What are the purposes of inner classes

Question

I am reviewing the concept of inner classes in java. so far from what I\'ve understood and applied java inner classes has a link or access to the methods and fields of its o

Answer 1

Lets say you have a type and its a class, called OuterClass, in a package called "com.custom.classes".

Then here is how you begin to need an inner class or static class:

Case 1:

1. you need to package a group of classes
2. but also kind of need certain global variables exposed to all these classes at that package level
3. you understand you can do no such things with packages but realise that you could achieve this with inheritance, where the parent class members can act as global variables that become available for all of its child class instances.
4. but you don't like the idea that you need to inherit the parent class and also that you need to instantiate the child class to access the global variables. Thats like asking to buy a coffee shop in order to have a coffee.
5. and so you realise that you can create an OuterClass with the static members and house all the classes in this OuterClass as inner class or static class as needed and lo! The OuterClass static members become available as global variables for these nested classes and you could even access them without instantiating them.

This code should explain better

public class InnerClassTester{
  public static void main(String []args){

    // without the need to instantiate any class
    // static class without instantiation
    System.out.println(OuterClass.NestedStaticClass1.incrGlobalNum()); // outputs 1

    // static class instantiated
    OuterClass.NestedStaticClass2 koolObj = new OuterClass.NestedStaticClass2();
    // works with instantiation as well
    System.out.println(koolObj.incrGlobalNum()); // outputs 2

    // inner classes always need to be instantiated
    // and they can only be instantiated from within an instance of outer class
    // think of them as instance member of outer class and this would make sense
    OuterClass.NestedInnerClass1 koolObj2 = new OuterClass().new NestedInnerClass1();
    // works with inner classes as well
    System.out.println(koolObj2.incrGlobalNum()); // outputs 3
 }
}

class OuterClass{
    // global variable thats only accessible for select classes (or nested classes)
    // we also learn a purpose for private static fields
    private static int privateGlobalValue = 0;

    // classes to be grouped
    // static class
    public static class NestedStaticClass1{
        // no need to instantiate this class to access/update the global value
        public static int incrGlobalNum(){
            return ++privateGlobalValue;
        }
    }

    public static class NestedStaticClass2{
        // instantiate and still manipulate the global value
        public int incrGlobalNum(){
            return ++privateGlobalValue;
        }
    }

    // inner class
    public class NestedInnerClass1{
        // instantiate and still manipulate the global value
        public int incrGlobalNum(){
            return ++privateGlobalValue;
        }
    }

}

Does this remind you of closures in Javascript ? :)

---

Most applications of nested classes see it being applied on basis of design decisions. What that means is, every case of a nested class can be replaced with other designs.

But having said that, it is also true that we can also replace the inheritance pattern with composition pattern (and it is gaining momentum lately) although an inheritance pattern is definitely better when the dependencies between the classes is so much so that composing the dependencies entirely would be ugly.

Case 2:

1. you need to implement 2 interfaces, IShark and IMosquito, with the same signature, a public bite method, on the OuterClass.
2. but you want to display 2 different messages since a shark's bite is a tad different from that of a mosquito's.
3. however you know that's not possible since only one bite method can be implemented
4. you know you can create 2 different classes in the same package that implement either interfaces and also implement separate bite methods and have them composed in OuterClass.
5. but you wanted to get it done within OuterClass because it was your design decision to encapsulate the bite behaviour within it, maybe because there was a dependency on a private variable within the class.
6. soon you realise you can implement both the interfaces via private static inner classes and make it appear to the outside world as though it was composed.

Take a look at this code:

// no additional classes in the package

public class InterfaceTester{

     public static void main(String []args){

        // same class returns 2 instances - both compliant to 
        // either interfaces and yet different output

        IShark shark = OuterClass.getSharkInstance();
        System.out.println(shark.bite()); // outputs "Die fast bosedk!"

        IMosquito mosquito = OuterClass.getMosquitoInstance();
        System.out.println(mosquito.bite()); // outputs "Die slow bosedk!"
     }

}

interface IShark{
    public String bite();
}

interface IMosquito{
    public String bite();
}

class OuterClass implements IShark{

    // dependency of inner class on private variable
    private static String dieSlow = "Die slow bosedk!";
    private static String dieFast = "Die fast bosedk!";

    private static OuterClass outerInst;
    private static InnerClass innerInst;

    // private constructor to stop regular instantiation
    private OuterClass(){}

    // get a shark !
    public static IShark getSharkInstance(){
        return outerInst != null ? outerInst : new OuterClass();
    }

    // get a mosquito !
    public static IMosquito getMosquitoInstance(){
        return innerInst != null ? innerInst : new InnerClass();
    }
    // an implementation of bite
    public String bite(){
        return dieFast;
    }

    // inner class that implements the second interface
    private static class InnerClass implements IMosquito{
        // different implementation of bite
        public String bite(){
            return dieSlow;
        }
    }

}

These kind of design decision cases are numerous and all of the answers above list several such cases. So it would not be wrong to think that this feature was introduced more as a new pattern than as a feature or functionality.