This rule raises an issue when the `Collections.EMPTY_*` fields are used instead of the `Collections.empty*()` methods.

## Why is this an issue?

Generic types (types with type parameters) have been introduced into Java with language version 1.5. If type parameters are specified for a class or method, it is still possible to ignore them to keep backward compatibility with older code, which is called the raw type of the class or interface.

Using raw type expressions is highly discouraged because the compiler cannot perform static type checking on them. This means that the compiler will not report typing errors about them at compile time, but a `ClassCastException` will be thrown during runtime.

In Java 1.5, generics were also added to the Java collections API, and the data structures in `java.util`, such as `List`, `Set`, or `Map`, now feature type parameters. `Collections.EMPTY_LIST`, `Collections.EMPTY_SET`, and `Collections.EMPTY_MAP` are relics from before generics, and they return raw lists, sets, or maps, with the limitations mentioned above.

## How to fix it

Use:

- `Collections.emptyList()` instead of `Collections.EMPTY_LIST`
- `Collections.emptySet()` instead of `Collections.EMPTY_SET`
- `Collections.emptyMap()` instead of `Collections.EMPTY_MAP`

In addition, there are variants of `Collections.empty*()` available also for other collection interfaces, such as `Collections.emptyIterator()`, `Collections.emptyNavigableMap()`, `Collections.emptySortedSet()`.

### Code examples

#### Noncompliant code example

    List<String> collection1 = Collections.EMPTY_LIST;     // Noncompliant, raw List
    Set<Float> collection2 = Collections.EMPTY_SET;        // Noncompliant, raw Set
    Map<Int, String> collection3 = Collections.EMPTY_MAP;  // Noncompliant, raw Map

#### Compliant solution

    List<String> collection1 = Collections.emptyList();    // Compliant, List<String>
    Set<Float> collection2 = Collections.emptySet();       // Compliant, Set<Float>
    Map<Int, String> collection3 = Collections.emptyMap(); // Compliant, Map<Int, String>

## Resources

### Documentation

- Oracle - Java™ Platform, Standard Edition 8 API Specification, Class Collections
- Oracle - The Java™ Tutorials - Raw Types

### Articles & blog posts

- Baeldung - The Basics of Java Generics