corejava第10版

corejava第10版/v2ch01/optional/OptionalTest.java

@@ -0,0 +1,63 @@
+package optional;
+
+import java.io.*;
+import java.nio.charset.*;
+import java.nio.file.*;
+import java.util.*;
+
+public class OptionalTest
+{
+   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+"));
+
+      Optional<String> optionalValue = wordList.stream()
+         .filter(s -> s.contains("fred"))
+         .findFirst();
+      System.out.println(optionalValue.orElse("No word") + " contains fred");
+
+      Optional<String> optionalString = Optional.empty();
+      String result = optionalString.orElse("N/A");
+      System.out.println("result: " + result);
+      result = optionalString.orElseGet(() -> Locale.getDefault().getDisplayName());
+      System.out.println("result: " + result);
+      try
+      {
+         result = optionalString.orElseThrow(IllegalStateException::new);
+         System.out.println("result: " + result);
+      }
+      catch (Throwable t)
+      {
+         t.printStackTrace();
+      }
+
+      optionalValue = wordList.stream()
+         .filter(s -> s.contains("red"))
+         .findFirst();
+      optionalValue.ifPresent(s -> System.out.println(s + " contains red"));
+
+      Set<String> results = new HashSet<>();
+      optionalValue.ifPresent(results::add);
+      Optional<Boolean> added = optionalValue.map(results::add);
+      System.out.println(added);
+
+      System.out.println(inverse(4.0).flatMap(OptionalTest::squareRoot));
+      System.out.println(inverse(-1.0).flatMap(OptionalTest::squareRoot));
+      System.out.println(inverse(0.0).flatMap(OptionalTest::squareRoot));
+      Optional<Double> result2 = Optional.of(-4.0)
+         .flatMap(OptionalTest::inverse).flatMap(OptionalTest::squareRoot);
+      System.out.println(result2);
+   }
+
+   public static Optional<Double> inverse(Double x)
+   {
+      return x == 0 ? Optional.empty() : Optional.of(1 / x);
+   }
+
+   public static Optional<Double> squareRoot(Double x)
+   {
+      return x < 0 ? Optional.empty() : Optional.of(Math.sqrt(x));
+   }
+}