corejava第10版

corejava第10版/v2ch01/parallel/ParallelStreams.java

@@ -0,0 +1,58 @@
+package parallel;
+
+import static java.util.stream.Collectors.*;
+
+import java.io.*;
+import java.nio.charset.*;
+import java.nio.file.*;
+import java.util.*;
+import java.util.stream.*;
+
+public class ParallelStreams
+{
+   public static void main(String[] args) throws IOException
+   {
+      String contents = new String(Files.readAllBytes(
+            Paths.get("../gutenberg/alice30.txt")), StandardCharsets.UTF_8);
+      List<String> wordList = Arrays.asList(contents.split("\\PL+"));
+
+      // Very bad code ahead
+      int[] shortWords = new int[10];
+      wordList.parallelStream().forEach(s -> 
+         {
+            if (s.length() < 10) shortWords[s.length()]++;
+         });
+      System.out.println(Arrays.toString(shortWords));
+
+      // Try again--the result will likely be different (and also wrong)
+      Arrays.fill(shortWords, 0);
+      wordList.parallelStream().forEach(s -> 
+         {
+            if (s.length() < 10) shortWords[s.length()]++;
+         });
+      System.out.println(Arrays.toString(shortWords));
+
+      // Remedy: Group and count
+      Map<Integer, Long> shortWordCounts = wordList.parallelStream()
+         .filter(s -> s.length() < 10)
+         .collect(groupingBy(String::length, counting()));
+
+      System.out.println(shortWordCounts);
+
+      // Downstream order not deterministic
+      Map<Integer, List<String>> result = wordList.parallelStream().collect(
+         Collectors.groupingByConcurrent(String::length));
+
+      System.out.println(result.get(14));
+
+      result = wordList.parallelStream().collect(
+         Collectors.groupingByConcurrent(String::length));
+
+      System.out.println(result.get(14));
+
+      Map<Integer, Long> wordCounts = wordList.parallelStream().collect(
+         groupingByConcurrent(String::length, counting()));
+
+      System.out.println(wordCounts);
+   }
+}