JAVA EE: Java Tutorial: Generics in java | Java Generics [Upper bounded wildcard in Java Generics]

Tuesday 16 May 2017

Java Tutorial: Generics in java | Java Generics [Upper bounded wildcard in Java Generics]

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Java Tutorial: Generics in java | Java Generics [Upper bounded wildcard in Java Generics]

GenericDemo1.java

import java.util.ArrayList;
import java.util.List;

public class GenericDemo1
{

    public static void main(String[] args)
    {
        List<Number> numberList= new ArrayList<Number>();
        numberList.add(2.2);
        numberList.add(3.3);
        numberList.add(5.5);
        double sum = sum(numberList);
        System.out.println("sum = " + sum);
        
        List<Integer> integerList= new ArrayList<Integer>();
        integerList.add(2);
        integerList.add(3);
        integerList.add(5);
        sum = sum(integerList);
        System.out.println("sum = " + sum);

    }

    public static double sum(List<Number> list)
    {
        double sum = 0;
        for (Number n : list)
        {
            sum += n.doubleValue();
        }
        return sum;
    }

}

Output

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
    The method sum(List<Number>) in the type GenericDemo1 
    is not applicable for the arguments (List<Integer>)

    at GenericDemo1.main(GenericDemo1.java:20)

GenericDemo2.java

import java.util.ArrayList;
import java.util.List;

public class GenericDemo2
{

    public static void main(String[] args)
    {
        List<Number> numberList= new ArrayList<Number>();
        numberList.add(2);
        numberList.add(3);
        numberList.add(5);
        double sum = sum(numberList);
        System.out.println("sum = " + sum);
        
        List<Integer> integerList= new ArrayList<Integer>();
        integerList.add(20);
        integerList.add(33);
        integerList.add(50);
        sum = sum(integerList);
        System.out.println("sum = " + sum);

    }

    public static double sum(List<? extends Number> list)
    {
        double sum = 0;
        for (Number n : list)
        {
            sum += n.doubleValue();
        }
        return sum;
    }

}