Why written nested classes in Android

About

When I am working on my android project, I am quite curious about why templates from Android Studio prefers Nested classes design. After digging deeper in this issue, I believe this is worth for me to write a blog to record them down.

What is a Nested classes?

For detail please read this tutorial and this answer. Basically, Nested classes are classes defined inside of another class. For example,

class OuterClass {
    ...
    class InnerClass {
        ...
    }
}

InnerClass is a kind of Nest classes.

There are 4 types of inner classes in general:

• static class: declared as a static member of another class
• inner class: declared as an instance member of another class
• local inner class: declared inside an instance method of another class
• anonymous inner class: like a local inner class, but written as an expression which returns a one-off object

Why using nested classes?

According to the tutorial, pros for using nested classes:

• Able to access container’s private attributes, methods for non static inner class
• Saves some typing in certain cases for anonymous inner class like click handlers
• Better code organizations for related classes
• Enhance performances in certain cases like reusing identical handlers

According to this answer and this answer, cons for using nested classes:

• Waste memories as there are implicit references from inner class to outer class
• High potential causing memory leaks when using non static inner class

Idea of how memory is leaked

• Objects are unable to be garbage collected if there are references to them
• Inner class always holds a implicit reference to outer class
• If there is a reference to the inner class object outside of the outer class,
  • Inner class object is unable to be collected
  • Outer class object is unable to be collected too as it is pointed by the inner class object
  • This is a serious leak in android activities (outer class) as they have associated many resources like views and context etc

Words before end…

I strongly suggested you to go through the answers I pasted in this blog especially this one. The one I highlighted answering why and how memory leaks in nested class very well.

References:

• https://code.tutsplus.com/tutorials/learn-java-for-android-development-inner-classes--mobile-3530
• https://stackoverflow.com/questions/10864853/when-exactly-is-it-leak-safe-to-use-anonymous-inner-classes/10968689#10968689
• http://stackoverflow.com/questions/10112163/inner-classes-android-vs-java
• http://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class

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