Singleton Design Pattern is by far the most common design pattern and the first one that come across to anyone when think about Design Patterns. It's so commonly used that there are people who can write the pattern in their sleep.

Android Studio Default Template

classTest{privatestaticfinalTest ourInstance=newTest();staticTestgetInstance(){return ourInstance;}privateTest(){}}

The below is the template that we generally use in our team.

Our Template

classTest{privatestaticfinalTest ourInstance;staticTestgetInstance(){if(ourInstance==null){            ourInstance=newTest();}return ourInstance;}privateTest(){}}

I faced a situation where I was required to create a Singleton per activity, or a Singleton class that holds an instance of Activity context. Usually when you try to declare objects like 'Activity' or 'Context' as a static field in any Java class, Android Studio warns about memory leak via static objects.

Do not place Android context classes in static fields; this is a memory leak.

To avoid that memory leakage,

1. Do not store context (That's obvious! isn't it).

2. UseWeakReference to store the context.

privatefinalWeakReference<Activity> mActivityContextRef;publicstaticvoidnewInstance(Activity activity){if(ourInstance==null){        ourInstance=newTest(activity);}}privateTest(Activity activity){    mActivityContextRef=newWeakReference<>(activity);}

Some of the best articles on Weak References:

1. Romain Guy's Article
2. Articles from Google Developers Experts-1
3. Articles from Google Developers Experts-2

Back to the earlier scenario that I described, I had to create anSingletons per Activity. Combining with our singleton template with the concept ofWeakReference, I came up with this.

publicstaticvoidnewInstance(Activity activity){if(ourInstance==null){            ourInstance=newTest(activity);}else{if(activity.equals(ourInstance.mActivityContextRef.get())){LogUtils.debug(TAG,"newInstance: Same Activity; No need to create new instance");}else{LogUtils.debug(TAG,"newInstance: New Activity; Create new instance");                ourInstance=null;                ourInstance=newTest(activity);}}}privateTest(Activity activity){        mActivityContextRef=newWeakReference<>(activity);}

This solution helped us to solve the issue we were facing.

I don't argue that this is perfect solution and in fact aWord of Caution When I was talking to one of my senior colleagues regarding the aboveSingleton per Activity, he questioned me thatWhy would anyone require anActivity context in a Singleton POJO class. That got me thinking and I realized that the class in question was handlingtoo many responsiblities and refactored usingMVP design pattern. I will write that in another post.

This post is also available on DEV.