Question

I tried the line below:

HashMap <Temp,<? extends Action> > p= new HashMap<Temp,  <? extends Action>>()

Eclispe is giving me an error

Multiple markers at this line
– Syntax error on token “,”, Type expected after
this token
– Syntax error on token “,”, Type expected after
this token

Any reasons why i am getting this error and how to remove this ?

Answer 1

Program to interfaces!

Map<Temp, ? extends Action> p = new HashMap<Temp, ? extends Action>();

Its better practice.

I noticed that perhaps this wasn’t answered correctly. The problem is explained in detail here. What will work for you is perhaps

Map<Temp, ? super Action> p = new HashMap<Temp, Action>();

Hope that helps.

Third edit, I’m new to writing comments so note sure what the process is anyhow here is an example:

public void doSomething ()
{
    Map<A, ? super A> p = new HashMap<A, A>();

    A a1 = new A();
    A a2 = new A();

    // AS A CONSUMER
    // Not a problem because at runtime we'll know we can accept A.
    p.put(a1, new B());
    p.put(a2, new C());

    // AS A PRODUCER
    // what do I cast to? No way of knowing if its B or C.
    for (Object a : p.values())
    {
    }

    // Same deal as above is it B or C?
    p.get(a1);
}

class A{}

class B extends A {}

class C extends B {}