Java 8 GroupingBy with Object Collectors

Question

I would like to pass a stream of objects myClassto group using Collectors.groupingBy(). But instead of retrieving Map<String, List<myClass>>, I would like to group it by object myOutputand get Map<String, myOutput>. I tried to create a custom collector:

List<myClass> myList = new ArrayList<myClass>();
myList.add(new myClass("a", 1));
myList.add(new myClass("a", 2));
myList.add(new myClass("b", 3));
myList.add(new myClass("b", 4));

Map<String,myOutput> myMap = myList.stream().collect(Collectors.groupingBy(myClass::getA, Collectors.of(myOutput::new, myOutput::accept, myOutput::combine)));

myClass:

protected String a;
protected int b;

public myClass(String aA, int aB)
{
  a = aA;
  b = aB;
}

public String getA()
{
  return a;
}

public int getB()
{
  return b;
}

myOutput:

protected int i;

public myOutput()
{
  i = 0;
}

public void accept(myClass aMyClass)
{
  i += aMyClass.getB();
}

public myOutput combine(myOutput aMyOutput)
{
  i += aMyOutput.getI();
  return this;
}

public int getI()
{
  return i;
}

But with this code there is a problem with the collector:

Collectors.of(myOutput::new, myOutput::accept, myOutput::combine)

I know that in this case, the reduction will be much easier, but let it be assumed that there are many operations in the myOutput object.

What happened to this collector?

Answer 1

. Collector.of static factory ( Collectors.of).

    Map<String,myOutput> myMap = 
        myList.stream()
              .collect(Collectors.groupingBy(
                myClass::getA, 
                Collector.of(myOutput::new, myOutput::accept, myOutput::combine)
              ));

, , . . a , a, b. Collectors.summingInt(mapper), mapper b:

Map<String,Integer> myMap = 
    myList.stream()
          .collect(Collectors.groupingBy(
            myClass::getA, 
            Collectors.summingInt(myClass::getB)
          ));