將對象從開關盒中取出

Question

我有一個菜單驅動的程序，提示用戶輸入整數以創建BST。一切工作都很棒，但如果用戶返回到主菜單並選擇創建新的二叉搜索樹，則新輸入實際上只是添加到舊樹中。我知道這是因爲我在switch語句之外創建了我的樹，但是，如果我在第一個switch語句內部創建它，以便創建一棵新樹，其他switch語句將無法訪問該樹...關於我應該在哪裏創建我的樹的任何想法？

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

聲明變量而不初始化它。然後創建一個新的樹，每當用戶進入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; 
    //... 
} 

或者因爲你創造你自己的BinaryTree類，你可以創建一個clear()方法來刪除從樹中每個節點和調用。