When and Why Use `` in Java Generics?

Generics in Java: When and How to Use

In Java, generics allow developers to write code that operates on different data types without having to create multiple versions of the same code. When using generics, it's crucial to understand the difference between using and .

When to Use

The syntax indicates that a generic type parameter can be any subtype of the specified type. For example, in the code below:

Map<String, Class<? extends Serializable>> expected = null;

The expected map can hold Class objects that represent any class that implements Serializable or any of its subclasses.

Why Does Cause Compilation Errors?

In the example provided:

Map<String, Class<java.util.Date>> result = null;
assertThat(result, is(expected));

The result map specifies that it can only hold Date class objects. When checking the type of result, the assertThat method expects a Matcher that fits the type of result. However, the expected map's type, which is Matcher, does not match the expected type, Map>.

Downsides of Switching to

Changing the assertThat method signature to may have unintended consequences, including:

• Breaking compatibility with existing code that uses Matcher directly.
• Potentially allowing invalid matches to occur.

Genericizing the assertThat Method

The genericization of the assertThat method in JUnit is designed to ensure that a compatible Matcher is provided. However, since the Matcher class doesn't require a generic type, the genericization may appear redundant. However, it provides type safety by ensuring that the Matcher can handle the actual type of the result being tested.

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