我想知道這段代碼有什么問題

Question

我想將 if 語句用於帶有字符串變量的多項選擇......但是，我無法輸入我選擇的答案或字母，即使我還沒有輸入任何內容，它也會立即輸出“輸入錯誤”。 . 請問這段代碼有什么問題嗎？

    System.out.println("Pick your choice: \na. Add Node\nb. Delete Node\nc. Insert Node");
    String letterChoice = scan.nextLine();

    if(letterChoice.equalsIgnoreCase("A")){
        System.out.println("Enter node value: ");
        int nodeValue = scan.nextInt();
        list.addNodes(nodeValue);
    }
    else if(letterChoice.equalsIgnoreCase("B")){
        System.out.println("Which node to delete? Node #: ");
        int thisNode = scan.nextInt();
        list.deleteNode(thisNode);
    }
    else if(letterChoice.equalsIgnoreCase("C")){
        System.out.println("Input node number: ");
        int thisNode = scan.nextInt();
        int prevNode = thisNode - 1;

        System.out.println("Input node value: ");
        int nodeValue = scan.nextInt();

        list.insertNode(nodeValue, prevNode);
    }
    else{
        System.out.println("Input Error");
    }

除非我更改我的代碼，否則我無法在這里找到以前問題的幫助...我不想，所以我不得不問...謝謝...

完整代碼...

    import java.util.*;

    public class DoublyLinkedList {


        static DNode root;
        static DNode temp;
        static DNode current;


        public void addNodes(int data){

            DNode dNode = new DNode(data);

            if(root==null){

                root = dNode;
                root.previousNode = null;
                root.nextNode = null;

            }else{

                current = root;

                while(current.nextNode!=null){

                    current = current.nextNode;

                }

                current.nextNode = dNode;
                dNode.previousNode = current;
                dNode.nextNode = null;

            }

        }

        public void insertNode(int data, int after){

            DNode dNode = new DNode(data);

            int ithNode = 1;

            current = root;

            while(ithNode != after){

                current = current.nextNode;

                ithNode++;

            }

            temp = current.nextNode;

            current.nextNode = dNode;
            dNode.nextNode = temp;
            temp.previousNode = dNode;
            dNode.previousNode = current;

        }

        public void deleteNode(int nodeNumber){

            int ithNode = 1;

            current = root;

            if(nodeNumber==1){

                root = current.nextNode;
                current.nextNode = null;
                current.previousNode = null;

            }else{

                while(ithNode != nodeNumber){

                    current = current.nextNode;

                    ithNode++;

                }

                if(current.nextNode == null){

                    current.previousNode.nextNode = null;
                    current.previousNode = null;

                }else{

                    current.previousNode.nextNode = current.nextNode;
                    current.nextNode.previousNode = current.previousNode;

                }

            }

        }

        public void print(){

            current = root;

            System.out.print("The Linked List: ");

            do{

                System.out.print(" " + current.data + " ");


                current = current.nextNode;

            }while(current!=null);

        }

        public void printBackwards(){

            current = root;

            while(current.nextNode!=null){

                current = current.nextNode;

            }

            System.out.print("Inverse: ");

            do{

                System.out.print(" " + current.data + " ");

                current = current.previousNode;

            }while(current.previousNode!=null);

            System.out.print(" " + current.data + " " );

        }

        public static void main(String[] args){

            DoublyLinkedList list = new DoublyLinkedList();
            Scanner scan = new Scanner(System.in);

            String letterChoice = "";
            int nodesNum;

            System.out.print("Input number of nodes: ");
            nodesNum = scan.nextInt();

            for(int x = 1; x <= nodesNum; x++){
                System.out.print("Input value of node #" + x + ": ");
                int value = scan.nextInt(); 
                list.addNodes(value);
            }

            list.print();


            System.out.println("Pick your choice: \na. Add Node\nb. Delete Node\nc. Insert Node");
            letterChoice = scan.nextLine();

            if(letterChoice.equalsIgnoreCase("A.")){
                System.out.println("Enter node value: ");
                int nodeValue = scan.nextInt();
                list.addNodes(nodeValue);
            }
            else if(letterChoice.equalsIgnoreCase("B.")){
                System.out.println("Which node to delete? Node #: ");
                int thisNode = scan.nextInt();
                list.deleteNode(thisNode);
            }
            else if(letterChoice.equalsIgnoreCase("C.")){
                System.out.println("Input node number: ");
                int thisNode = scan.nextInt();
                int prevNode = thisNode - 1;

                System.out.println("Input node value: ");
                int nodeValue = scan.nextInt();

                list.insertNode(nodeValue, prevNode);
            }
            else{
                System.out.println("Input Error");
            }

            list.print();
            System.out.println();
            list.printBackwards();

            System.out.println();
            System.out.println("The number of DNodes in the Linked List is " + DNode.noOfLinkedList);


        }

    }


     class DNode {

        static int noOfLinkedList = 0;

        int data;

        DNode previousNode;
        DNode nextNode;

        DNode(int data){

            this.data = data;
            noOfLinkedList++;

        }

    }

感謝所有的幫助，但要么是我的 jdk 或 cmd 有問題，要么沒有任何幫助......或者我只是沒有讓自己清楚......看，這是我的問題......

它運行，是的，但它只是跳到輸出“輸入錯誤”的其他部分......

Answer 1

請記住， Scanner#nextInt方法不會消耗輸入的最后一個換行符，因此在下一次調用 Scanner#nextLine 時會消耗該換行符。

看看這個： http : //docs.oracle.com/javase/7/docs/api/java/util/Scanner.html#nextLine%28%29

快速解決

System.out.print("Input number of nodes: ");
nodesNum = scan.nextInt();
scan.nextLine(); //this force the reading of the last newline after reading the integer...
        
...
System.out.println("Pick your choice: \na. Add Node\nb. Delete Node\nc. Insert Node");
final String letterChoice = scan.nextLine();