问题

public class DequeCyclic implements Deque{
    private int left, right, size, capacity;
    private Object[] queue;
    // 构造一个空队列
    // @param s 是数组的最终大小
    public DequeCyclic(final int s) {
        left = right = size = 0;
        queue = new Object[s];
        capacity = s;
    }

    public boolean isEmpty() {
        return (size == 0);
    }
    
    public void pushLeft(Object c) throws Overflow {
        if (isFull()) {
            throw new Overflow("队列已满");
        } else if (left == 0 && right == 0 && size == 0) {
            queue[0] = c;
        } else {
            left = (left + 1) % queue.length;
            queue[left] = c;
        }
        size++;
    }

    public static void main(String[] args) {
        DequeCyclic[] ab = new DequeCyclic[10];
        ab.isEmpty(); // 这是问题所在
    }
}

运行时出现错误信息：
DequeCyclic.java:115: 错误: 找不到符号
ab.isEmpty();
^
符号: 方法 isEmpty()
位置: 类型为 DequeCyclic[] 的变量 ab
1 个错误

并且我也想知道为什么 pushLeft 方法有这行代码：
left = (left+1) % queue.length;

<details>
<summary>英文:</summary>

public class DequeCyclic implements Deque{
    private int left, right, size, capacity;
    private Object[] queue;
    //Constructing an empty Queue
    //@param s is the final size of the array

public DequeCyclic(final int s) {
    left= right = size = 0;
    queue = new Object
展开收缩
;
    capacity = s;
    
}

public boolean isEmpty() {
    return (size == 0);
}
public void pushLeft(Object c) throws Overflow{
    if (isFull()) {
        throw new Overflow ("the queue is full");
    }
    else if (left == 0 && right == 0 && size ==0) {
        queue[0] = c;
    }
    
    else {
        left = (left+1) % queue.length;
        queue[left] = c;
    }
    size++;
    }
public static void main(String[] args){
    DequeCyclic[]  ab = new DequeCyclic[10];
    ab.isEmpty();//this is the problem
    }
}

When I run this, I do not why it gave me that 
DequeCyclic.java:115: error: cannot find symbol
        ab.isEmpty();
          ^
  symbol:   method isEmpty()
  location: variable ab of type DequeCyclic[]
1 error
And I also wanna know why does the pushLeft has this line
left = (left+1) % queue.length;




</details>


# 答案1
**得分**: 0

你的问题在这里：

`DequeCyclic[]  ab = new DequeCyclic[10];`

这行代码的作用是什么？
它初始化了一个由大小为10的`DequeCyclic`组成的数组，命名为`ab`。

你可能想要的是：

`DequeCyclic  ab = new DequeCyclic(10);`

你调用了`DequeCyclic`的构造函数，并将参数10传递给它，将其赋值给变量`ab`。

接下来，`left = (left+1) % queue.length;` 是什么意思？

如果`left+1 < queue.length`，那么它等同于 `left++`。
否则，一旦 `left+1 == queue.length`，由于取余运算符，left 将变为零。

<details>
<summary>英文:</summary>

Your problem is here:

`DequeCyclic[]  ab = new DequeCyclic[10];`

What does this line do?
It initializes an array (size=10) of `DequeCyclic`s with the name `ab`.

What you probably want is:

`DequeCyclic  ab = new DequeCyclic(10);`

You call the constructor of `DequeCyclic` with the parameter 10, assigning it to the variable `ab`.

Next, what does `left = (left+1) % queue.length;` do?

It `left+1 &lt; queue.length`, it is equivalent to `left++`.
Else, as soon, as `left+1 == queue.length`, thanks to the modulo-operator, left will be zero.


</details>