Why is there no type conversion exception in this code?

Question

This is from Thinking in Java

class Snow {}
class Powder extends Snow {}
class Light extends Powder {}
class Heavy extends Powder {}
class Crusty extends Snow {}
class Slush extends Snow {}

public class AsListInference {
    public static void main(String[] args) {
        //The book says it won't compile, but actually it does.
        List<Snow> snow2 = Arrays.asList(new Light(), new Heavy());
    }
}

Here is my Java enviroment:

java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

Because there should not be one. Light and Heavy are subclasses of Snow, and can therefore be added to a List of Snow. — hotzst
– hotzst, Commented Oct 17, 2015 at 11:05
In Java 7 this code gives me a compilation error - Type mismatch: cannot convert from List<Powder> to List<Snow>. — Eran
– Eran, Commented Oct 17, 2015 at 11:09

Tunaki · Accepted Answer · 2015-10-17 11:13:19Z

6

Actually, the book is right. The difference here is the Java version.

Thinking in Java targets Java 5/6 (as per the cover). For this version of Java (and with Java 7 also), the snippet won't compile with javac. The error is:

incompatible types: java.util.List<Powder> cannot be converted to java.util.List<Snow>

With Java 8, this compiles just fine: the type-inference system was improved.

answered Oct 17, 2015 at 11:13

Tunaki

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chrylis -cautiouslyoptimistic- · Accepted Answer · 2015-10-17 11:09:18Z

2

The book is apparently expecting the compiler to determine the type of the right-hand side as List<Powder>, which is not a List<Snow>. However, since Arrays.<T>asList(T ...) has its own self-contained scope for T, the compiler is able to infer that the correct bound should be Snow and that both Light and Heavy are Snow.

I don't have a Java 7 compiler on hand, but Java 8 did bring some improvements in generic type inference that may be what lets the compiler solve this particular type bound.

answered Oct 17, 2015 at 11:09

chrylis -cautiouslyoptimistic-

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