CODEWITHSUNDEEP
javagenericswildcard
gstackoverflow
gstackoverflow

Reputation: 37034

Are super and extends exclusive in generic wildcard?

I am reading book for scjp preparation. I have read that extends in wild card is inclusive operator but super is exclusive.But I cannot find related example. I tryed to write some code.

    List<? super Number> list1 = new ArrayList<Number>();
    List<? extends Number> list2= new ArrayList<Number>();

Both strings compiles good.

I am suppose that I knew something was wrong. Or maybe it is a book bug.

Please help to understand what was it in the book?

Upvotes: 1

Views: 581

Answers (1)

kopytko
kopytko

Reputation: 203

It's a bug in the book, both super and extends operators are inclusive - they accept the type defined in the wildcard. You can see this in both your example and in the following example

    List<? super Number> list1;
    list1 = new ArrayList<Number>();  // ok - inclusive
    list1 = new ArrayList<Object>();  // ok - wildcard accepts the super classes
    list1 = new ArrayList<Integer>(); // compilation error

    List<? extends Number> list2;
    list2 = new ArrayList<Number>();  // ok - inclusive
    list2 = new ArrayList<Object>();  // compilation error
    list2 = new ArrayList<Integer>(); // ok - wildcard accepts the subclasses

Upvotes: 2

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