学习java自定义异常，需要以某种方式更改实现

Question

我正在学习java，我应该为一个固定队列的类添加一个异常处理程序。似乎界面需要更改，但我不确定如何更改。

代码：

//ICharQ.java
package qpack;

public interface ICharQ {
    void put(char ch);

    char get();

    void reset();
}

//QExcDemo.java
package qpack;

class QueueFullException extends Exception {
    int size;

    QueueFullException(int s) { size = s; }

    public String toString() {
        return "\nQueue is full. Max size is " + size;
    }
}

class QueueEmptyException extends Exception {
    public String toString() {
        return "\nQueue is empty.";
    }
}


//Excerpt from IQClasses.java
package qpack;

class FixedQueue implements ICharQ {
        private char q[];
        private int putloc, getloc;

        public FixedQueue(int size) {
                q = new char[size+1];
                putloc = getloc = 0;
        }

        public void put(char ch)
         throws QueueFullException {
                if (putloc == q.length-1)
                        throw new QueueFullException(q.length-1);


                putloc++;
                q[putloc] = ch;
        }

        public char get()
         throws QueueEmptyException {
                if (getloc == putloc)
                        throw new QueueEmptyException();

                getloc++;
                return q[getloc];
        }

        public void reset() {
                putloc = getloc = 0;
        }
}

编译器输出...

qpack/IQClasses.java:22: error: get() in FixedQueue cannot implement get() in ICharQ
public char get() 
            ^
   overridden method does not throw QueueEmptyException
qpack/IQClasses.java:12: error: put(char) in FixedQueue cannot implement put(char) in ICharQ
public void put(char ch) 
            ^
   overridden method does not throw QueueFullException

2个错误

Answer 1

在FixedQueue中，您有检查过的异常。

    public void put(char ch)
     throws QueueFullException {

    public char get()
     throws QueueEmptyException {

这意味着这些方法的接口必须有相同的“抛出”。

顺便说一句，我会让QueueFullException和QueueEmptyException扩展IllegalStateException，这不是一个检查异常，但我仍然会将它添加到您接口的throws子句中。

为了进行比较，您可以查看Queue，我将尽可能多地跟踪它抛出的命名和异常。

我会考虑把你的FixedBuffer变成一个环形缓冲区，也被称为Circular Buffer，这样你的队列就不会因为到达末尾而耗尽空间。

这就是我如何通过基于队列来设置接口。

public interface CharQueue {

    /**
     * Inserts the specified element into this queue if it is possible to do so
     * immediately without violating capacity restrictions, returning
     * <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
     * if no space is currently available.
     *
     * @param ch the char to add
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws IllegalStateException if the element cannot be added at this
     *         time due to capacity restrictions
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean add(char ch) throws IllegalStateException;

    /**
     * Inserts the specified element into this queue if it is possible to do
     * so immediately without violating capacity restrictions.
     * When using a capacity-restricted queue, this method is generally
     * preferable to {@link #add}, which can fail to insert an element only
     * by throwing an exception.
     *
     * @return <tt>true</tt> if the element was added to this queue, else
     *         <tt>false</tt>
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     */
    boolean offer(char ch);

    /**
     * Retrieves and removes the head of this queue.  This method throws an exception if this
     * queue is empty.
     *
     * @return the head of this queue
     * @throws NoSuchElementException if this queue is empty
     */
    char remove() throws NoSuchElementException;

    /**
     * Removes all of the elements from this collection.
     * The collection will be empty after this method returns.
     */
    void clear();
}

这样，您可以在心理上(如果不是在代码中)将Queue<Character>替换为CharQueue，正如文档所指出的那样，offer比add更可取，并且您可能希望根据自己的需求选择其中之一。

Answer 2

在FixedQueue类中，方法put抛出QueueFullException，但接口ICharQ中没有指定这一点。get和QueueEmptyException也是如此。

您可以执行以下任一操作：

• 也可以在接口中指定这些异常，
• 或者使这两个异常都扩展RuntimeException而不是Exception