从父对象创建子对象的地图

Question

I have a list of parent objects where I want to count the occurrence of children objects. I know that I can use instanceof operator like shown below to count the occurrence of each object type. However, I wanted to use a HashMap instead of if-else branches. I try to create Map<? extends Parent, Integer> Map<? extends Parent, Integer> but it didn't work. Any suggestions?

class Parent {
// parent class
}

class ChildA extends Parent {
 // child class
}

class ChildB extends Parent {
 // child class
}

class ChildC extends Parent{
 // child class
}

int countChildA = 0;
int countChildB = 0;
int countChildC = 0;
for (Parent child : children)
{
    if (child instanceof ChildA)
    {
        countChildA++;
    }
    else if (child instanceof ChildB)
    {
        countChildB++;
    }
    else if (child instanceOf ChildC)
    {
        countChildC++;
    }
}

// what I'm looking for
Map<? extends Parent, Integer> map = new HashMap<>();
for (Parent child : children)
{
    map.put(child, child.getValue(child)++);
}

Answer 1

You need the key of your Map to be Class (the type of the Parent instance):

Map<Class<? extends Parent>, Integer> map = new HashMap<>();

And instead of:

map.put(child, child.getValue (child)++);

use:

if (map.containsKey(child.getClass())) {
    map.put(child.getClass(), map.get (child.getClass())+1);
} else {
    map.put(child.getClass(), 1);
}

or

map.put(child.getClass(), map.getOrDefault(child.getClass(), 0) + 1);