Java 8 adding values of multiple property of an Object List

Question

Lets say I have a class below with getters and setters but with only default constructor.

Note: I'm not allowed to change the structure of this class.

class Target {

    private String year;
    private String month;
    private String name;
    private double target;
    private double achieved;


    public String getYear() {
        return year;
    }

    public void setYear(String year) {
        this.year = year;
    }

    public String getMonth() {
        return month;
    }

    public void setMonth(String month) {
        this.month = month;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getTarget() {
        return target;
    }

    public void setTarget(double target) {
        this.target = target;
    }

    public double getAchieved() {
        return achieved;
    }

    public void setAchieved(double achieved) {
        this.achieved = achieved;
    }    
}

I have to add the Target and Achieved columns values based on year and Name.

Year Month Name Target Achieved
2018  8    Joey 50.00   10.00
2018  9    Joey 200.00  100.00
2018  9    Fred 200.00  150.00
2018  9    Fred 20.00   50.00

So the output would be:

Year Month Name Target  Achieved
2018  8    Joey 50.00   10.00
2018  9    Joey 200.00  100.00
2018  9    Fred 220.00  200.00

I've seen an example on how I could achieve something like this if I had a constructor that accepts parameters but I'm not so clear on the concept Group by and sum objects like in SQL with Java lambdas?:

How do I achieve this with just the default constructor to get same type List<Target> but with calculated values of multiple columns?

Answer 1

It seems that you need to group based on three things: Year, Month and Name, so this could look like this:

Collection<Target> merged = yourListOfTargets
            .stream()
            .collect(Collectors.toMap(
                    t -> List.of(t.getYear(), t.getMonth(), t.getName()),
                    Function.identity(),
                    (left, right) -> {
                        left.setTarget(left.getTarget() + right.getTarget());
                        left.setAchieved(left.getAchieved() + right.getAchieved());
                        return left;
                    }))
            .values();

As Federico mentions in comments, this will alter your elements in the initial List. You might be OK with it, but if you are not, you need to replace Function.identity() with a copying Function that would create a new Target from an existing one.