Experiment with Intersection Types

Author: Nicolai Parlog

README.md

@@ -12,13 +12,14 @@ You can read more from me on [codefx.org](http://codefx.org) ([Java 10 tag](http
 * download [OpenJDK 10](http://jdk.java.net/10/) (not Oracle JDK!) and unpack it
 * to use with Maven simply set `target` and `release` to `10` (see [`pom.xml`](pom.xml#L13-L14))
 * to use with IntelliJ:
-    * install [2017.3 EAP](https://www.jetbrains.com/idea/nextversion/)
+    * install [2017.3 EAP](https://www.jetbrains.com/idea/nextversion/) or later
     * go to an occurrence of `var`, hit _ALT + Enter_, and "Enable support for beta java version"
     * in project and module settings, make sure that language level _X_ is selected
 
 ## Language Changes
 
 * [local-variable type inference with `var`](src/main/java/org/codefx/demo/java10/lang/var/VariableTypeInference.java) ([blog post](http://blog.codefx.org/java/java-10-var-type-inference/), [video](https://www.youtube.com/watch?v=Le1DbpRZdRQ), [JEP 286](http://openjdk.java.net/jeps/286))
+	* experiments with [intersection types](src/main/java/org/codefx/demo/java10/lang/var/IntersectionTypes.java) ([blog post](http://blog.codefx.org/java/intersection-types-var))
 
 ## JVM Capabilities
 

src/main/java/org/codefx/demo/java10/lang/var/IntersectionTypes.java

@@ -0,0 +1,75 @@
+package org.codefx.demo.java10.lang.var;
+
+import java.io.Closeable;
+import java.io.IOException;
+import java.util.Iterator;
+import java.util.Optional;
+import java.util.Random;
+import java.util.Scanner;
+import java.util.function.Predicate;
+import java.util.stream.Stream;
+import java.util.stream.StreamSupport;
+
+import static java.util.stream.StreamSupport.stream;
+
+public class IntersectionTypes {
+
+	public static void main(String[] args) throws Exception {
+		boolean empty = new Random().nextBoolean();
+		doItWithGenerics(empty);
+		doItWithVar(empty);
+	}
+
+	private static <T extends Closeable & Iterator<String>> void doItWithGenerics(boolean empty) throws IOException {
+		T elements = createCloseableIterator(empty);
+		Optional<String> dollarWord = firstMatch(elements, s -> s.startsWith("$"));
+		System.out.println(dollarWord.orElse("none"));
+	}
+
+	private static void doItWithVar(boolean empty) throws IOException {
+		var scanner = createCloseableIterator(empty);
+		Optional<String> dollarWord = firstMatch(scanner, s -> s.startsWith("$"));
+		System.out.println(dollarWord.orElse("none"));
+	}
+
+	@SuppressWarnings("unchecked")
+	private static <T extends Closeable & Iterator<String>> T createCloseableIterator(boolean empty) {
+		if (empty)
+			return (T) new Empty();
+		else
+			return (T) new Scanner(System.in);
+	}
+
+	private static <E, T extends Closeable & Iterator<E>> Optional<E> firstMatch(
+			T elements, Predicate<? super E> condition)
+			throws IOException {
+		try (elements) {
+			return stream(elements)
+					.filter(condition)
+					.findAny();
+		}
+	}
+
+	private static <E> Stream<E> stream(Iterator<E> elements) {
+		return StreamSupport.stream(((Iterable<E>) () -> elements).spliterator(), false);
+	}
+
+	static class Empty<E> implements Closeable, Iterator<E> {
+
+		@Override
+		public void close() throws IOException {
+
+		}
+
+		@Override
+		public boolean hasNext() {
+			return false;
+		}
+
+		@Override
+		public E next() {
+			return null;
+		}
+	}
+
+}