Java Generics：从泛型中检索类型 object

Question

I have a generic class Class1<K, V> which takes two types. Now I want to maintain a map which maps String to Class1 objects ie Map<String, Class1>. Can I retrieve the types (K,V) of each Class1 Object while I iterate through Map?

ie lets think of code as below

Map<String, Class1> map;
map.put("1", Class1<String, Integer> obj1);
map.put("2", Class1<String, Double> obj2);
map.put("3", Class1<Integer, Double> obj3);

Now when I iterate over the map... Can I somehow get the types(K, V) of each Class1 object??

Answer 1

It's not possible according to JVM specification.

Generic types are available only at compile time. At runtime ther're only Object .

All you have is 2 options:

1. Add class definition variable to your objects and us it when you need this info.
2. Eg for List<Object> as well as for other collections if it's not empty, you can get an item from it and get item.getClass() .

PS Buy the way, your approach is not correct to solve this issue. You could implement an interface like:

interface TypeConverter {
    boolean isValid(String str);   // call it before to check if current converter valid for required transform or not

    Class1 converts(String str);  // call it to convert 
}

And for each required types you have to create separate converter (this is Strategy Pattern). And your final code could look like this:

final TypeConverter converters = new ArrayList<>();
converters.add(new OneTypeConverter());
converters.add(new TwoTypeConverter());

public static Class1 convert(String str) {
    for(TypeConverter convereter : converters)
        if(converter.isValid(str))
            return converter.convert(str);
  
    throw new RuntimeException("No suitable converter found");
}