Android SDK: How to Define a Database with SQLiteOpenHelper
Defining a database with SQLiteOpenHelper
The first step in working with SQLite in Android is to define your database and table structure, and the best practice for this is to create a Java class that extends a class called SQLiteOpenHelper. I'll describe how to do this in this project named CreateDatabase. In this version of my project, I moved my XML parser classes to a new package that ends with .xml, and I've created another new package that ends with .db. This is where I'll place my java classes that manage my database.
I'll right-click on the new package and create a new Java class. This database will contain tours data. So I'll name it ToursDBOpenHelper. I'll set its superclass, I'll click Browse, and I'll type sqliteo, and I'll select the first class that appears, SQLiteOpenHelper, and click OK. I'll make sure I have the option to create method stubs for Inherited abstract methods and click Finish.
That will generate the Java class and add two methods named onCreate and onUpgrade. I'll come back to these methods in a moment. When you create the class, you'll automatically see a warning. I'll move the cursor over the warning icon on the left, and I'll see that it's telling me that I must define an explicit constructor method. I'll click on the icon and I'll choose the first quick fix to add a constructor method with four arguments. I'll come back to that constructor method in a moment, too, but for the moment, I'll just save my changes and make sure that the errors go away.
You need a lot of bits of information to create a database and its table structures. That includes the name of the database, the names of the tables, and the names of columns. It's a common practice to define all of these as constants, and it takes a good bit of typing. So to make it go a little bit more quickly, I've included a file called typinghelp.txt in this project. I'll open the file and I'll select all of it's contents and copy it to the clipboard. Then I'll come back to my class, ToursDBOpenHelper, I'll expand it to full screen, I'll place the cursor inside the class declaration and before my new constructor, and I'll paste my constants into place.
Let's do a quick review of all of these constants. First, there's a LOGTAG that I'll use to output to the LogCat console. Then there's the name of the database. You can name your database file anything you want. It's common to use a file extension of .db, but it's not required. The database version is required. It's an integer value and it always starts at 1. Each time you change the structure of the database, you should increment this value by 1. You can't decrement it.
The rest of the constants define the names of the table and the columns I'll be creating. And finally, there's a constant named Table Create that defines an SQL statement that will create my table. The name of the table will be Tours. It'll have five columns. The primary key column will be an integer and it will auto-increment and then there are three text columns and one numeric column. For more information about the available data types in SQLite, see the SQLite documentation and other information that's included in the Android API docs documentation.
Now let's go to that constructor method. When I generated it, it was given four arguments, but that's more than it really needed. The explicit constructor method will only be called by my own code so I can determine how I structure it and I'd like to simplify it. I'm going to remove the last three arguments so this constructor method only receives the context. That's how this class will be connected to the current activity, but when I called the super class' constructor method, I do need to pass those four values in.
I'll still pass in the context argument, but the next value is the database name. And I'll use this constant, the database name that I defined up here. I'll select it and copy it and then paste it into the constructor method. I don't need to pass in a factory value, so I'll set that to null, but I do need to pass in a version. Once again, I'll use one of my constants. This time I'll use DATABASE_VERSION.
Each time I want to work with the database in my code, I'll create an instance of this open helper class, and I'll call this constructor method. I'll pass in the context, but the values for the database name and the database version will come from the constants that are defined in this class. That's all the work I need to do on the constructor method. So I'll get rid of the TODO comment. Now let's go to the onCreate and onUpgrade methods. These methods will be called automatically by the Android SDK.
Each time I say to the Android SDK I want to get a connection to my database, Android will determine whether the database exists or not. If it doesn't exist yet, Android will call the onCreate method. If the database already exists, but I've indicated through the database version value that I'm changing the version, that is that I've incremented it, then the onUpgrade method will be called. I will never call these two methods directly. They'll only be called by the SDK.
In the onCreate method, you should add code that creates your database tables and if you like, you can also add code to add data. I'm just going to create the table and its structure. To do that, I'll remove the TODO comment, and I'll use the database argument that's being passed in. It's named db, and I'll call a method called execute SQL or execSQL for short, and I'll pass in my constant that contains the SQL command that will create the table, that'll be TABLE_CREATE.
So, that command is called and my table is created. Then I'll give myself a little bit of LogCat output. I'll call the Log class, and I'll make sure I've included the import. I'll use the I method. I'll pass in my LOGTAG constant and a literal string as a message, Table has been created, and that's all I need to do in the onCreate method. When the onUpgrade method is called, I'll receive arguments named oldVersion and newVersion, and I might want to write some very finely-tuned code that examines those values and upgrades the database in some complex way, but again, I'm going to keep this simple.
I'm just going to drop the existing table, the tours table, and then I'll recreate it. So I'll move the cursor into the onCreate method. I'll call db.execSQL and I'll pass in this explicit SQL Command, DROP TABLE IF EXISTS, and then I'll append to that the name of the table, TABLE_TOURS. Then once I've dropped the table, in order to recreate it, I'll simply call my onCreate method.
This is the one exception to what I just said that I wouldn't call the onCreate method directly. I'll call it within this class but not from the rest of the application. I'll call the onCreate method. I'll pass in the db argument, and now I've recreated the table structure. So, that's what a basic open helper class looks like. It typically defines the name of the database and the version and then assigns constants for all the tables and column names and also useful SQL commands to create the tables.
This open helper class only defines a single table, but in a complete application, you can define as many tables as you want. Now I have one warning which I see here. So I'll go to my Problems tab and I'll see that I have an import statement left over from my original constructor method signature. So I'll delete that and save those changes. I'm going to add some code to my MainActivity to use this code that I've just created. I'll go to my MainActivity class. Now, eventually I'm going to have a special class called a data source that deals directly with the database Open Helper and I'll show you that in the next video, but to keep things a little bit simpler, I'll call the database open helper class directly from my activity in this exercise.
So I'll go to my MainActivity, and first I'll add a field that will represent an instance of my open helper class. I'll declare it using the Superclass data type, SQLiteOpenHelper, and I'll name it dbhelper. Then I'll also create a reference to a database object. That database type will be SQLiteDatabase, and I'll name that database. Now I'll go down to my onCreate method, and I'll place the cursor right here before I call the ArrayAdapter code and I'll instantiate the dbhelper object.
I'll say dbhelper = new, and then I'll use my new class constructor method and I'll pass in this as the context, and then I'll get a reference to the database. I'll call database = dbhelper. And then I'll call a method that this class has inherited from SQLite database. The name of the method will be getWritableDatabase. This returns a reference to the connection to the database, and I'll be able to use that connection to do things like inserting data, retrieving data, updating, and deleting.
Simply by calling the method, that will trigger the onCreate method of my database open helper class, and in turn that will create the table structure. I'll save my changes, and now I'm ready to test. Before I test, I'll go to my LogCat window. I'll reset my perspective to bring back the LogCat window. Then I'll expand it to full screen. Notice that I've added a tag filter, so that I'll only be seeing messages that come in with that tag.
Now, I'll run the application in the emulator. As the application comes to the screen, it executes the code that opens my database open helper class, the onCreate method is called, and the result is that the database and the table have been created. To prove that the database is there, let's look at it in the File Explorer. I'll go to my DDMS perspective, and in the File Explorer tab, I'll go to the data folder and then from there, to the sub-folder named data.
Then I'll go to the package that represents my application, com.exploreca.tourfinder. I'll open that and I have a new databases folder that didn't exist before, and there's the database that's been created. This is where your databases will be automatically placed, in internal storage, inside the same parent folder that contains any internal files. You can see it easily when you're working with the emulator, but just as with any other files that are stored in internal storage, you won't be able to easily reach them with a real device.
If you root the device and you change the permissions, then you should be able to see them. But to keep it simple, use the emulator when you're testing this. So, now we have a database open helper class and we're ready to go to the next step in following best practices in working with SQLite and that's creating a class called a data source. I'll show you how to do that in the next video.
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