/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
        Map < String, Double > map1 =
                Map.of(
                        "X1" , 1d ,
                        "X2" , 1d
                );
 
        Map < String, Double > map2 =
                Map.of(
                        "X1" , 2d ,
                        "X2" , 2d ,
                        "X3" , 7d
                );
 
        List < Map < String, Double > > listOfMaps = List.of( map1 , map2 );
        Map < String, Double > mergedMap = new HashMap <>();
        listOfMaps.forEach( 
                map -> map.forEach( ( k , v ) -> mergedMap.merge( k , v , Double :: sum ) ) 
        );
 
        System.out.println( "mergedMap.toString() = " + mergedMap );
	}
}

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