TIL #7: Java Lambda Puzzler

Today, a colleague asked me how they can pass a java.util.Stream to a function that accept an java.lang.Iterable.

Let’s suppose we have a following function that accepts an Iterable.

public static void doSth(Iterable<String> iterable){
        iterable.forEach(System.out::println);
 }

The calling function has a Stream that it want to pass to the doSth function.

public static void main(String[] args) throws IOException {
        Stream<String> lines = Files.lines(Paths.get("src", "main", "resources", "names.txt"));
}

One way we could easily achieve this is by collecting the Stream into a Collection like List. As List is an Iterable so we can pass it. This is should below

Stream<String> lines = Files.lines(Paths.get("src", "main", "resources", "names.txt"));
doSth(lines.collect(Collectors.toList()));

This works but what if Stream is big and collecting into an in-memory data structure like List leads to java.lang.OutOfMemoryError: Java heap space.

I googled around and found a nice Lambda hack.

Stream<String> lines = Files.lines(Paths.get("src", "main", "resources", "names.txt"));
doSth(lines::iterator);

I have worked a lot on Java 8 but first time I looked at it I couldn’t figure out how it works. If you know Java 8, give it a shot.

The magic here is that Iterable interface has a single abstract method iterator.

public interface Iterable<T> {
    Iterator<T> iterator();
}

This means we can write it as following Lambda function.

Iterable<T> iterable = () -> iterator();

In our case, Stream has an iterator method. So, we can convert Stream to Iterable by defining a lambda function as shown below.

Stream<String> lines = Files.lines(Paths.get("src", "main", "resources", "names.txt"));
doSth(() -> lines.iterator());

You can refactor the Lambda to a method reference.

Stream<String> lines = Files.lines(Paths.get("src", "main", "resources", "names.txt"));
doSth(lines::iterator);

Can we use () -> iterator() in the for-each loop

I wondered if we can use the lambda expression in the for-each loop

for (String str : () -> lines.iterator()) {
    System.out.println(str);
}

This looks like a valid use case. As for-each loop works with types that implement Iterable interface. But, it turns out the code does not compile. It gives Lambda expression not implemented here error.

The answer for this is mentioned in the JSR335

Deciding what contexts are allowed to support poly expressions 
is driven in large part by the practical 
need for such features:

The expression in an enhanced for loop is not in a 
poly context because, as the construct is currently defined, 
it is as if the expression were a receiver: 
exp.iterator() (or, in the array case, exp[i]). 
It is plausible that an Iterator could be wrapped 
as an Iterable in a for loop via a 
lambda expression (for (String s : () -> stringIterator)), 
but this doesn't mesh very well with the semantics of Iterable.

Another Interesting thing to note is that Iterable is not marked as @FunctionalInterface. I don’t know the exact reason why Iterable is not marked as @FunctionalInterface. My guess is that because Iterable has special semantics in Java so they didn’t explicitly marked it @FunctionalInterface.

Programmatically Generating Database Schema with Hibernate 5

Today, I was working with a code base that was using Hibernate‘s schema generation facilities to create the database schema for a customer on the fly. The application creates separate database schema for each customer. Then, depending on the client specific identifier it connects to the correct database. This blog will not talk about how you should design your application with multi tenant database. I will cover that in a future post. In this post, I will share the code snippet that helped me generate database schema with Hibernate 5. In this post, I am using MySQL database as an example. The same code snippet should also work with other RDBMS as well.
Read More »

Realtime People Counter with Google’s Cloud Vision API and RxJava

Welcome to the ninth blog of 52 Technologies in 2016 blog series. Recently, Google released Cloud Vision API that enables developers to incorporate image recognition in their applications. Image Recognition allow developers to build applications that can understand content of images. Google’s Cloud Vision API is very powerful and support following features:

  1. Image categorization: The API can help classify images into categories. You can build powerful applications like Google Photos that do automatic categorization.
  2. Inappropriate content detection: The API can detect inappropriate content in an image like nudity, violence, etc. It uses Google Safe search capabilities underneath.
  3. Emotion detection: This allows you to detect happy, sad or moderate emotions in an image.
  4. Retrieve text from the image: This allows you to extract text in multiple languages from the images.
  5. Logo detection: It can help you identify product logos within an image.

There are many possible applications that you can build using this powerful API. In this tutorial, we will learn how to build a realtime people counter. The application will subscribe to a twitter stream for a topic and would return number of people found in each image. We can then use this data to get advanced statistic like number of people in a time frame using RxJava buffer capabilities.

You can read full blog https://github.com/shekhargulati/52-technologies-in-2016/blob/master/09-cloudvision/README.md

Day 5 — Default and static interface methods in Java 8

We all understand that we should code to interfaces. Interfaces give client a contract which they should use without relying on the implementation details(i.e. classes). Hence, promoting loose coupling. Designing clean interfaces is one of the most important aspect of API design. One of the SOLID principle Interface segregation talks about designing smaller client-specific interfaces instead of designing one general purpose interface. Interface design is the key to clean and effective API’s for your libraries and applications.Read More »

Day 4 — Let’s write Null free Java code

Every Java developer whether beginner, novice, or seasoned has in his/her lifetime experienced NullPointerException. This is a true fact that no Java developer can deny. We all have wasted or spent many hours trying to fix bugs caused by NullPointerException. According to NullPointerException JavaDoc, NullPointerException is thrown when an application attempts to use null in a case where an object is required.. This means if we invoke a method or try to access a property on null reference then our code will explode and NullPointerException is thrown. You can follow the 7 Days with Java 8 series at https://shekhargulati.com/7-days-with-java-8/Read More »

Day 3: Let’s collect data using Stream API

On day 2, you learned that Stream API can help you work with collections in a declarative manner. We looked at the collect method, which is a terminal operation that collects the result set of a stream pipeline in a List. The collect method is a reduction operation that reduces a stream to a collection. The collect method takes a Collector which let us implement functionalities like group by, partitioning, very easily.Read More »

Day 2 — Let’s learn about Streams

On day 1, we learnt how lambdas can help us write clean concise code by allowing us to pass behavior without the need to create a class. Lambdas is a very simple language construct that helps developer express their intent on the fly by using functional interfaces. The real power of lambdas can be experienced when an API is designed keeping lambdas in mind i.e. a fluent API that makes use of Functional interfaces(we discussed them on day 1).Read More »