Java generics unchecked casts - wildcard framework
I'm trying to get my head around generics, but I block with wildcard constraints and drops. For example:
List<? extends Number> l = new ArrayList<>();
List<PositiveInteger> p = new ArrayList<PositiveInteger>();
p.add(new PositiveInteger(10));
l = p;
NegativeInteger ni = (NegativeInteger) l.get(0);// runtime java.lang.ClassCastException:
System.out.println(ni);
NegativeNumber and PositiveNumber increase the number. So the question is, why is there no warning at compile time when casting an item from a list to NegativeNumber? Is there a way to prevent this type of exception?
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Ok, the same will happen if you do:
Object a;
Integer b = new Integer(5);
a=b
Double c = (Double)a;
ClassCastException occurs because you are trying to convert a class to another class that is not a parent / child class
regarding the question of preventing this:
the compiler doesn't know what will happen at l
runtime. but you can use List<PositiveInteger>
a wildcard instead to tell the compiler what will be in l
. if you need to use List<? extends Number>
, you can create a method that tries to pass an object, this way you can determine if there is an object List<PositiveInteger>
or List<NegativeInteger>
like here :
if(l instanceof List<NegativeInteger>){
//Do something
}
so if I change your code accordingly:
List<? extends Number> l = new ArrayList<>();
List<PositiveInteger> p = new ArrayList<PositiveInteger>();
p.add(new PositiveInteger(10));
l = p;
Object ni = l.get(0);
if(ni instanceof NegativeInteger){
NegativeInteger tmp = (NegativeInteger)ni;
System.out.println(ni);
}
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When you define a Generic type using type wildcards <? extends Number>
, you should not be throwing objects into Number subclasses, because the generic type says you will get an instance of the subclasses Number
. Like this piece of code:
Number n = new PositiveInteger(10);
NegativeInteger ni = (NegativeInteger ) n;
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Once you do the casting, you tell the compiler "I know what I'm doing", so it won't complain as long as it's possible to create an object (NegativeNumber extends Number -> ok).
If you really need to do the cast (which you want to avoid, generics are all it takes to compile and avoid "unsafe" dumps), you can test it first like this:
Number number = l.get(0);
if (number instanceof NegativeInteger){
NegativeInteger ni = (NegativeInteger) l.get(0);
...
}
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