思路分析
首先，在KeywordCounter类中，定义了一个包含所有Java关键字的字符串数组KEYWORDS，用于存储所有关键字。然后创建了一个Scanner对象input，用于从标准输入读取用户的输入。接下来创建了一个StringBuilder对象sb，用于存储读取的每一行输入内容。同时，还创建了一个HashMap对象map，用于存储每个关键字的出现次数。
在一个无限循环中，通过input.nextLine()方法逐行读取用户输入的内容，并判断是否等于"exit"，如果是则跳出循环。否则，将读取到的内容传递给processLine()方法进行处理，并将处理结果添加到sb中。
接下来，将sb转换为一个字符串content。对content进行预处理，首先调用removeCommentsAndStrings()方法去除注释和字符串，并将处理后的内容重新赋值给content。然后使用正则表达式content.replaceAll("[^a-zA-Z]", " ")将非字母字符替换为空格，得到只包含字母的单词。
将content按照空格进行分割，将分割后的单词存储在words数组中。接着调用countKeywords()方法，遍历words数组，如果单词在KEYWORDS数组中，则将该单词作为键，存储在map中，并增加对应关键字的计数。
最后，调用printKeywordCounts()方法，对map中的结果进行排序，并逐个输出关键字及其出现次数。
运行结果示例
代码
import java.util.Arrays;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Set;
import java.util.regex.Matcher;
import java.util.regex.Pattern;public class KeywordCounter {private static final String[] KEYWORDS = {"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"};public static void main(String[] args) {Scanner input = new Scanner(System.in);StringBuilder sb = new StringBuilder();Map<String, Integer> map = new HashMap<>();while (true) {String line = input.nextLine();if ("exit".equals(line)) {break;}sb.append(processLine(line)).append(" ");}String content = sb.toString();content = removeCommentsAndStrings(content);content = content.replaceAll("[^a-zA-Z]", " ");String[] words = content.split("\\s+");countKeywords(words, map);printKeywordCounts(map);}private static String processLine(String line) {if (line.matches("(.*)//(.*)")) {return line.split("//", 2)[0];} else {return line;}}private static String removeCommentsAndStrings(String content) {content = content.replaceAll("/\\*(?:.|[\\n\\r])*?\\*/", " ");content = content.replaceAll("\".*?\"", " ");return content;}private static void countKeywords(String[] words, Map<String, Integer> map) {for (String word : words) {if (Arrays.asList(KEYWORDS).contains(word)) {map.put(word, map.getOrDefault(word, 0) + 1);}}}private static void printKeywordCounts(Map<String, Integer> map) {Set<String> keySet = map.keySet();String[] keys = keySet.toArray(new String[0]);Arrays.sort(keys);for (String key : keys) {//System.out.println(map.get(key) + "\t" + key);System.out.println(key + ":" + "\t" + map.get(key) + "次");}}
}