Java Hashmap实现

Question

我想使用hashmap来计算文件中多个字符串的出现次数。我该怎么做呢？另外，我能以类似的方式计算唯一字符串的数量吗？举例非常感谢。

Answer 1

例如，这是一个程序，它将读取文件中的单词并计算遇到Java关键字的次数。

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);
    }
}

要回答有关唯一字符串的问题，您可以采用类似的方式调整我使用HashMap的方式。

1. 创建一个新的HashMap，称之为uniqueStrings
2. 从文件中读取字符串时，检查跟踪计数的HashMap是否包含当前字符串
   • 如果没有，则将其添加到uniqueStrings
   • 如果是，请将其从uniqueStrings
   中删除
3. 阅读完文件后，uniqueStrings
中只有唯一的字符串

如果您有疑问，请告诉我。

我希望这会有所帮助 赫里斯托斯

Answer 2

要跟踪唯一字符串，您无需跟踪文件中出现的次数。相反，您可以使用HashSet代替HashMap来提高代码清晰度。

注意：HashSet由HashMap内部支持，最终对象用作键值对中的值。