When programming for Android, you have to take into account that your application will be killed, paused and recreated without your consent. This is because of the nature of phones which have less memory and move from one task to another without the ability to support everything at the same time.

The example I’m going to take is screen rotation. When the screen rotates, the visible activity gets recreated. How do I know that? I have two Spinners on it, one with an adapter that changes according to the other’s selection. With each rotation I lost the current selection of the second Spinner. I added some Toasts to the code and found that onCreate() is being called upon orientation change. The first thing that comes to mind is to save the selection somewhere and restore it after the recreation.

There is a facility for that. Have you seen the parameter that your onCreate() gets?
protected void onCreate(Bundle savedInstanceState)
This savedInstanceState is a ‘bundle’ which means a space that Android gives your application to save some info between recreations of your activities. There are also two methods you can use to put the save and restore code in:

@Override
public void onSaveInstanceState(Bundle savedInstanceState)
{
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putInt("numberOfThings", num);
    savedInstanceState.putInt("anotherNumber", numnum);
}

@Override
public void onRestoreInstanceState(Bundle savedInstanceState)
{
    super.onRestoreInstanceState(savedInstanceState);
    num = savedInstanceState.getInt("numberOfThings");
    numnum = savedInstanceState.getInt("anotherNumber");
}

This didn’t help me completely because it was only then that I realized that onCreate is called for every rotation and in onCreate I initialized my UI elements to their default values. I tried putting num = savedInstanceState.getInt("numberOfThings") in onCreate to load the previous values but got a null reference exception on the first run because savedInstanceState is not initialized if this is the first run. The complete solution was to ditch onRestoreInstanceState and use only onCreate(). I set the default values in variables, and then did:

if (savedInstanceState != null)
{
    // Load variables here and overrite the default values
}

The rest of the initialization stayed as it was.

So the complete solution is:

• Save the values you need in class members using onSaveInstanceState
• In onCreate() restore the values if savedInstanceState != null and use them for initialization

That’s all folks.

Well, at least anything that can be printed. It’s quite easy.

1. Create a string stream
2. “Print” the value to the stream
3. Get a string from the string stream

I like to wrap it in a template function like that:

#include <string>
#include <sstream>

template <typename T>
string to_string(T x)
{
    stringstream ss;
    ss<<x;
    return ss.str();
}

You can make the function inline, and get the argument as a reference to const if you want.

Have you got a simpler generic solution?