java - 从开关盒中取出对象

Question

我有一个菜单驱动程序，提示用户输入整数以创建 BST。第一次一切都很好，但是如果用户返回主菜单并选择创建一个新的二叉搜索树，新的输入实际上只是添加到旧树上。我知道这是因为我在 switch 语句之外创建树，但是，如果我在我的第一个 switch 语句中创建它，以便每次创建新树时，其他 switch 语句将无法访问树...关于我应该在哪里创建我的树的任何想法？

package model;

import java.io.*; 
import java.util.*; 


public class menu {
    public static void main(String[] args){
        int integerInput;
        int inputOption;
        int myCategoryChoice;

        InOrder inOrder;
        PreOrder preOrder;
        PostOrder postOrder;
        Context context;

        BinaryTree myIntTree;

        myIntTree = new BinaryTree();
        inOrder = new InOrder();
        preOrder = new PreOrder();
        postOrder = new PostOrder();

        Scanner userInput = new Scanner(System.in);
        do{
            System.out.println("=======================================");
            System.out.println("     Binary Search Tree Traversal!     ");
            System.out.println("=======================================");
            System.out.println("Options:                               ");
            System.out.println("  1.  Create a new binary search tree  ");
            System.out.println("  2.  Traverse the tree                ");
            System.out.println("  3.  Quit                             ");
            System.out.println("=======================================");

            inputOption = KeyIn.inInt("Please select an option from the menu: ");

            switch(inputOption){
                case 1:
                    System.out.println("You've selected to create a new binary tree." + "\n");
                    Scanner scan = new Scanner(System.in);
                    String tempInput;
                    Boolean repeat = true;
                    try{
                        System.out.println("Please enter as many integers as you'd like, hit 'Q' when you are finished." + "\n");
                        do{

                            tempInput = scan.next();
                            if(!tempInput.equals("Q") && !tempInput.equals("q")){
                                integerInput = Integer.parseInt(tempInput);
                                myIntTree.insert(integerInput);
                                repeat = true;
                            }
                            else
                                repeat = false;

                        }while(repeat);

                    }catch (NumberFormatException e) {
                            System.out.println("Invalid input. Not an integer");
                }
                break;

                case 2:
                    System.out.println("Let's traverse the tree we just made!. ");
                    myCategoryChoice = KeyIn.inInt("Please select a traversal method: (1) Pre-Order (2) In-Order (3) Post-Order");

                    Boolean done = false;

                    do{
                        switch(myCategoryChoice){
                            case 1:
                                System.out.println("You've selected a pre-order traversal." + "\n");

                                context = new Context(preOrder);
                                context.traverse(myIntTree);

                                done = true;                        
                                break;

                            case 2:
                                System.out.println("You've selected an in-order traversal." + "\n");


                                context = new Context(inOrder);
                                context.traverse(myIntTree);

                                done = true;
                                break;

                            case 3:
                                System.out.println("You've selected a post-order traversal." + "\n");

                                context = new Context(postOrder);
                                context.traverse(myIntTree);

                                done = true;
                                break;
                        }
                    }while(!done);
                    break;
                case 3:
                    System.out.println("Goodbye!");
                    break;
                default:
                    System.out.println("Invalid selection.");
                    break;              
            }
        } while(inputOption != 3);
    }
}

Answer 1

声明变量而不初始化它。然后在用户输入时创建一个新树1：

public static void main(String[] args){
    //...
    BinaryTree myIntTree = null;

    inOrder = new InOrder();
    preOrder = new PreOrder();
    postOrder = new PostOrder();
    //...

    case 1:
        myIntTree = new BinaryTree();
    //...
    case 2:
        if(myIntTree == null) break;
    //...
}

或者，由于您创建了自己的BinaryTree类，您可以创建一个clear()方法来从树中删除每个节点并调用它。