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java - 如何修改此代码以写入文件？

问题描述

我正在尝试修改此堆算法，以显示字符串输入的所有可能排列。我正在尝试修改代码，以便能够与带有编写器文件的 Scanner 类一起使用。当我尝试写入一个新文件时，它不会添加所有 24 个字符串，而是添加前 4 个。因为它是一个 void 方法，我不能使用 pw.println(obj.heapPermutation(a, a.长度，a.长度））。有什么建议可以解决这个问题吗？

谢谢

PS我在网上找到了这段代码，我承认它不是我的。

import java.util.Scanner;
import java.io.*;
import static java.lang.System.*;

class HeapAlgo 
{ 




    void heapPermutation(String a[], int size, int n) throws IOException
    { 
       FileWriter fw = new FileWriter("note.txt");
       PrintWriter pw = new PrintWriter(fw);


 // if size becomes 1 then prints the obtained 
 // permutation 
       if (size == 1) 
          for (int i=0; i<n; i++)
          { 
              System.out.println(a[i] + "");



          } 


          for (int i=0; i<size; i++) 
          { 
             heapPermutation(a, size-1, n); 

// if size is odd, swap first and last 
// element 
                if (size % 2 == 1) 
                { 
                    String temp = a[0]; 
                    a[0] = a[size-1]; 
                    a[size-1] = temp; 
                } 

// If size is even, swap ith and last 
// element 
                else
                { 
                   String temp = a[i]; 
                   a[i] = a[size-1]; 
                   a[size-1] = temp; 
                } 


         }


  } 


public static void main(String args[]) throws IOException 
{ 

   HeapAlgo obj = new HeapAlgo(); 
   String a[] = new String["abcd","bbbb","cccc","dddd"];
   obj.heapPermutation(a, a.length, a.length);

标签： javajava.util.scannerpermutationfilewriterprintwriter

解决方案

你PrintWriter在 heapPermutation 方法中被初始化，因为它是递归调用的，heapPermutation(a, size-1, n)每次都会被覆盖。我相信默认行为是替换文件，而不是附加到它。

您应该创建一个构造函数来初始化它，PrintWriter因此它不会每次都重新初始化。

class HeapAlgo {
    // create a writer at the class level
    private PrintWriter _pw;

    // Create a constructor to assign the writer
    public HeapAlgo(PrintWriter pw) {
       this._pw = pw;
    }

    void heapPermutation(String a[], int size, int n) throws IOException { 
    // if size becomes 1 then prints the obtained 
    // permutation 
        if (size == 1) 
            for (int i=0; i<n; i++) { 
                System.out.println(a[i] + "");
                this._pw.print(a[i] + ""); // print here I belive?
            }

        for (int i=0; i<size; i++) { 
            heapPermutation(a, size-1, n); 

            // if size is odd, swap first and last 
            // element 
            if (size % 2 == 1) { 
                String temp = a[0]; 
                a[0] = a[size-1]; 
                a[size-1] = temp; 
            }

            // If size is even, swap ith and last 
            // element 
            else { 
               String temp = a[i]; 
               a[i] = a[size-1]; 
               a[size-1] = temp; 
            } 
        }
    }
}

public static void main(String args[]) throws IOException {
    FileWriter fw = new FileWriter("note.txt");
    PrintWriter pw = new PrintWriter(fw);
    HeapAlgo obj = new HeapAlgo(pw); // Pass in a writer
    String a[] = new String["abcd","bbbb","cccc","dddd"];
    obj.heapPermutation(a, a.length, a.length);

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