
{
    Node Top;

    public void push(int d)
    {
        Node temp = new Node(d);
        temp.next = Top;
        Top = temp;
        return;
    }

    public Node pop()
    {
        Node temp = Top;
        Top = Top.next;
        return temp;
    }

    public Node top()
    {
        if (isEmpty())
        {
            System.out.println("The Stack is Empty");
        }
        return Top;
    }

    public void display()
    {
        if (isEmpty())
        {
            System.out.print("The Stack is Empty");
        }
        Node curr = Top;
        while (curr != null)
        {
            System.out.print(curr.data + " ");
            curr = curr.next;
        }
        System.out.println();
        return;
    }

    public boolean isEmpty()
    {
        return Top == null;
    }
}