Java – how to create a generic type safe HashMap by class type?

See English answers > java map with values limited by key's type parameter5

Later, I want to pass the class type and get a reference to that particular object Simple example:

Map<Class<?>,?> values = new HashMap<>();

public <T> t get(Class<T> type) {
    return values.get(type);
}


//pet and car do not share any interface or parent class
class Pet;
class Car;

//error: not applicable for arguments
values.put(Pet.class,new Pet());
values.put(Car.class,new Car());

Usage:

values.get(Pet.class);

How to create such a general hash diagram and lookup function accordingly?

Solution

You need to store certain types of objects. If you want to place any other objects in the map, start from here:

Map<Class<?>,Object> values = new HashMap<>();

This must be done because? It is not a specific object, but for the type of map storage, the object is

So the following code snippet works without any warnings:

Map<Class<?>,Object> values = new HashMap<>();
values.put(Pet.class,new Car());

The trick now is to get the object. We do it as follows:

@SuppressWarnings("unchecked")
private <T> T get(Class<T> clazz) {
    return (T)values.get(clazz);
}

Now, your goal is to ensure that the mapping contains pairs that do not appear wrong at run time If you use car Class add a new car () instance, then you will encounter an error

So the following example code:

values = new HashMap<>();
    values.put(Pet.class,new Pet());
    values.put(Car.class,new Car());

    System.out.println("get(Pet.class).getClass() = " + get(Pet.class).getClass());
    System.out.println("get(Car.class).getClass() = " + get(Car.class).getClass());

Will print:

get(Pet.class).getClass() = class testproject8.Pet
get(Car.class).getClass() = class testproject8.Car