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.

 

 

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.

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.

When in doubt, refactor at the bottom

When in doubt, refactor at the bottom by Eric NormandEric Normand (The Practical Dev)

But every ten-line bit of repeated code has nine two-line bits and eight three-line bits. There's probably something there to extract. Start there, with smaller abstractions. Start refactoring at the bottom!

I like Eric Normand’s idea: when in doubt, refactor few lines of code rather than more lines. Extract 2 or 3 lines and give them a name (method or function). I am aware he usually uses Clojure where you often see short functions. But it applies to other programming languages as well.

How to Benchmark Alternative SQL Queries to Find the Fastest Query

How to Benchmark Alternative SQL Queries to Find the Fastest Query (Java, SQL and jOOQ.)

Tuning SQL isn’t always easy, and it takes a lot of practice to recognise how any given query can be optimised. One of the most important slides of my SQL training is the one summarising "how to be fast"

Lukas Eder from JOOQL posts some code to benchmark SQL statement. Comes in flavours of PostgreSQL, Oracle and SQL-Server. Handy.