Optionally typechecked StateMachines

Posted by & filed under Java.

Many things can be modelled as finite state machines. Particularly things where you’d naturally use “state” in the name e.g. the current state of an order, or delivery status. We often model these as enums. enum OrderStatus { Pending, CheckingOut, Purchased, Shipped, Cancelled, Delivered, Failed, Refunded } Enums are great for restricting our order status… Read more »

HTML in Java

Posted by & filed under Java.

Another use of lambda parameter reflection could be to write html inline in Java. It allows us to create builders like this, in Java, where we’d previously have to use a language like Kotlin and a library like Kara. String doc = html( head( meta(charset -> "utf-8"), link(rel->stylesheet, type->css, href->"/my.css"), script(type->javascript, src -> "/some.js") ),… Read more »

Lambda parameter names with reflection

Posted by & filed under Java.

Java 8 introduced a compiler flag -parameters, which makes method parameter names available at runtime with reflection. Up to now, this has not worked with lambda parameter names. However, Java 8u60 now has a fix for this bug back-ported which makes it possible. Some uses that spring to mind (and work as of recent 8u60ea… Read more »

Anonymous Types in Java

Posted by & filed under Java.

Java allows casting to an intersection of types, e.g. (Number & Comparable)5. When combined with default methods on interfaces, it provides a way to combine behaviour from multiple types into a single type, without a named class or interface to combine them. Let’s say we have two interfaces that provide behaviour for Quack and Waddle…. Read more »

Lambda Type References

Posted by & filed under Java.

We’re used to type erasure ruining our day in Java. Want to create a new T() in a generic method? Normally not possible. We can pass a Class<T> around, but that doesn’t work for generic types. We can’t write List<String>.class One exception to this was using super type tokens. We can get the generic type… Read more »

Yield Return in Java

Posted by & filed under c#, Java.

A feature often missed in Java by c# developers is yield return It can be used to create Iterators/Generators easily. For example, we can print the infinite series of positive numbers like so: public static void Main() { foreach (int i in positiveIntegers()) { Console.WriteLine(i); } }   public static IEnumerable<int> positiveIntegers() { int i… Read more »

Adding toList() to Java Streams

Posted by & filed under Java.

The Java Streams API is lovely, but there are a few operations that One repeats over and over again which could be easier. One example of this is Collecting to a List. List<String> input = asList("foo", "bar"); List<String> filtered = input .stream() .filter(s -> s.startsWith("f")) .collect(Collectors.toList()); There are a few other annoyances too. For example… Read more »

Work around Java “same erasure” errors with Lambdas

Posted by & filed under Java.

A common frustration with Java is the inability to overload methods when the method signatures differ only by type parameters. Here’s an example, we’d like to overload a method to take either a List of Strings or a List of Integers. This will not compile, because both methods have the same erasure. class ErasureExample {… Read more »

Implicit Conversions with Identity Functions

Posted by & filed under Java.

What use is a method that just returns its input? Surprisingly useful. A surprising use is as a way to convert between types. There’s a well known trick that’s often used to work around Java’s terrible array literals that you may have come across. If you have a method that takes an array as an… Read more »

Builder Pattern with Java 8 Lambdas

Posted by & filed under Java.

The builder patten is often used to construct objects with many properties. It makes it easier to read initialisations by having parameters named at the callsite, while helping you only allow the construction of valid objects. Builder implementations tend to either rely on the constructed object being mutable, and setting fields as you go, or… Read more »