Category Archives: Coding

Developers Beginner Guide to Public Static Foo

We recently asked out students to create a simple method in Java return if a given number is a prime number or not.

The task included a list of requirements the students had to fulfill. However, after correcting I realized that for many beginners some concepts are not fully clear – even if those concepts have been taught before. These are things an experienced developer sometimes often forgets because one has used these concepts over the years and became second nature of the developers.

Let’s have a look at what we asked for:

  1. Implement a method which returns if a given number is prime or not.
  2. The method has to be accessible from callers outside of the current class.
  3. The method has to be called without the corresponding class to be instantiated.
  4. Name the given number ‘n’.

These points mainly addressed the understanding of basic language concepts of the programming language taught so far.

There are different approaches, how to test number for being prime. In the given example we use the simple 6k +/- 1 optimization method.

public static boolean isPrime(int n) {
if (n <= 3)
return (n > 1);

if (n % 2 == 0 || n % 3 == 0)
return false;

int i = 5;
while (i * i <= n) {
if (n % i == 0 || n % (i+2) == 0)
return false;
i = i + 6;
}
return true;
}

While we have been very indulgent with the algorithms “invented” we have been very strict about the language concepts and syntax used in the solutions. number four of the previous task list should be, however, clear. The thing is called ‘n’.

I reflected this quite a while, trying to come up with explanations to the various issues I have seen in the solutions.

Methods as Return Values

Return values. We asked for a method that returns if a given number is prime or not. First of all, you need to define the type of the return value. In the given example it was boolean. Simply because you want to return a yes or no or – as we say in programming jargontrue or false.

public static boolean isPrime(int n) {
...
return false;
...
return true;
}

Also you make sure that all execution paths in your method provide a return statement. The value returned is either true or false.

For someone new to programming, the concept of methods, functions and so on is not always intuitive. In this case I usually try to come up with basic math. I show a mathematical function like f(x) = x + 1. As long as you know something about math it should be clear what this function does. You provide a value for x and you will receive a value increased by one. Then I write this down as code.

int f(int x) {
return x + 1;
}

You have written your very first method. It eventually does exactly what the mathematical function did before. Well done. Usually I celebrate this with my students.

It doesn’t matter if we are talking about object oriented or functional programming languages. The idea behind methods and functions is always the same. The syntax differs from language to language but the concept stays the same.

In addition I extend this example to the mathematical formula f(g(x)) = x + 1 with g(x) = x *2. Again I show how both methods would look in code.

int g(int x) {
return x * 2;
}

int f(int x) {
return x +1;
}

Now I write down a few lines to show how this would work.

int x = 5; 
int y = f(g(x));

While most compilers would refuse to compile this without some additional syntactical sugar, to the students the concepts should be quite clear by now.

Accessibility – Private Property vs. Public Services

If you are not used to programming yet, accessibility can give you a major headache. Who can access what? Over time you will understand concepts like information hiding, encapsulation or abstraction. All of these concepts will make use of accessibility. Although there are slightly different meanings and keywords in a variety of programming language, the idea is always the same. While there is a tutorial available for Java, this is one thing very hard to understand for those learning programming.

I usually try to find some real-world examples: public phone, a phone within the flat with is protected (to be used by family members only) by walls and doors and my private iPhone.

But also public services to be used by everybody, protected classes to be used only for those members in the course and private property not to be accessed by anybody else than me.

I realized that these concepts are slightly more difficult to non-native English speakers until I talked about the examples above which might include translation to some point.

Static – Just use it

I even don’t know where to start how to explain this to somebody not familiar with object-orientation and higher programming languages. I came from Assembler, C, Pascal, BASIC, LISP to Perl, C++ and finally to Java and .NET (VB, C# some F#), some Ruby, Python and various other languages I just don’t have in mind.

If found

The static keyword denotes that a member variable, or method, can be accessed without requiring an instantiation of the class to which it belongs.

and

In C# terms, “static” means “relating to the type itself, rather than an instance of the type”. You access a static member using the type name instead of a reference or a value, e.g. Guid.NewGuid().

and hell ya, this explains everything to a novice. So I explain it a very simple but inaccurate way: When using the static keyword on a method, you can just run the code. If using it on a variable, you can just use the variable. To some point you can think of static as “just use it”.

If you are new to programming and come across this article, I hope this helps .

tl;dr

There’s no short version. Just trying explaining public static foo to someone without a clue about programming.

Fighting Qt Creator develpoing for SailfishOS

Prologue

For some days, I fight quite an epic battle with Qt Creator. A battle, I am losing right now. I recently started developing a SailfishOS app though. A spare time project which actually drives me mad. To be fair, one should say SailfishOS is quite at an early stage compared to other platforms. However, the SDK, the tool chain, including Qt Creator, VirtualBox integration, build and deploy process already work as a charm. It works out of the box. Even for someone like me, pampered by Visual Studio, Eclipse and corresponding platforms existing for a decade and more.

There are some shortcomings in documentation, though. As a developer myself, I know how hard it is to get some good documentation in place. Therefore, kudos for everything available already. Although, I spend 95% percent in digging the darkest places of the internet to find some information. I have to move out of my comfort zone, which is good. I learn a lot by digging around, which is even better. However, I do not get my project out of the starting blocks, which is bad.

Teh Facts

Following the tutorials available, I tried to create a SailfishOS dialog with two text fields as below.

import QtQuick 2.0
import Sailfish.Silica 1.0

Dialog {
    id: addDeviceDialog
    property string deviceName
    property string deviceIdentifier 

    anchors.fill: parent

    DialogHeader {
        title: qsTr("Add a Device")
    }

    TextField {
        id: deviceNameField
        placeholderText: qsTr("Device Name")
    }

    TextField {
        id: deviceIdentifierField
        placeholderText: qsTr("Device ID")
    }

    onDone: {
        if (result == DialogResult.Accepted) {
            deviceName = deviceNameField.text
            deviceIdentifier = deviceIdentifierField.text
        }
    }
}

Qt Creator, however, responds with

Could not resolve the prototype ‘TextBaseItem’ of ‘TextBase’. (M301)

I encounter this issue wit both, TextField and TextArea.

M301

I can remove this issue by adding

import QtQuick.Controls 1.0

But then Qt Creator responds with

Qml module not found

Modules seem to be installed and paths are set. I probably miss something over here or I do deal with the IDE in a terrible wrong way.

QtQuick.Controls 1.0I will dig further and contact the SailfishOS mailing list, looking forward to get some help there to update this article soon.

[Update, 13.07.2014]

There was an almost instant reply on my request on the mailing list, though. You can completely ignore this issue. Gt Creator will build and deploy without any issues. In my very case, I had an additional issue, where I forgot to set the width property of the text field. Once I did this, I was able to see the text field on my deployed app.

Stacking Using

A few days ago, Benjamin pointed me to a feature in C#, I was not aware of, yet. You can stack using statements. In the following I will show an example where this might come in very handy.

I recently worked on a project where we had to process large text files (with large I mean 14GB each). In a pre-processing step we cleaned up the files, processed each line, validated it against a set of rules and then either wrote it back to a output file or a temporary which required further processing steps. Due to the complexity of the validation we decided to go with a small C# program that does the job.

A fast and convenient way was utilizing a set of StreamReaders and StreamWriters and applying using statements a to read and write the files. As Anoop pointed out, not using usings is one of a common mistakes .NET developers should avoid. Eventually, the code looked similar to the following example by cascading the using statements.

using (StreamReader reader = new StreamReader(@"c:input.txt"))
{
    using (StreamWriter successWriter = new StreamWriter(@"c:success.txt"))
    {
        using (StreamWriter failWriter = new StreamWriter(@"c:fail.txt"))
        {
            // all the magic happens here
        }
    }
}

That would be the way many developers would write the code, and if you have a look at the MSDN documentation about using statements  this seems to be the way to do it. But the C# 4.0 Language Specification does give you an hint that there is more you could do.

For a using statement stmt of the form:

using ( resource-acquisition ) embedded-statement

The definite assignment state of v at the beginning of resource-acquisition is the same as the state of v at the beginning of stmt.

The definite assignment state of v on the control flow transfer to embedded-statement is the same as the state of v at the end of resource-acquisition.

What’s not obvious here, is the fact that you can stack using statements utilizing one code block. In this case you embedded-statement is another using statement. In fact that’s not different how most of us use cascading for and foreach loops. In fact, this is not a new feature of C# but something you might not have considered before.

using (StreamReader reader = new StreamReader(@"c:input.txt"))
using (StreamWriter successWriter = new StreamWriter(@"c:success.txt"))
using (StreamWriter failWriter = new StreamWriter(@"c:fail.txt"))
{
    // all the magic happens here
}

Writing the code like this reduces a lot of noise and indentation in your code, keeps the resource acquisition tight and might be definitely worth keeping in mind.