Why does the `assertThat` method fail to compile when using `Map

Java Generics: vs. in Method Signatures

Java generics allow you to create type-safe collections and methods that can operate on a range of data types. When using generics, understanding the difference between and is crucial.

Consider the following example:

Map<String, Class<? extends Serializable>> expected = null;
Map<String, Class<Date>> result = null;
assertThat(result, is(expected));

This example generates a compilation error with the following message:

Error: cannot find symbol method assertThat(java.util.Map<java.lang.String,java.lang.Class<java.util.Date>>, org.hamcrest.Matcher<java.util.Map<java.lang.String,java.lang.Class<? extends java.io.Serializable>>>)

Why does this version fail to compile?

The reason lies in the use of in the Matcher class. In its original form, assertThat requires a Matcher that matches the exact type of T. However, in our example, we have a Map of String to Class, where the key is of type String and the value is of type Class. Since our result map contains Class, which is a subtype of Class, the compiler cannot guarantee that the Matcher is compatible with the expected map.

What is the downside of changing assertThat to Matcher?

Changing assertThat to Matcher allows us to pass in a Matcher that matches a supertype of T. While this may seem like a simple change, it can lead to unexpected behavior. For instance, if we have a Matcher that matches a List of Strings, we could pass it to a method that expects a Matcher for a List of Objects. In this case, the Matcher would not be able to correctly match the actual parameter, potentially leading to incorrect results.

Is there a point to genericizing the assertThat method in JUnit?

The genericizing of assertThat in JUnit aims to ensure type safety and prevent mismatches between expected and actual types. However, as discussed above, it can also lead to potential issues if not used carefully.

Recommendation

When deciding between and in method signatures, it is important to consider the following:

• Use when the method requires a supertype of T to be passed in.
• Use when the method requires an exact type match for T.

By following these recommendations, you can avoid compilation errors and ensure type safety when using Java generics.

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