Iterate though the contents of a Map<String,List<Object>> and assign the values of the List<Object> into separate List<Double>

John Stef :

I have a list of objects that contain coordinates and a name. An example of the printed list can be seen below:

[X=0.1, Y=0.1, name=Series1]
[X=0.1, Y=0.2, name=Series1]
[X=0.1, Y=0.3, name=Series1]
[X=0.1, Y=0.4, name=Series2]
[X=0.1, Y=0.5, name=Series2]

What I'm trying to achieve is to group them together based on the name which can be done with the following Java 8 operation:

Map<String, List<Coordinates>> newList = coordinateList.stream()
           .collect(Collectors.groupingBy(Coordinates::getName));

Output:

Series1=[[X=0.1, Y=0.1],[X=0.1, Y=0.2],[X=0.1, Y=0.3]]
Series2=[[X=0.1, Y=0.4],[X=0.1, Y=0.5]]

The final step which I have not yet been able to achieve, is to iterate through the Map, create lists of X's and Y's so I can finally create a graph (chart.addSeries(name, xData, yData);).

My approach to this so far has been the following:

    List<Double> xData = new LinkedList<Double>();      
    List<Double> yData = new LinkedList<Double>();

    newList.forEach((k, v) -> {
        //TO-DO
    });

Inside the loop I would like to assign X's and Y's to the corresponding lists but with no success so far.

One of the problems is that X and Y are associated with a Coordinates object, so I cannot just add them to the list which is of type Double. Another problem I faced is that I cannot get X and Y separately from the list. I have tried getX() and getY() but with no luck.

Example of what I would like the lists to contain:

For the key Series1 the generated lists would be the following:

xData=[0.1,0.1,0.1]
yData=[0.1,0.2,0.3]

For the next key Series 2:

xData=[0.1,0.1]
yData=[0.4,0.5]

azro :

In the forEach, (k, v) represents the key and the value of each pair, so you need to get the Coordinates and then its x and y. Also as you don't need specificly a LinkedList, use an ArrayList.

As you want a Series for each pair, your xData and yData should be in the loop :

for (Map.Entry<String, List<Coordinates>> serie : newList.entrySet()) {
    List<Double> xData = new ArrayList<>();
    List<Double> yData = new ArrayList<>();

    for (Coordinates coord : serie.getValue()) {
        xData.add(coord.getX());
        yData.add(coord.getY());
    }

    chart.addSeries(serie.getKey(), xData, yData);
}

---

Choose first option as it's easier to use, don't use forEach to quickly :

newList.forEach((key, value) -> {
    List<Double> xData = new ArrayList<>();
    List<Double> yData = new ArrayList<>();

    value.forEach(c -> {
        xData.add(c.getX());
        yData.add(c.getY());
    });

    chart.addSeries(key, xData, yData);
});

---

With Stream api you can move to :

newList.forEach((key, value) -> {
   List<Double> xData = value.stream().map(Coordinates::getX).collect(Collectors.toList());
   List<Double> yData = value.stream().map(Coordinates::getY).collect(Collectors.toList());
   chart.addSeries(key, xData, yData);
});