StackQueue.java

package datastructure.stackqueue;

interface Stack {
    void push(Object obj);
    Object pop();
    boolean isEmpty();
}

class ArrayStack implements Stack {
    private Object[] theArray;
    private int topOfStack;
    private static final int DEFAULT_CAPACITY = 10;

    public ArrayStack() {
        theArray = new Object[DEFAULT_CAPACITY];
        topOfStack = -1;
    }

    public boolean isEmpty() {
        return topOfStack == -1;
    }

    public void push(Object obj) {
        if(!isFull()) {
            theArray[++topOfStack] = obj;
        }
    }

    public Object pop() {
        if(!isEmpty()) {
            return theArray[topOfStack--];
        } else {
            return null;
        }
    }

    public boolean isFull() {
        return (topOfStack == theArray.length);
    }
}

public class StackQueue {

    ArrayStack inStack = new ArrayStack();
    ArrayStack outStack = new ArrayStack();

    public void enQueue(Object obj) {
        inStack.push(obj);
    }
    
    public Object deQueue() {
        if(outStack.isEmpty()) {
            while(!inStack.isEmpty()) {
                outStack.push(inStack.pop());
            }
        }
        return outStack.pop();
    }

    public static void main(String[] args) {
        StackQueue queue = new StackQueue();
        queue.enQueue("first");
        queue.enQueue("second");
        queue.enQueue("third");
        queue.enQueue("fourth");

        System.out.println("1." + queue.deQueue());
        System.out.println("2." + queue.deQueue());
        System.out.println("3." + queue.deQueue());
        System.out.println("4." + queue.deQueue());
    }
}

Review before 7/15
Thanks