java - 无法实例化 Queue 类型。为什么是这样？

Question

这是我用于堆栈/队列分配的主要方法。我的队列一直有错误，但我的 Stack 没有。堆栈类似乎工作得很好。我完全卡住了。它说“无法实例化类型队列”。非常感激任何的帮助!

 public class mainMeth {

      public static void main(String[] args) throws FileNotFoundException {
            File Polish = new File("fILE4INPUT.txt");
            File out = new File("outfile.txt");

            Scanner f = new Scanner(Polish);
            Queue inputQ = new Queue();
            Stack stack2 = new Stack();
            Queue outputQ = new Queue();

            String word;
            Character ch;

            while (f.hasNext()) {
                String myString = f.nextLine();

                for (int count = 0; count < myString.length(); count++) {
                    ch = myString.charAt(count);
                    inputQ.addtoRear(ch);
                }
                while (!inputQ.ismtQ()) {
                    ch = inputQ.remfront();

                    if (isAlpha(ch)) {
                        // System.out.println(ch);
                        outputQ.addtoRear(ch);
                    } else {
                        if (isOperator(ch)) {

                            if (stack2.ismt()) {
                                stack2.push(ch);

                            } else {
                                if (valueOf(ch) > valueOf(stack2.top())) {
                                    stack2.push(ch);
                                } else {
                                    outputQ.addtoRear(stack2.pop());
                                    stack2.push(ch);
                                }
                            }
                        }
                    }
                }
                while (!stack2.ismt()) {
                    outputQ.addtoRear(stack2.pop());
                }
                System.out.println(outputQ.toString() + "\n\n");
                while (!outputQ.ismtQ()) {
                    outputQ.remfront();
                }

            }

        }

        public static boolean isAlpha(Character ch) {
            boolean retVal = false;
            if (ch >= 'A' && ch <= 'Z' || ch >= 'a' && ch <= 'z')
                retVal = true;


            return (retVal);
        }

        public static boolean isOperator(Character ch) {
            boolean retVal = false;
            if (ch == '+' || ch == '-' || ch == '/' || ch == '*')
                retVal = true;

            return (retVal);
        }

        public static int valueOf(Character ch) {
            int retval = 0;
            if (ch == '/' || ch == '*')
                retval = 2;
            else
                retval = 1;
            return retval;
        }


 }      

Answer 1

在Interfaces Java 文档部分:

Interfaces cannot be instantiated—they can only be implemented by classes or extended by other interfaces.

那么你不能直接实例化 interface Queue<E> .但是，您仍然可以引用实现 Queue 的对象。按接口(interface)类型的接口(interface)，例如:

// As I saw that you are adding Characters to your queue
Queue<Character> inputQ = new PriorityQueue<Character>();

您可以根据您的要求选择适当的实现，这里是所有具体和已知实现类的列表，来自 java docs :

ArrayBlockingQueue, ArrayDeque, ConcurrentLinkedDeque, ConcurrentLinkedQueue, DelayQueue, LinkedBlockingDeque, LinkedBlockingQueue, LinkedList, LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, SynchronousQueue