Develop Reference

How should I store common strings?

I can store them in res/strings
<resources>
<string name="str1">app</string>
</resources>
And I can store them in static const
public static final string str1 = "app"
Which one is better?
I mean that which one will use more memory and which one will make package larger.

If you have to change some text in your app's view, then you don't have to find where you have written that text. You simply go to the strings.xml in res folder and can change the string. For that reason, the first option will always be the better option.
If you want a string that will be used as a key in your app then you can store it in a static constant variable. Like: If you want to pass data through intent or in any other way, at that time you have to pass a key. That key will be a string and can be stored in a static constant variable.

it's really depend if you treat strings as constants or not.
If you string are constants (e.g. string that will be sent as key of intent) then you can either create some Constants class or define it as a constant in your activity/fragment class. If those string are texts that will be displayed on your application UI then they should be defined as resources in your application.
BTW all the resources string can be localized to different languages
If you need more info or examples please let me know.

The package size:
I believe that "Ran Hassid"'s comment describes well about the package size.
Memory usage:
Just create an android apk file with some static final strings and then decompile it. You will see that the variable has no initial value in the compiled version and the value is replaced in all references!
For example check this sample from my decompiled apk which is also obfusicated by proguard:
public static String f10850f;
...
...
...
hashMap.put(C1554f.m14687a("\u0014]SCVQ^V"), str);
here is the actual code I have written:
public static String packageName="\u0014]SCVQ^V";
...
...
...
hashMap.put(EncryptorClass.decrypt(packageName), str);
In this case, the static final String variable in my written code has value of "\u0014]SCVQ^V" but it is not used in run-time and just copy/pasted in related pieces of code.
So using static final String variables needs no memory because variable has no initial value.
Have a look at android docs about using static final variables
Also take a look at this post regarding memory of variables with no value
Overally, I use static final variables and string resources for different purposes like code encryption, language support and whether I want to access the variable using a Context or not.

Best way to use getString()

I'm having some problems on where and how to place and call the getString().
I tried at the beginning of my activity where I usually define it but always get an error. Should it be only after onCreate()?
Some of my strings I use it very often so I don't want to use getString() every time I use them.
Is this the correct way?
private String helloWorld = getString(R.string.hello_world);
Thank you very much! :)

at the top of your file do:
private String helloWorld;
And then, in onCreate(),
do:
helloWorld = getString(R.String.hello_world).
The problem is that getString needs a reference to the activity context, which has not been associated with your class until onCreate is called.

Problems accessing my strings.xml items… i got numbers and not the string value

i found this problem some time ago, but i solve it using this: getString(), or this: getResources().getString()
but now, for this case, it doesn't works, i think it's because i need to get the string values on a NON ANDROID ACTIVITY CLASS. I need the resource values on a remote connection class, that doesn't extends any kind of activity or service.
how i can acces to the variables from my strings.xml on this normal class?
this is the code where i get the error (it gets an integer, and not the string value)
String a =R.string.totalpermission;

Take a look at these two answers (are the same XD):
How to obtain AssetManager without reference to Context?
How can I get a resource content from a static context?
Just an advice: try to read some basic concepts... it seems you don't understand what the R class is and how to use it. Trust me, you waste less time studying than trying to figure out how things work.

I'll add something to existing answers since I found it very useful.
To get your strings you have to use a Context. Your activity will work just great.
String string = getString(R.string.myString);
But if you have something more complex... for exemple
R.string.result -> "You %1$s %2$d cats"
String result = getString(R.string.result, killed ? "killed": "saved", count);
That would give you a result like that:
You saved 10 cats or You killed 2 cats... and so on. You can pass parameters and positional arguments in strings will get replaced by your arguments in getString.

All Android resources are referenced via a resource ID, like R.string.totalpermission. You can see those numbers in R.java (although there's no reason to ever do that).
In cases of strings, you can easily get those using Context.getString. Bonus: You can even pass optional arguments and add dynamic strings that way. You always have a context - how are you getting called? If you really don't have a context, you can create one for the package your resources are in.

naming convention for passing data through extras in android

when passing extras such as Intent.putExtra("myName", myName), what's the convention for the name of the extra?
ie: if passing data between two activities, both would put/extract data under the id "myName", but should I just hardcode "myName" everywhere, or keep the value in the R.values.string?

Hardcoding is definitely not an ideal solution.
The convention used in the Android framework is to create public static final constants named EXTRA_FOO (where FOO is the name of your key) like Intent.EXTRA_ALARM_COUNT
The actual value of the constant is a name spaced string to avoid conflicts: "android.intent.extra.ALARM_COUNT"
If you don't want to create dependencies between your Activities with constants, then you should consider putting these keys into string values within your strings.xml file. I tend to follow the same naming convention when defining the keys in xml:
<string name="EXTRA_MY_NAME">com.me.extra.MY_NAME</string>
It still reads like a static constant from the Java side:
getString(R.string.EXTRA_MY_NAME);

The only thing I saw in documentation is that extra keys should start from package name. However I do not fully follow this and the app works Ok so far.
I would prefer to use R.string.some_key within the code just to have it clean and dry.

Cannot get resource from android application

I am building an Android app, and I have a problem to get resource from string.xml.
I need to have a URL in String.xml that will be used several times through the app.
I tried Resources.getText("my_url"), but this does not work. Eclipse complains when I use this.
Do you think this is the best way to do ?

What you probably want is:
String myUrl = getString(R.string.my_url);
The getString() method is in Context which means that it's available directly in your Activity class since Activity is a subclass of Context. (The getString() method is also in Resources but it's easier to call on it directly on your Activity.)
What happens with your XML resources is that each is given a unique integer ID and this is added to the automatically generated R class as a public static final int. You then use these IDs to reference the resources. Have a look in the gen folder in your Eclipse project and you'll find the R class in there.

Do you ever refer this page: https://developer.android.com/guide/topics/resources/available-resources.html ?
If you want to retrieve the String represented by a resource ID, you can call the Context.getString() method.
Or, you have to post Eclipse's complains.