Java哈希表實現

Question

我想使用哈希表來計算文件中幾個字符串的出現次數。我會如何去做這件事？另外，我能夠以相似的方式計算唯一字符串的數量嗎？例子將不勝感激。

Answer 1

作爲一個例子，下面是一個程序，它將讀取文件中的單詞並計算遇到Java關鍵字的次數。

import java.io.BufferedReader; 
import java.io.File; 
import java.io.FileNotFoundException; 
import java.io.FileReader; 
import java.io.IOException; 
import java.util.Map; 
import java.util.HashMap; 

public class CountKeywords { 

    public static void main(String args[]) { 

     String[] theKeywords = { "abstract", "assert", "boolean", "break", "byte", "case", "catch", "char", "class", "const", "continue", "default", "do", "double", "else", "enum", "extends", "false", "final", "finally", "float", "for", "goto", "if", "implements", "import", "instanceof", "int", "interface", "long", "native", "new", "null", "package", "private", "protected", "public", "return", "short", "static", "strictfp", "super", "switch", "synchronized", "this", "throw", "throws", "transient", "true", "try", "void", "volatile", "while" }; 

     // put each keyword in the map with value 0 
     Map<String, Integer> theKeywordCount = new HashMap<String, Integer>(); 
     for (String str : theKeywords) { 
      theKeywordCount.put(str, 0); 
     } 

     FileReader fr; 
     BufferedReader br; 
     File file = new File(args[0]); // the filename is passed in as a String 

     // attempt to open and read file 
     try { 
      fr = new FileReader(file); 
      br = new BufferedReader(fr); 

      String sLine; 

      // read lines until reaching the end of the file 
      while ((sLine = br.readLine()) != null) { 

       // if an empty line was read 
       if (sLine.length() != 0) { 

        // extract the words from the current line in the file 
        if (theKeywordCount.containsKey(sLine)) { 
         theKeywordCount.put(sLine, theKeywordCount.get(sLine) + 1); 
        } 
       } 
      } 

     } catch (FileNotFoundException exception) { 
      // Unable to find file. 
      exception.printStackTrace(); 
     } catch (IOException exception) { 
      // Unable to read line. 
      exception.printStackTrace(); 
     } finally { 
       br.close(); 
      } 

     // count how many times each keyword was encontered 
     int occurrences = 0; 
     for (Integer i : theKeywordCount.values()) { 
      occurrences += i; 
     } 

     System.out.println("\n\nTotal occurences in file: " + occurrences); 
    } 
} 

要回答關於唯一字符串的問題，可以採用類似方式來調整我使用HashMap的方式。

1. 創建一個新的HashMap，稱之爲uniqueStrings
2. 從文件中讀取字符串時，檢查，保持計數的軌道HashMap中包含當前字符串
   • 如果沒有，再加入它uniqueStrings
   • 如果確實如此，那麼從uniqueStrings
3. 刪除它，你就大功告成了讀取文件後，你將只有唯一字符串uniqueStrings

如果您有任何問題，請告訴我。

我希望這會有所幫助。
Hristo

Answer 2

爲了跟蹤唯一字符串，您不需要跟蹤文件中出現的次數。相反，您可以使用HashSet代替HashMap以實現代碼清晰度。

注意：HashSet內部支持HashMap，最終對象用作鍵值對中的值。