Why shouldn't static variables be used as global variables in Android development

For mobile devices with small memories, memory usage and care of applications is essential. If an application uses images, it can easily result a memory leak if they cannot be collected by garbage collector. A reason why GB couldn't collect unused objects is that those objects refers to a static variable. So, if you are setting a property of a local variable with a static one, you need to be double careful.

This post which explains how to use static variables in an android app. To give a brief explanation about this fundamental way;  a local variable with a short life time and as a child of an object with big size(local var: textview, parent: activity) shouldn't refer to a static variable. This is because of that they will not be collected by garbage collector even if they are garbage and not used by the app. A few instance of activities in garbage can easily cause a memory leak in devices with small memories. It is also not very neat way for android development. So, avoid strong reference of static vars to such objects.

Using Static variables is a way of defining and using global variables as long as the app lives. But it is not very appropriate as it can result such disappointing endings. So, a question may come in minds: how should we define global variables? Which one is the best way? There are various ways to follow, but in my opinion, inheriting Application class and defining lifetime-long variables in it is the best one.

This is an example code of it.

public class MyApplication extends Application {

    private String var;

    public String getVar() {
        return var;
    }

    public void setVar(String pVar) {
        this.var = pVar;
    }
}

Application should use the newly implemented class instead of App class of Android library. To provide it, set name property of application tag in AndroidManifest.xml.

application android:name="MyApplication" android:icon="@drawable/icon" android:label="@string/app_name"

Because from each activity, application class is reachable, public methods of Application class can be easily called:

//set
((MyApplication) this.getApplication()).setVar("foo");  

//get      
String s = ((MyApplication) this.getApplication()).getVar();

It is quite perfect way to define variables whose lifetimes are same with the app.  I think this is a must-known way which is particularly essential for android development.

Thanks to Jeff in stackoverflow. This work is originated from this post.