Why doesnt the following code get compiled? I know changing it to String works but why we are unable to typecast from string to object?
List<Object> c4 = new ArrayList<String>(); Add a comment |
5 Answers
When using generics, the inheritance concept is little bit different. You need to use wildcards and sub typing to achieve inheritance with generics.
As per oracle tutorial
Note: Given two concrete types A and B (for example, Number and Integer),
MyClass<A>has no relationship toMyClass<B>, regardless of whether or not A and B are related. The common parent ofMyClass<A>andMyClass<B>is Object.
4 Comments
MaheshVarma
That link is so helpful
MaheshVarma
List<? extends Object> c4 = new ArrayList<String>() may cause errors at runtime?
Aniket Thakur
No it will not. Again as I said in my comment you cannot add anything to the List in this case.
kosa
Generics are compile time type safety stuff. After compilation generics will be replaced with corresponding nearest objects (type erasure) docs.oracle.com/javase/tutorial/java/generics/erasure.html.
Although String is a subclass of Object, List<String> is not a subclass of List<Object>.
The reason is that if it were a subclass, it would allow the following:
List<String> listA = new ArrayList<String>(); List<Object> listB = listA; // normally a compile error listB.add(new Integer(1)); // it would allow this String s = listA.get(0); // boom! ClassCastException 
Comments
Assuming this worked
List<Object> c4 = new ArrayList<String>(); you could then go add
c4.add(new YourCrazyObjectThatIsNotAString()); which doesn't make sense because c4 should really be an ArrayList of Strings.
Generics were invented in part so that this wouldn't happen. Bohemian and Nambari have the technical reason.
answered Dec 18, 2013 at 5:16
Sotirios Delimanolis
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Comments
List<Object> c4 = new ArrayList<String>(); Now you can add anything that is Object which is technically every Object. Which is incorrect because List is really of String. Hence it is not allowed.
So lets say you add some custom Object to the list. Then while fetching the object it will try to typecast to String which will throw incorrect cast exception. To avoid this ambiguity java does not allow it.
In general
List<SomeObject> l = new ArrayList<SubClassOfSomeObjectClass>; is not allowed. However you can do something like
List<? extends SomeObject> l = new ArrayList<SubClassOfSomeObject>; 2 Comments
MaheshVarma
In this case List<? extends Object> c4 = new ArrayList<String>(); also there is a possibility of exception right ?
Aniket Thakur
Nope! This will compile and run just fine. Only drawback of this is you cannot add anything to the List(except null of-course).
Consider following code:
List<String> ls = new ArrayList<String>(); List<Object> lo = ls; lo.add(new Integer(5)); String x = ls.get(0); Generics are invented to prevent some of class-cast exceptions. But if it was possible to assign List<String> to an List<Object> then you could do everything to break their power.
