Develop Reference

Android, should all code be written in an Activity

I am more familiar with iOS development than Android and I am wondering if all code should be written in an Activity rather than having a "model" class.
I have a couple screens each with a few checkboxes and I want them all to behave the same on click, I am trying to figure out how I would do this without writing repeating code in each activity. Thanks!

No you should not. If you are familiar with java, think of an activity as a extension of main with OO added.
In your particular example you can create a class with a method like:
<MethodName>(View <checkboxClickedName>){ //your code here }. and then add this to the checkbox in the XML android:onClick="<MethodName>", you may need the full package path (e.g. com.example.app.)
Note: if some of the commands/objects you need are only available within an activity you should create this in an calss that extends Activity or preferably within the running activity.

You could have a base class that extends activity that contains the methods that you want executed on click (either implemented or abstract). Use this new base class instead of activity when making new activities. In the layout xml, you can set the onclick of each checkbox to be the method in the base activity you want executed.

The best practice would be to use a single activity and switch fragments as if they were your screens. Then, the activity could simply implement the listener interface that the fragments would re-use.
Since you have multiple activities this becomes a little bit harder. To really re-use a single listener, I can think of a single (not so beautiful) option. Create a static listener and lazy load it:
public class MainActivity extends Activity {
private static View.OnClickListener sCheckboxClickListener;
public static View.OnClickListener getCheckboxClickListener() {
if (sCheckboxClickListener == null) {
sCheckboxClickListener = new View.OnClickListener() {
#Override
public void onClick(View v) {
// Work with v
}
};
}
return sCheckboxClickListener;
}
}
And in each of your activities call:
findViewById(R.id.checkbox1)
.setOnClickListener(MainActivity.getCheckboxClickListener());

how to use the widgets assigned in a activity in another activity?

I am creating an android project . In that i i got the ids from the XML file and wrote the click events in a (class or activity) . I want to use the widgets from another class without getting the id's again. like (Button btn=(Button)findViewById(R.id.button1);) i want to use this code in one class
But I want to use the Button btn in another class and also the click event should work.

If you want to use any View in more than one Activity you can create a class BaseActivity that extends Activity and include all the common Views in that Activity and then extend this BaseActivity to all the Activity in which you want to use common views/layouts/headers/footers. For a Pseudo code you can check my answer here.

android common OnItemClickListener and separate button listeners

In my app I have around 12 activities. The first layout of my activity is a list view. By clicking each list it gets redirected to other activities. I have implemented the Listview with OnItemClickListener and is working good.
In all the other 11 activities, I have a common title bar with a logo and a button named as S. When the user clicks on the S button, I am showing the same list of items in the First activity of my app, within the current activities by splitting the page. Now instead of writing a separate OnItemClickListener for each activity, how to write it once and use in multiple activities?
In the same way I have placed 3 button in include layout and I am using this in various activities, how to write a separate common button action, it can be used in multiple activities.

While #SamirMangroliya's method works, this is an alternate method you can use, and the one I've been using for a while.
Create your listener in another class file (say, MyClickListener.java):
public class MyClickListener implements OnItemClickListener {
// This can be OnClickListener, OnTouchListener, whatever you like.
// Implement your method here:
public onItemClick(...) {
// Your selection process, etc.
}
}
Then in each of your Activity objects, all you have to do is:
myObject.setOnItemClickListener(new MyClickListener());

how to write a separate common button action, it can be used in multiple activities.
you should create one Activity(called BaseActivity),
class BaseActivity extends Activity{
#Override
public void setContentView(int layoutResID) {
super.setContentView(layoutResID);
Button btn1 = (Button)findViewById(r.id.btn1);
.
.
.
//now setonclickListner here...
}
}
then extends it ...class MyActivity extends BaseActivity

It sounds like you should implement ActionBar navigation. For a more detailed description about how to do it the Android way, see the Design guide. To support versions of Android between 1.6 and 2.3, link in the ActionBarSherlock library.

create common class which includes intents of header and footer icons throughout whole application in android

i stuck with a unusual problem
i have a restaurant application which includes menu icons fixed at the bottom of every layout,
my problem is i dont want to create onsetclicklistner() method of every icon on each of my activity class....
please give some suggestion so that i can make a common class where i can put all my footer icon click event and activities in it and use it on my every activity class with different setcontentview...
hope you all get my question...
looking forward for your reply

You don't need to setup onClickListeners programmatically. You can also put them in the layout XML like this:
<Button android:id="#+id/my_button"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="#string/my_button_text"
android:onClick="myOnClick"
/>
Then you can declare a base activity class that contains this method in it, like this:
public class BaseActivity extends Activity {
public void myOnClick(View v) {
// Do whatever you want to do when the button is clicked
}
}
Then you write all of your activity classes so that they derive from BaseActivity instead of Activity.

How do I create common code for parts of Android activities?

In my application there are 14 activities. Out of that 9 activity contains custom title bar and tab pane. so here I need to write this common code at one place instead of redundant code in each activity that contain custom title bar and tab pane code (i.e layout and it's activity specific code)
What are the possible ways to do this?

The common way is:
Create a super class called, for instance, CommonActivity which extends Activity
Put the boilerplate code inside that class
Then make your activities extend CommonActivity instead of Activity:
Here a simple example:
public class CommonActivity extends Activity{
public void onCreate(Bundle b){
super.onCreate(b);
// code that is repeated
}
protected void moreRepeatitiveCode(){
}
}
And your current activities:
public class AnActivity extends CommonActivity{
public void onCreate(Bundle b){
super.onCreate(b);
// specific code
}
}

Hmm.. Common code doesn't always need to be in Activity class but just regular class. Than we could call those methods according to our needs referring to the common code class.
Am I right with this example?
Of course in case we need it like Activity, above proposal would work perfectly if we take care of Activity lifecycle and we don't forget to add it to manifest file.
In general Activities should just create UI, handle events occurrences and delegate business logic and/or other actions to the other components in our App.
Cheers