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 ?
Have this:
HashMap <Temp, ? extends Action > p= new HashMap<Temp, *>();
You need two types for a map. Type for key, and a type for value. You cannot surround the second (value) type with < & > because it is just not allowed.
I also recommend programming to an interface, note the declared type is an interface:
Map<Temp, ? extends Action > p = new HashMap<Temp, ? extends Action>();
EDIT
As pointed out, you cannot instantiate a variable with a wildcard generic type. Please change * to the type that you need.
This will compile :
Map<Temp, ? extends Action> p = new HashMap<Temp, Action>();
Or with Java 7:
Map<Temp, ? extends Action> p = new HashMap<>();
You had extra < and > in the value type parameter, causing a syntax error. Past that, you can't instantiate a generic type using a wildcard as a type parameter.
You'll also notice I changed the variable to Map instead of HashMap - it's good practice to program to interfaces instead of implementations, like winged's answer mentions.
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 {}
? super Action? Isn't that different from the variable the OP's trying to assign to?
<>around? extends Action...and you'll be closer to being right.