## Why is this an issue?

Assertions comparing incompatible types always fail, and negative assertions always pass. At best, negative assertions are useless. At worst, the developer loses time trying to fix his code logic before noticing wrong assertions.

Dissimilar types are:

- comparing a primitive with null
- comparing an object with an unrelated primitive (E.G. a string with an int)
- comparing unrelated classes
- comparing an array to a non-array
- comparing two arrays of dissimilar types

This rule also raises issues for unrelated `class` and `interface` or unrelated `interface` types in negative assertions. Because except in some corner cases, those types are more likely to be dissimilar. And inside a negative assertion, there is no test failure to inform the developer about this unusual comparison.

Supported test frameworks:

- JUnit4
- JUnit5
- AssertJ

### Noncompliant code example

    interface KitchenTool {}
    interface Plant {}
    class Spatula implements KitchenTool {}
    class Tree implements Plant {}

    void assertValues(int size,
                      Spatula spatula, KitchenTool tool,  KitchenTool[] tools,
                      Tree    tree,    Plant       plant, Tree[]        trees) {

      // Whatever the given values, those negative assertions will always pass due to dissimilar types:
      assertThat(size).isNotNull();           // Noncompliant; primitives can not be null
      assertThat(spatula).isNotEqualTo(tree); // Noncompliant; unrelated classes
      assertThat(tool).isNotSameAs(tools);    // Noncompliant; array & non-array
      assertThat(trees).isNotEqualTo(tools);  // Noncompliant; incompatible arrays

      // Those assertions will always fail
      assertThat(size).isNull();                       // Noncompliant
      assertThat(spatula).isEqualTo(tree);             // Noncompliant

      // Those negative assertions are more likely to always pass
      assertThat(spatula).isNotEqualTo(plant); // Noncompliant; unrelated class and interface
      assertThat(tool).isNotEqualTo(plant);    // Noncompliant; unrelated interfaces
    }

## Resources

- {rule:java:S2159} - Silly equality checks should not be made