Java wildcard difference in 7 and 8

Question

I have the following code, which works fine on Java 8:

List<Class<?>> KEY_NAME_CLASSES = Collections.singletonList(String.class);

But when I try to use the Java 7 compiler, I get an error:

incompatible types: java.util.List<java.lang.Class<java.lang.String>> cannot be converted to java.util.List<java.lang.Class<?>>

Why? Is there some way to use such wildcards in Java 7?

Answer 1

Type inference differs significantly in Java-7 and Java-8. In short, to determine the expression type Java-7 uses only the expression itself while Java-8 can use the surrounding context. So the type of the expression Collections.singletonList(String.class) in Java-7 is the most precise type which could be determined from this expression, namely List<Class<String>>. Java-8 is smarter: it also looks that this expression is assigned to another yet compatible type List<Class<?>>, so it sets the type of Collections.singletonList(String.class) to List<Class<?>> as well.

To make this code working in Java-7 you should specify the generic type explicitly:

List<Class<?>> KEY_NAME_CLASSES = Collections.<Class<?>>singletonList(String.class);