Generics upper bound and lower bound in java and generics: in, out android kotlin

Rmvivek

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Mar 14, 2022

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• Unbounded Wildcards: List<?> represents a list of unknown type
• Upper Bounded Wildcards: List<? extends Number> represents a list of Number or its sub-types such as Integer and Double
• Lower Bounded Wildcards: List<? super Integer> represents a list of Integer or its super-types Number and object.

Eg:

We can declare sample classes

interface Madurai { }
abstract class Hosur implements Madurai { }
abstract class Arni {}
class Chennai extends Arni { }
class Vellore extends Chennai implements Madurai {}
class Erode extends Vellore { }
class Salem extends Arni {}

We can Define List<? super T> and List<? extends T>

static void add(List<? super Vellore> list) {do logic}->access only base generic type
 static void print(List<? extends Chennai> list) {do logic}->access only drived generic typepublic class City {
  static void add(List<? super Vellore> list) {do logic}
   static void print(List<? extends Chennai> list) {do logic}
    public static void main(String[] args) {
        List<Object> list0 = new ArrayList<Object>();
        List<Arni> list = new ArrayList<Arni>();
        List<Chennai> list1 = new ArrayList<Chennai>();
        List<Vellore> list2 = new ArrayList<Vellore>();
        List<Madurai> list3 = new ArrayList<Madurai>();
        List<Salem> list4 = new ArrayList<Salem>();
        List<Erode> list5 = new ArrayList<Erode>();
        List<Hosur> list6 = new ArrayList<Hosur>();
        add(list0);
        add(list);
        add(list1);
        add(list2);
        add(list3);
        add(list4);//err
        add(list5);//err
        add(list6);//err
        print(list);//err
        print(list0);//err
        print(list1);
        print(list2);
        print(list3);//err
        print(list4);//err
        print(list5);
        print(list6);//err
    }
}

Kotlin Generics: in, out

1. Array<in String > corresponds to Java Array<? Super String>

means we can pass an array of charsequence or an array of object

2. Array<out Any> corresponds to Java Array<? extends Object>

for the example

in->assing the value, set the value , write the value

// generic interface 
interface Road<in R>{
    fun setCity(points:R)
}//class implement from interfaceclass RoadSerial():Road<Int>
{
    override fun setCity(points: Int) {
      val roadNo=points
    }
}

out-> fetch the value, return the value, read the value

interface City<out T>{
 fun getCity():T
}class Area():City<String>
{
    override fun getCity(): String {
        return "Arni"
    }
}

Generics combine of in and out

interface Country<out R,in M>{
    fun setCity(points:M)
    fun getCity():R
}class CityDetail:Country<String,Int>
{
    override fun setCity(points: Int) {
        val roadNo=points
    }    override fun getCity(): String {
        return "Arni"
    }
}