├── LICENSE
├── pom.xml
├── README.md
└── src
    ├── test
        └── java
        │   └── io
        │       └── herrmann
        │           └── generator
        │               └── GeneratorTest.java
    └── main
        └── java
            └── io
                └── herrmann
                    └── generator
                        └── Generator.java


/LICENSE:
--------------------------------------------------------------------------------
 1 | The MIT License (MIT)
 2 | 
 3 | Copyright (c) 2014 Michael Herrmann.
 4 | 
 5 | Permission is hereby granted, free of charge, to any person obtaining a copy
 6 | of this software and associated documentation files (the "Software"), to deal
 7 | in the Software without restriction, including without limitation the rights
 8 | to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 9 | copies of the Software, and to permit persons to whom the Software is
10 | furnished to do so, subject to the following conditions:
11 | 
12 | The above copyright notice and this permission notice shall be included in all
13 | copies or substantial portions of the Software.
14 | 
15 | THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
16 | IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
17 | FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
18 | AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
19 | LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
20 | OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
21 | SOFTWARE.
22 | 
23 | 


--------------------------------------------------------------------------------
/pom.xml:
--------------------------------------------------------------------------------
 1 | <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 2 |   xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 3 |   <modelVersion>4.0.0</modelVersion>
 4 | 
 5 |   <groupId>io.herrmann</groupId>
 6 |   <artifactId>generator</artifactId>
 7 |   <version>1.0-SNAPSHOT</version>
 8 |   <packaging>jar</packaging>
 9 | 
10 |   <name>java-generator-functions</name>
11 |   <description>An implementation of a Python-like yield(...) method in Java.</description>
12 |   <url>https://github.com/mherrmann/java-generator-functions</url>
13 | 
14 |   <licenses>
15 |     <license>
16 |       <name>MIT License</name>
17 |       <url>http://www.opensource.org/licenses/mit-license.php</url>
18 |       <distribution>repo</distribution>
19 |     </license>
20 |   </licenses>
21 | 
22 |   <developers>
23 |     <developer>
24 |       <name>Michael Herrmann</name>
25 |       <email>michael@herrmann.io</email>
26 |     </developer>
27 |   </developers>
28 | 
29 |   <scm>
30 |     <connection>scm:git:git@github.com:mherrmann/java-generator-functions.git</connection>
31 |     <url>scm:git:git@github.com:mherrmann/java-generator-functions.git</url>
32 |     <developerConnection>scm:git:git@github.com:mherrmann/java-generator-functions.git</developerConnection>
33 |   </scm>
34 | 
35 |   <properties>
36 |     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
37 |   </properties>
38 | 
39 |   <dependencies>
40 |     <dependency>
41 |       <groupId>junit</groupId>
42 |       <artifactId>junit</artifactId>
43 |       <version>4.11</version>
44 |       <scope>test</scope>
45 |     </dependency>
46 |   </dependencies>
47 | </project>


--------------------------------------------------------------------------------
/README.md:
--------------------------------------------------------------------------------
 1 | java-generator-functions
 2 | ========================
 3 | 
 4 | An implementation of Python-like generator functions in Java. This repository contains a single class, `Generator` with a method `yield(...)` which can be used to mimic the behaviour of the `yield` keyword in Python.
 5 | 
 6 | Examples
 7 | --------
 8 | The following is a simple generator that yields `1` and then `2`:
 9 | 
10 | ```java
11 | Generator<Integer> simpleGenerator = new Generator<Integer>() {
12 |     public void run() throws InterruptedException {
13 |         yield(1);
14 |         // Some logic here...
15 |         yield(2);
16 |     }
17 | };
18 | for (Integer element : simpleGenerator)
19 |     System.out.println(element);
20 | // Prints "1", then "2".
21 | ```
22 | 
23 | Infinite generators are also possible:
24 | 
25 | ```java
26 | Generator<Integer> infiniteGenerator = new Generator<Integer>() {
27 |     public void run() throws InterruptedException {
28 |         while (true)
29 |             yield(1);
30 |     }
31 | };
32 | ```
33 | 
34 | The `Generator` class lies in package `io.herrmann.generator`. So you need to `import io.herrmann.generator.Generator;` in order for the above examples to work.
35 | 
36 | Usage
37 | -----
38 | 
39 | This package is hosted as a Maven repository with the following url:
40 | 
41 |     http://dl.bintray.com/filipmalczak/maven
42 | 
43 | To use it from Maven, add the following to your `pom.xml`:
44 | 
45 | ```xml
46 | <project>
47 |     ...
48 |     <repositories>
49 |         ...
50 |         <repository>
51 |             <id>java-generator-functions</id>
52 |             <url>http://dl.bintray.com/filipmalczak/maven</url>
53 |         </repository>
54 |     </repositories>
55 |     ...
56 |     <dependencies>
57 |         <dependency>
58 |             <groupId>io.herrmann</groupId>
59 |             <artifactId>java-generator-functions</artifactId>
60 |             <version>1.0</version>
61 |         </dependency>
62 |     </dependencies>
63 | </project>
64 | ```
65 | 
66 | For Gradle:
67 | 
68 | ```gradle
69 | compile(group: 'io.herrmann', name: 'java-generator-functions', version: '1.0')
70 | ```
71 | 
72 | Caveats and Performance
73 | -----------------------
74 | The `Generator` class internally works with a Thread to produce the items. It does ensure that no Threads stay around if the corresponding Generator is no longer used. However:
75 | 
76 | **If too many `Generator`s are created before the JVM gets a chance to garbage collect the old ones, you may encounter `OutOfMemoryError`s. This problem most strongly presents itself on OS X where the maximum number of Threads is significantly lower than on other OSs (around 2000).**
77 | 
78 | The performance is obviously not great but not too shabby either. On my machine with a dual core i5 CPU @ 2.67 GHz, 1000 items can be produced in < 0.03s.
79 | 
80 | Contributing
81 | ------------
82 | Contributions and pull requests are welcome. Please ensure that `mvn test` still passes and add any unit tests as you see fit. Please also follow the same coding conventions, in particular the line limit of 80 characters and the use of tabs instead of spaces.
83 | 


--------------------------------------------------------------------------------
/src/test/java/io/herrmann/generator/GeneratorTest.java:
--------------------------------------------------------------------------------
  1 | package io.herrmann.generator;
  2 | 
  3 | import org.junit.Test;
  4 | 
  5 | import java.util.ArrayList;
  6 | import java.util.Arrays;
  7 | import java.util.Iterator;
  8 | import java.util.List;
  9 | 
 10 | import static org.junit.Assert.assertEquals;
 11 | import static org.junit.Assert.assertTrue;
 12 | 
 13 | public class GeneratorTest {
 14 | 	@Test
 15 | 	public void testEmptyGenerator() {
 16 | 		assertEquals(new ArrayList<Object>(), list(new EmptyGenerator()));
 17 | 	}
 18 | 	private class EmptyGenerator extends Generator {
 19 | 		@Override
 20 | 		protected void run() {
 21 | 		}
 22 | 	}
 23 | 	public static <T> List<T> list(Iterable<T> iterable) {
 24 | 		List<T> result = new ArrayList<T>();
 25 | 		for (T item : iterable)
 26 | 			result.add(item);
 27 | 		return result;
 28 | 	}
 29 | 	@Test
 30 | 	public void testOneEltGenerator() {
 31 | 		List<Integer> oneEltList = Arrays.asList(1);
 32 | 		assertEquals(oneEltList, list(new ListGenerator<Integer>(oneEltList)));
 33 | 	}
 34 | 	private class ListGenerator<T> extends Generator<T> {
 35 | 		private final List<T> elements;
 36 | 		public ListGenerator(List<T> elements) {
 37 | 			this.elements = elements;
 38 | 		}
 39 | 		protected void run() throws InterruptedException {
 40 | 			for (T element : elements)
 41 | 				yield(element);
 42 | 		}
 43 | 	}
 44 | 	@Test
 45 | 	public void testTwoEltGenerator() {
 46 | 		List<Integer> twoEltList = Arrays.asList(1, 2);
 47 | 		assertEquals(twoEltList, list(new ListGenerator<Integer>(twoEltList)));
 48 | 	}
 49 | 	@Test
 50 | 	public void testInfiniteGenerator() {
 51 | 		InfiniteGenerator generator = new InfiniteGenerator();
 52 | 		testInfiniteGenerator(generator);
 53 | 	}
 54 | 	public void testInfiniteGenerator(InfiniteGenerator generator) {
 55 | 		int NUM_ELTS_TO_INSPECT = 1000;
 56 | 		Iterator<Integer> generatorIterator = generator.iterator();
 57 | 		for (int i=0; i < NUM_ELTS_TO_INSPECT; i++) {
 58 | 			assertTrue(generatorIterator.hasNext());
 59 | 			assertEquals(1, (int) generatorIterator.next());
 60 | 		}
 61 | 	}
 62 | 	private class InfiniteGenerator extends Generator<Integer> {
 63 | 		@Override
 64 | 		protected void run() throws InterruptedException {
 65 | 			while (true)
 66 | 				yield(1);
 67 | 		}
 68 | 	}
 69 | 	@Test
 70 | 	public void testInfiniteGeneratorLeavesNoRunningThreads() throws Throwable {
 71 | 		InfiniteGenerator generator = new InfiniteGenerator();
 72 | 		testInfiniteGenerator(generator);
 73 | 		generator.finalize();
 74 | 		assertEquals(Thread.State.TERMINATED, generator.producer.getState());
 75 | 	}
 76 | 
 77 | 	private class CustomRuntimeException extends RuntimeException {}
 78 | 
 79 | 	private class GeneratorRaisingException extends Generator<Integer> {
 80 | 		@Override
 81 | 		protected void run() throws InterruptedException {
 82 | 			throw new CustomRuntimeException();
 83 | 		}
 84 | 	}
 85 | 
 86 | 	@Test(expected = CustomRuntimeException.class)
 87 | 	public void testGeneratorRaisingExceptionHasNext() {
 88 | 		GeneratorRaisingException generator = new GeneratorRaisingException();
 89 | 		Iterator<Integer> iterator = generator.iterator();
 90 | 		iterator.hasNext();
 91 | 	}
 92 | 
 93 | 	@Test(expected = CustomRuntimeException.class)
 94 | 	public void testGeneratorRaisingExceptionNext() {
 95 | 		GeneratorRaisingException generator = new GeneratorRaisingException();
 96 | 		Iterator<Integer> iterator = generator.iterator();
 97 | 		iterator.next();
 98 | 	}
 99 | 
100 | }


--------------------------------------------------------------------------------
/src/main/java/io/herrmann/generator/Generator.java:
--------------------------------------------------------------------------------
  1 | package io.herrmann.generator;
  2 | 
  3 | import java.util.Iterator;
  4 | import java.util.NoSuchElementException;
  5 | 
  6 | /**
  7 |  * This class allows specifying Python generator-like sequences. For examples,
  8 |  * see the JUnit test case.
  9 |  *
 10 |  * The implementation uses a separate Thread to produce the sequence items. This
 11 |  * is certainly not as fast as eg. a for-loop, but not horribly slow either. On
 12 |  * a machine with a dual core i5 CPU @ 2.67 GHz, 1000 items can be produced in
 13 |  * &lt; 0.03s.
 14 |  *
 15 |  * By overriding finalize(), the class takes care not to leave any Threads
 16 |  * running longer than necessary.
 17 |  */
 18 | public abstract class Generator<T> implements Iterable<T> {
 19 | 
 20 | 	private class Condition {
 21 | 		private boolean isSet;
 22 | 		public synchronized void set() {
 23 | 			isSet = true;
 24 | 			notify();
 25 | 		}
 26 | 		public synchronized void await() throws InterruptedException {
 27 | 			try {
 28 | 				if (isSet)
 29 | 					return;
 30 | 				wait();
 31 | 			} finally {
 32 | 				isSet = false;
 33 | 			}
 34 | 		}
 35 | 	}
 36 | 
 37 | 	static ThreadGroup THREAD_GROUP;
 38 | 
 39 | 	Thread producer;
 40 | 	private boolean hasFinished;
 41 | 	private final Condition itemAvailableOrHasFinished = new Condition();
 42 | 	private final Condition itemRequested = new Condition();
 43 | 	private T nextItem;
 44 | 	private boolean nextItemAvailable;
 45 | 	private RuntimeException exceptionRaisedByProducer;
 46 | 
 47 | 	@Override
 48 | 	public Iterator<T> iterator() {
 49 | 		return new Iterator<T>() {
 50 | 			@Override
 51 | 			public boolean hasNext() {
 52 | 				return waitForNext();
 53 | 			}
 54 | 			@Override
 55 | 			public T next() {
 56 | 				if (!waitForNext())
 57 | 					throw new NoSuchElementException();
 58 | 				nextItemAvailable = false;
 59 | 				return nextItem;
 60 | 			}
 61 | 			@Override
 62 | 			public void remove() {
 63 | 				throw new UnsupportedOperationException();
 64 | 			}
 65 | 			private boolean waitForNext() {
 66 | 				if (nextItemAvailable)
 67 | 					return true;
 68 | 				if (hasFinished)
 69 | 					return false;
 70 | 				if (producer == null)
 71 | 					startProducer();
 72 | 				itemRequested.set();
 73 | 				try {
 74 | 					itemAvailableOrHasFinished.await();
 75 | 				} catch (InterruptedException e) {
 76 | 					hasFinished = true;
 77 | 				}
 78 | 				if (exceptionRaisedByProducer != null)
 79 | 					throw exceptionRaisedByProducer;
 80 | 				return !hasFinished;
 81 | 			}
 82 | 		};
 83 | 	}
 84 | 
 85 | 	protected abstract void run() throws InterruptedException;
 86 | 
 87 | 	protected void yield(T element) throws InterruptedException {
 88 | 		nextItem = element;
 89 | 		nextItemAvailable = true;
 90 | 		itemAvailableOrHasFinished.set();
 91 | 		itemRequested.await();
 92 | 	}
 93 | 
 94 | 	private void startProducer() {
 95 | 		assert producer == null;
 96 | 		if (THREAD_GROUP == null)
 97 | 			THREAD_GROUP = new ThreadGroup("generatorfunctions");
 98 | 		producer = new Thread(THREAD_GROUP, new Runnable() {
 99 | 			@Override
100 | 			public void run() {
101 | 				try {
102 | 					itemRequested.await();
103 | 					Generator.this.run();
104 | 				} catch (InterruptedException e) {
105 | 					// No need to do anything here; Remaining steps in run()
106 | 					// will cleanly shut down the thread.
107 | 				} catch (RuntimeException e) {
108 | 					exceptionRaisedByProducer = e;
109 | 				}
110 | 				hasFinished = true;
111 | 				itemAvailableOrHasFinished.set();
112 | 			}
113 | 		});
114 | 		producer.setDaemon(true);
115 | 		producer.start();
116 | 	}
117 | 
118 | 	@Override
119 | 	protected void finalize() throws Throwable {
120 | 		producer.interrupt();
121 | 		producer.join();
122 | 		super.finalize();
123 | 	}
124 | }
125 | 


--------------------------------------------------------------------------------