Question

can anyone tell me why the put is giving me an error?

static HashMap<Class, ? extends BaseSystem> systems;

public <T extends BaseSystem> EntityManager()//constructor
{
    systems=new HashMap<Class, T>();
} 

public static <T extends BaseSystem> void addSystem(Class c, T system)//store a system for later reference
{
    systems.put(c, system);
}

Answer 1

The most likely problem is that the T in the first method and the T in the second method are not automagically considered the same. In other works you have two different types which both extends BaseSystem but could be unrelated.

You need to define T once on the class and use it consistently or just use BaseSystem and not use generics.

BTW: On a more basic level, don’t confuse static initialisation and instance intialisation. If you are setting a static field in a constructor you have a design issue.

Is there any relationship between the class and the T system?