adding to an empty list

Question

When I execute the below code it works without any problem.

List<String> singletonList = Collections.singletonList("Hello");
List<String> s = Collections.emptyList();
singletonList.addAll(s);

However, when I try to do following it gives me compile error. Why?

List<String> singletonList = Collections.singletonList("Hello");
singletonList.addAll(List<String> Collections.emptyList());

Collections.emptyList is a type-safe way of creating an empty list. But why doesn't my program compile then? I know I cannot add to immutable list (UnsupportedOperationException) but adding an empty list is allowed. Actually I was testing this and I noticed above thing.

after reading the answers, i'm curious to know what java version you are using. — maxx777
– maxx777, Commented Apr 17, 2014 at 19:07
When you get a compile error and you ask a question about it, always include the exact error message in your question - that makes it a lot easier to help you. — Jesper
– Jesper, Commented Apr 17, 2014 at 19:29

Bhesh Gurung · Accepted Answer · 2014-04-17 19:02:27Z

4

Collections.emptyList() returns List<Object> which isn't (does not have any type relationship with) List<String> so casting is not allowed.

What you need is following:

singletonList.addAll(Collections.<String>emptyList());

answered Apr 17, 2014 at 19:02

Bhesh Gurung

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6 Comments

ParagJ Over a year ago

Thanks. Your code works. But why casting is not allowed? I can write Object o = "abc";

Sotirios Delimanolis Over a year ago

@ParagJ A String is an Object, but a List<String> is not a List<Object>

Bhesh Gurung Over a year ago

There has to be a type relationship (parent-child, child-parent) for the casting to work. e.g. String is an Object.

ParagJ Over a year ago

And why this works then ? List<String> s = Collections.emptyList();

Bhesh Gurung Over a year ago

That's when @SotiriosDelimanolis' answer comes into picture. There is some thing called automatic type inference, where compiler tries to infer the return type based on the context. In version 7 it works in the declaration, but as you can see in his answer, in version 8, in works in method call also.

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Sotirios Delimanolis · Accepted Answer · 2014-04-17 19:05:21Z

2

In Java 8, the type inference is improved. You can do

List<String> singletonList = Collections.singletonList("Hello");
singletonList.addAll(Collections.emptyList());

The type argument for the invocation of emptyList() will be inferred from the context it is used in, ie. it expects a Collection<? extends String>.

answered Apr 17, 2014 at 19:05

Sotirios Delimanolis

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