Java method references recap

In the last post I reviewed Java lambda expressions. They represent a concise syntax to implement functional interfaces.

Enter Java method references. They represent a concise syntax to implement functional interface using existing methods. Like with lambda expressions, referenced methods are not allowed to throw checked exceptions.

Syntax

It’s simply “class-or-instance name” “::” “method name”, like

Types of method references

Reference to a static method

Static methods are referenced using the class name like in the example above.

Reference to an instance method of a particular object

Methods of a particular object are referenced using the variable name of that object:

Reference to an instance method of an arbitary object of a particular type

Instead of using an already existing object you can just state the class and a non-static method. Then the instance is an additional parameter. In the following example toURI is a method with no arguments that returns a String. The function of this method reference takes a File (the object) and returns a String:

Reference to a constructor

Constructors are references using its type and “new”:

Here the constructor of StringBuffer with String parameter is referenced. Return type is the type of the constructor, parameters of the function are the parameters of the constructors.

 

 

Sprint Goal in Scrum

Myth: Having A Sprint Goal Is Optional In Scrum (Scrum.org)

Sprint Goals are one of the more elusive parts of the Scrum Framework. Most teams know they are important, but few use them – for a variety of reasons. In this post, Barry Overeem and I bust the myth that Sprint Goals are optional in Scrum. And we make an effort to show what makes them so vital in the face of complex work. And more importantly, what you can do to start working with Sprint Goals from now on.

I really like this article because I am in a Scrum team where everyone works in his/her niche area. So far we haven’t used Sprint goals. I agree with the observation in the “What happens without Sprint Goals” – section, especially:

Without Sprint Goals, each Sprint implicitly has the same purpose: complete all the work. This makes all Sprints the same, and can make people (rightfully) complain that they are artificial;

So if you’re in a Scrum team that doesn’t use sprint goals, this is definitely a must read.

Agile back to basics with Robert C. Martin

Episode 125 – Agile back to basics (The Cynical Developer Podcast)

In this episode we talk to Robert C. Martin, many of you might know him as Uncle Bob, and we’re here to talk Agile and taking it back to basics.

A great podcast episode with Robert C. Martin on “Agile – back to the basics”. So what are, in the time of Scrum, LeSS, SaFE etc., the basics of Agile?

Apparently, “Uncle Bob” has a new book in the works, “Clean Agile” (duh!), covering this subject.

On point he makes is that often engineering practices like Pair Programming, Test Driven Design and Refactoring are overlooked. Especially Scrum does not comment on these. But without these practices, technical debt is likely to accumulate (see FlaccidScrum).

Java lambda expression recap

Lambda expressions in Java represent “functions”, something that take a number of parameters and produce at most one return value.

This could be expressed with anonymous classes but lambda expressions offer a more concise syntax.

Syntax

Lambda expression consist of a parameter list, an “arrow” and a body.

The parameter list is enclosed in round brackets. Types are optional. When the expression has exactly one parameter, the brackets can be omitted.

The body can either be an expression (that returns a value) or a block. A block is a sequence of statements, enclosed in curly braces.

Lambda expressions and types

In the Java type system, lambda expressions are instances of “functional interfaces”. A functional interface is an interface with exactly one abstract method.

Functional interfaces in java.util.function

The package java.util.function in the JDK contains a number of functional interfaces:

  • Function<T,U>  represents a function with one parameter of type T and return type U
  • Consumer<T>  represents a function with one parameter of type T and return type void
  • Supplier<T>  represents a function with no parameter and return type T
  • Predicate<T>  represents a function with one parameter of type T and return type boolean

Plus, variants with “Bi” prefix exists that have two parameters, like BiPredicate . More variants exists for using primitive types like DoubleToIntFunction .

User defined function interfaces

Any interface with exactly one abstract method can be used as type of a lambda expression. You mark this interface with @FunctionInterface .

Benefits

For me, the benefits of lambda expression are

  • concise syntax for anonymous classes that represent functional code
  • improved readability
  • encouragement of a more functional programming style

How static is a static inner class in Java?

Answer: not static at all. A static inner class behaves like a normal class except that it is in the namespace of the outer class (“for packaging convenience”, as the official Java tutorial puts it).

So as an example:

As opposed to a true inner (nested) class, you do not need an instance of Outer to create an instance of Inner:

and Inner instances have no special knowledge about Outer instances. Inner class behaves just like a top-level class, it just has to be qualified as “Outer.Inner”.

Why I am writing about this?

Because I was quite shocked that two of my colleagues (both seasoned Java developers) were not sure if a static inner class was about static members and therefore global state.

Maybe they do not use static inner classes.

When do I use static inner classes?

I use a static inner class

  1. when it only of use for the outer class and it’s independent of the (private) members of the outer class,
  2. when it’s conceptionally tied to the outer class (e.g. a Builder class)
  3. for packaging convenience.

Often, the visibility of the static inner class is not public. In this case there is no big difference whether I create a static inner class or a top-level class in the same source file. An alternative for the first code example therefore is:

An example for (2) is a Builder class:

If the Inner instance needs access to (private) members of the Outer instance then Inner needs to be non-static.

Checking your project dependencies for vulnerabilites

In the light of the recent case of introducing malicious code through a popular JavaScript module on npm, I like to mention snyk.io .

In a simple, free of charge scenario, snyk.io scans build or dependencies files on your github or gitlab projects and periodically reports vulnerabilities. Snyk supports Node, Ruby, Java, Scala and Python projects.

If you pay for snyk.io, you get a lot more integrations, CLI and API access etc.

In my own trial I found that even for fairly recent spring boot and apache camel dependency tree there a dozen of high-rated vulnerabilities! (Many of them by using “com.fasterxml.jackson.core:jackson-databind@2.9.1”). So the next question is if it’s advisable to upgrade to a secure patch of – say – jackson-databind although I use it only indirectly – in other words: will the depended framework still work with the secure patch version?

An open-source alternative is OWASP-Dependency-Check. It scans Java and .Net dependencies, has experimental support for Python, Ruby, PHP (composer), and Node.js applications. The tool seems to be JVM-based. There is a SonarQube-plugin. I have not tried it myself.

Things I learnt during my latest Javascript Code Kata

Sometimes I do a code kata at codewars.com. That is a fun way to solve computer science related problems, learn on the way to solve them and especially learn from the solutions of others.

Today I completed the kata “Make a spanning tree” using Javascript. I occasionally use Javascript to write an event handler or so but I don’t have much experience in “modern” Javascript. Here is what I learnt from looking at the solutions of others.

Destructuring

I know this from my Scala class and Clojure.

You can assign array elements to variables:

so “…rest” is assign the rest of the array.

This is nice syntactic sugar also when working with nested arrays. Eg when “edges” is an array of pairs:

There is object destructuring:

and even assigning to new variable names

See MDN web docs for more.

Spread operator to create an array using an array literal

Using an array literal to create an array from two other arrays:

Objects are associative arrays (aka Maps)

Although I already knew this, kind of, this refreshes my JS
knowledge.

First, you can add properties to Objects without declaring them in
the first place:

Second, instead of the dot-notation you can use array index
notation using the property name as the index:

One solution uses this in order to save the weighted edges in an
object just like i did in the proper Map object:

Third, methods are kind of properties, too. In the same solution,
“minOrMaxFunc” is cleverly choosen (“minOrMax” argument is either
“min” or “max”):

it creates an objects with two methods: “min” and “max” and then
accesses the one that is given in the argument. If “minOrMax” is
“min”, a reference of the “min” method is returned.

Strings are arrays

Destructuring works with strings:

and you can index strings:

“var” vs. “let”

Of course, the solutions written in “modern” JS use “let” and
“const” all over the place. I just reassured myself about the
difference between let and var:

First, variables declared in a block using “var” are visible
outside that block and are “known” before being declared:

a block might be a for-loop.

Variables declared using let are not visible outside the block and
are not “known” before declared:

Third, you might not redeclare a variable using let:

So basically, “let” is a sane way to declare variables.

Becoming a better programmer

Henry Stanley wrote a blog post “Becoming a dramatically better programmer“. He outlines areas where programmers who want to get better can focus on. That is learning skills directly on the one side and learning meta-skills on the other side. Meta-skills: learn about “deliberate practice” – do things that challenge you and get feedback -, identify common mistakes you make and learn to do deep work – focus on work with no distractions.

My advice for deliberate practice are sites like CodeWars where you can solve exercises (“katas”) in a couple of programming languages. It is challenging (CodeWars has martial art like grades of mastery) and you get feedback by being able to look at solution by other users and thus being able to compare them to your solution.

GOTO Berlin 2017

The last two days I spent at the GOTO Berlin 2017 conference. It’s a conference “by developers for developers”. Three out of four keynote speakers were women (last year four out of four); I have got the impression that inclusivness is an import part of the conference. There seem to be more female attendancees than on other tech conferences.

I enjoyed the conference: the keynotes, the talks, the food and the beverages, the people.

The first keynote on Thursday was held by Anita Sengupta about “The future of Mars exploration“. In the first part, she focused on the Curiosity mission. She developed the parachute that was used during the decent on Mars. Cool. Prof. Sengupta showed us some actual video shootage from the mission. In the second part of her talk she talked about the challenges that a human mission to mars would face, especially radiation.

The evening keynote on Thursday “Number crush” was held by Hannah Fry. She showcased some interesting data from human (and cow) behaviour. It’s hard to summarize her talk in a few sentences. Make sure to check out her website.

Raffaelo D’Andrea held the morning keynote on Friday on autonomous drones. He was part of Kiva Systems, a company that build robots that brings stuff in a warehouse to human packers. It got aquired by Amazon that uses this technology in its warehouses. It’s astonishing to see all these robots moving around bringing stuff from A to B. The main theme was the autonomous drones D’Andrea developed in a company called Verity studios. They are used on broadway, in Metallica concerts. A key concern for him are safeness (no drone is going to crash) and reliability. Very impressive!!!

Susan Landau held the evening keynote on Friday on “Cybersecurity in an Insecure Age“. She talked about end-to-end encryption and locked phones. A thing that was new to me is “tainted leaks” where documents from “a target” are stolen, messed with and then “leaked” in order to discredit the target and generally generate mistrust.

Attendance Joy Clark did some cool sketch note of the key notes and some talks.

I especially enjoyed the talks by Dan North (“How to break the rules” and “Agile revisited“) Gregor Hohpe (“Adopting DevOps? You are Aiming at the Wrong Target!” and “Enterprise Architecture = Architecting the Enterprise?“), Steve Smith (“Measuring Continuous Delivery“) and Adam Tornhill (“A Crystal Ball to Prioritize Technical Debt“).