Creating a List of Objects throwing compiler error

Question

I am currently studying generics as part of my programming class, and I'm having problems understanding why the following code throws a compiler error:

List<Object> objs = Arrays.asList(1,"2");

From what I'm aware, if you do not explicitly declare the type parameter for the method, for example Arrays.<Integer>asList(); then it is generated for you, using the most reasonable choice.

The following code:

List<Object> objs = Arrays.<Object>asList(1,"2");

works because i'm explicitly telling the compiler, "I want this method's type parameter to be Object", but I am curious why this is not done successfully automatically?

Do you compilation error for "List<Object> objs = Arrays.asList(1,"2");"? I just tried it and I didn't get any compilation error. — Karl
– Karl, Commented Jan 4, 2016 at 10:29
<? extends Object> List<? extends Object> java.util.Arrays.asList(? extends Object... arg0) — user3650602
– user3650602, Commented Jan 4, 2016 at 10:31
What Java version are you using? Type inference is slowly getting better and your example compiles fine with Java8. — sisyphus
– sisyphus, Commented Jan 4, 2016 at 10:34

ameishutovich · Accepted Answer · 2016-01-04 10:48:21Z

2

This issue appears because different type arguments were passed to a method Arrays.asList, so the compiler tried to find the intersection of all super types of your type arguments.

You created a list with String and int parameters. So compiler found only Serializable as common interface.

This will be compiled:

List<? extends Serializable> list = Arrays.asList(1, "2");

Reference to read: http://www.angelikalanger.com/GenericsFAQ/FAQSections/TechnicalDetails.html#FAQ404

answered Jan 4, 2016 at 10:48

ameishutovich

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Fred Porciúncula · Accepted Answer · 2016-01-04 10:58:01Z

2

List<Object> objs = Arrays.asList(1, "2") will only work with Java 8 :)

Even List<Object> objs = Arrays.asList("a", "b") will compile with Java 8.

Check these references:

edited Jan 4, 2016 at 10:58

answered Jan 4, 2016 at 10:34

Fred Porciúncula

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