Gadgetory

hello there my name is Gary Sims from Android authority if you've ever wanted to write an Android app you're probably going to have to do some programming now Java is the official language of the Android operating system not only is it used to create apps it's also used to write actually parts of the OS itself now when you want to write an app there are two aspects the first part is the java language and the second part is the Android software development kit and today we're going to be looking at the first part Java to start writing Java programs you need a way to compile source code and turn it into an executable for the Java Runtime the normal way to do this is to install the Java development kit which you can download from Oracle's website however if you aren't ready to install the Java development kit then you want a quick route to trying your first Java program then I recommend coding comm coding gives you access to a free virtual machine with all the compilers and tools you need to run a Java program as well as developing other languages like Go Python node and C to sign up just visit coding com type in your email address and a password of your choice and then click sign up the coding virtual machine comes with an IDE and command line access the web view of the virtual machine is divided into four parts on the left is the coding control panel with access to your account details the virtual machines you have created and so on next is the file manager which shows all the files and folders you have on your virtual machine next to that the screen is split into two the top half is an editor for writing your code and the bottom half gives you access to the command line now all the code for today's video is available on the android authority website in the written companion which will be linked in the video description below so our first program is the hello world program cut and paste the hello world code into the editor and save it as helloworld.java now in Java the file name of the source code and the class name must be the same the first line of the code declares a class called hello world so the source code must be called HelloWorld dot Java encoding to save the file hover the mouse over the tab for the code probably called untitled text and click on the little arrow then click save as enter HelloWorld dot Java as a file name and click Save at the bottom of the screen is the command line the virtual machine is running Linux and the terminal is running bash to compile the program type in Java C space HelloWorld dot Java the compiles will only take a second or two you can then run the program by typing Java HelloWorld the reward is the text hello world being displayed in the terminal congratulations so let's just take a moment to look at what happened first the source code the file does three things it declares a class called hello world it defines a method a function in the hello world class called main and that main method calls system dot out dot print line to output some text each Java program must define a method called main in at least one of its classes it is the entry point where the program starts executing once the program is written you then have to compile it and compile the program you first call Java C and then to run it you call Java the first is the compiler and hence the letter C at the end and the second is the Java Virtual Machine when writing computer programs you will often need to store data for temporary use for example in an android game you might want to store the score for the current player these bits of data are stored in variables a variable is a box in which you can put some data and then come back to it later to retrieve it since data comes in different forms a variable needs to define a type which tell Java what you want to store in it some of Java's primitive data types include inter for integer double for double precision floating point numbers and boolean x' for true or false let's look at the variable test program it's a simple program that sets the value of a variable prints out the variable to the console changes the variable and then print it out again cut and paste the code into the editor and then save it as a variable test Java to compile it type Java C variable test dot Java and to run it use Java variable test as you can see the program defines a variable called I and then gives it an initial value of 1 the value of I is printed to the console then I is set to a new value of I plus 24 now since I is already one that really means one plus 24 which is 25 and so the new value is then printed out onto the console try modifying the program yourself to use a double rather than int set I to something like 1.3 and increase its value by a decimal number like 24.9 and see what happens if you take a look at the print line method you will see that an integer is being added to a string the string being the value of I is : and then of course it does plus I what actually is happening here is that Java knows that the first part of the expression is a string so it generates a string value for I in other words it converts the 1 or the 25 into the characters 1 or 25 and then concatenate them to the string giving you the final string output the value of I is 1 or the value of I is 25 strings are an important part of any programming language including Java unlike int or boolean a string isn't a primitive type it's actually a class when you create a string variable you are creating a string object and notice it uses the capital S run the small s as an object it has certain properties like a value the string itself and its length strings can be manipulated in lots of different ways including being dissected concatenated compared and searched so let's take a look at the playing with strings source code cut and paste it into the web editor save it as playing with strings Java and compile it and run it using Java C and Java as we have done previously the first part of the program creates a string called hello and gives it a value of hello world although this might look similar to how you declare an assign an integer or a different primitive type actually there's a lot more going on here Java allows simple operators like equals and plus to be assigned to objects so really although it might look like you're just assigning the value hello world to the string what's actually happening is a new object of type string is being created and the value hello world is being passed to its constructor but we'll talk more about that in the object section the next part shows how you can concatenate strings in this case an exclamation point is added to the end of the string now since string is an object it can have methods so string dot substring is a method which returns part of the string in this case the first five characters string dot trim is another method that removes leading and trailing spaces the last part of the program demonstrates string dot to uppercase and string dot to lowercase methods computers are good at doing repetitive tasks as do repetitive tasks in a programming language you use a construct called a loop now Java has three types of loop the while loop the for loop and the do-while loop and they're all basically the same in that they keep on looping around until a certain condition is met so let's take a look at a loops example this particular one will show you how to print out the numbers 1 to 10 11 to 20 and 21 to 30 using three different types of loops as before copy of the source code from the written companion and put it into a file called loop store Java you'll then be able to compile it and run it as you have done the previous programs the for loop has three parts the first part is the initializer that's the int I is equal to one and it's only executed once what it does is declare an integer called I and sets its value to one then comes a test expression I is less than or equal to ten this expression will be tested every time the loop goes round if the result of the test is true it will go round again and in our example is used to see whether I is still less than or equal to ten after each iteration the third section the iterator will be executed in our example it increases the value of I by one note the I is equal to I plus one is actually the same as I plus plus the while loop is very similar to the for loop except it doesn't contain the initialization phase nor the iterator phase that means the initialization needs to be done separately in our example is the int J is equal to 11 which appears before the loop starts the iterator also needs to be coded separately in our example it is the j plus plus line which can be found inside the loop just after the call to print line a do-while loop is very similar to a while loop with one big difference the test to see if a loop should continue is at the end of the loop and not at the start this means that a do-while is guaranteed to execute at least once but a while loop doesn't even need to execute at all if the conditions aren't met the entrance into the loop like the while loop the initialization needs to happen outside the loop and in our example this is the int X is equal to twenty one line and the iterator needs to occur inside the loop X plus plus when X goes over 30 the loop will stop as I mentioned before Java is an object-oriented programming language and to really succeed in Android programming you're going to need to understand some important oo concepts the first one is what is an object an object is basically a set of methods or functions along with a set of data the data and the methods belong to the object and work for the object let's have a look at an example here is a program that create a very simple counter object the counter orbit has one piece of data the integer variable count and three methods besides the main method counter increment and get count leaving the first method out for the moment you can see that increment and get counter very simple the first one adds to the internal variable count and the second one just returns its value until now all the methods we have declared started with public void but if you notice the get count methods start with public int but the difference between void and int is this void declares that the method doesn't return anything there will be no result coming back out of the method but in tells us that the method will return a number specifically an integer you can actually create methods that return all kinds of data including objects notice that the first method has the same name as the class itself ie counter and it doesn't have a return type not even void this is a special method called constructor the constructor is called only once at the moment that the object is created it is used to initialize the object with default values and perform other necessary is initialization tasks in this example or it does is set count to zero now there's one last concept we need to look at before we come to a close and that's the idea of inheritance now in Java you can create an abstract class that represents an abstract idea and derived classes from it which are called subclasses in the written companion I show you how to create a class called animal it could be any animal and I derive from it a class called elk now when you do that what happens is some of the methods and some of the data that are in the superclass animal automatically get inherited now belong to the subclass elk go check out the source code in the written companion have a read about the explanations I give you there and see what you think well my name's Gary Sims from Android Authority I hope you've enjoyed this video if you did please do give it a thumbs up I really do recommend that you try out some of these examples I've given try out what's written in the written example go to coding com get yourself an account install the Java compiler if you need to on your PC or on your Mac just try it out can't do any harm it's good fun if you've got any questions why not leave them in the comments below we'll try our best to help you out if you've got any experience is about learning job you want to share with us leave them in the comments below if you've got maybe some books that you recommend also leave them in the comments below and as for me I'll see you in my next video