Develop Reference

Android Studio cannot resolve symbol

I'm just starting with programming Android apps and was working through the developers guide on android.com. When trying to display a text on a second activity it says it cannot resolve the symbol EXTRA_MESSAGE as you can see here:
As far as I can tell I did every step like the guide says. I also tried copy and pasting everything but it still doesn't work. What am I missing?

You're doing a static import of extra message from some random class in MainActivity. That's wrong, don't do that. Define EXTRA_MESSAGE as a public final static String, with whatever value you want (I'd suggest "message") in your MainActivity.

There is two ways of solving it.
1) Use same static variable (Its a dirty way). At DisplayMessageActivity use android.provider.AlarmClock.EXTRA_MESSAGE.
2) This approach I recommend you. Create public static final String field at MainActivity, remove android.provider.AlarmClock.EXTRA_MESSAGE, and use MainAcitivity field at both classes. Content of this variable does not matter as long as it is unique extra key.

What is the meaning of putting class name inside the brackets?

While observing an existing android application, I encountered this line of code:
((StoredVariables)this.getApplication()).getId
Why we are using this? What is the outcome of the code? Does it returns previously stored values from sharedPreference? No documentation found on internet to get an idea about it. Please explain.

You are probably a little newbie in Java also
I'm not sure about what is "this" in this.getApplication() and what the StoredVariable is, since it is not an android class...
But here are recommendations for you:
Look for StoredVariable in your class in imports section. You can find something like import my.project.StoredVariable;
Open the class (I'm 80% sure it will be interface) to see the class (this is already answer to your question)
See getId() method there
To know more about usage of StoredVariables. If this line of code is placed inside class that extends some Activity (e.g. AppCompatActivity, Activity e.t.c.)
Open AndroidManifest.xml and look into <application> tag to find android:name in there. Open class with that name (ctrl + click or cmd + click on class name)
This class is extending Application class (or subclass) and implementing StoredVariables interface, or it is directly StoredVariable class

Where can I find the source code of Android class SQLiteOpenHelper?

I want to take a look at the Android SQLiteOpenHelper class implementation, can somebody points me the location?

The link you want is:
https://android.googlesource.com/platform/frameworks/base/+/refs/heads/master/core/java/android/database/sqlite/SQLiteOpenHelper.java
But even better let me share a great tip:
This extension for Google Chrome
https://chrome.google.com/webstore/detail/android-sdk-search/hgcbffeicehlpmgmnhnkjbjoldkfhoin?hl=en-GB adds the ad (as in Android Developer) keyword to your searchbar so you can type ad TextView and it will bring you directly to TextView on this example AND will add a link "View Source" next to the class name inside the site https://developer.android.com/reference so for any class you want to see the source it's just a click away.

You can find all the existing versions here: http://grepcode.com/search?query=SQLiteOpenHelper&n=

Handling a core project and multiple derived from it in Android

I've searched around SO for this and found a few things, but I'm still not sure I fully understand, so I ask you for clarifications.
Here is what I need:
Have a project that has specific function: interrogate web service, display results in different views
Have a second, third and forth project that has exactly the same functionality as the first one, but only different graphic elements like splash screen image, icon, name, package name.
So, I have ProjectCore with activities and functionality. Project1 with a car icon and car image for splashscreen. Project2 with airplane icon and airplane image for splashscreen. Something like that. Each projects has a class with constants like'appId, appName, appServerURL"... All the web service call, data display is in Core as it's the same for all prohects, only the read is made from Constants class.
I was thinking of this approach
Make ProjectCore a Library project with a package like com.domain.core and dummy images
Make Project1, add reference to ProjectCore in it and with a package like com.domain.code.project1 and in resources folder, put images with same name as in core project
Make Project2 on the same principle like project1
Will this approach work ?
Thanks.
Later Edit. I've tried as mentioned before. For instance in Core project I had in drawable a file called splash.png. In Project1's and Project2's drawable folder I've put spash.png file with other images. This works fine. Running the Project1 and Project2 on my phone, started each app with it's own image. So far so good.
Then, because I have different constants I need to use in my App, I went into Core library project and added:
public class C {
public static String SomeConstant = "Project core!";
}
Here comes the problem, I need to have different constant values across Project1 and Project2. Because on Core project, the class is in com.domain.core.utils for instance... I can't add the same package in Project1 and Project2. How do I add the classes so I can update their values and be used on each project with particlar values ?
public class C {
public static String SomeConstant = "Project 1 constant!";
}
public class C {
public static String SomeConstant = "Project 2 constant!";
}
Thank you!

You want to create your functionality in a Library project and then have all of your Branded/OEM/3rdParty projects extend from this, overriding images and string resources where necessary.
When you need to use "Constants" you should instead have a single "run once" portion of your code (such as a splash screen) load these strings from resource files:
public static final String CONSTANT_ONE;
public void onCreate() { CONSTANT_ONE = getResources().getString(R.String.CONSTANT_ONE); }
EDIT
I'm unsure on how initialising a final value on onCreate() will perform. If final doesn't work well and you're worried about changing the variable during program execution then make the variable private (so only that class can assign to it) and then create a public static String getConstantOne() function.

Yes. Library projects are ideal for this, especially if only resources differ. I've used the exact approach that you've outlined with success...

Yes this should work fine. I did something a bit similar and I found occasionally you may have some circumstances where you want to call out from your library project to your application project. In these cases I used interfaces/abstract classes defined in the library project but implemented in application project...

Eval function in Dalvik

If I know a variable's pattern such as R.id.edit_x where x (1..N), how can I get a reference to a given EditText, like findViewByID(R.id.edit_1). Is there something like an "eval" function in Dalvik ? Thanks.

Try Java reflection. Discussion on retrieving static final fields via reflection is here - Accessing Java static final ivar value through reflection

hoha's answer is good. Another thing you can do is create a look-up table that maps 1..N to the resource IDs. (Presumably you know all the resource IDs ahead of time.)

maybe， you can check roboguice. it is a ioc framework for android and it's realy easy to use. i copy some code from the sample from the project to show how to use it:
public class AstroboyMasterConsole extends RoboActivity {
#InjectView(R.id.self_destruct) Button selfDestructButton;
#InjectView(R.id.say_text) EditText sayText;
#InjectView(R.id.brush_teeth) Button brushTeethButton;
#InjectView(tag="fightevil") Button fightEvilButton; // we can also use tags if we want
}
then you can you these injected variables in your code！