Recursive datastructure

Recursive datastructure.
Add prefab values for one of the following types: com.example.Foo.

One of the fields in Foo is of type Foo itself.

Recursive datastructure.
Add prefab values for one of the following types: com.example.Foo, com.example.Bar.

One of the fields in Foo is of type Bar, and one of the fields in Bar is of type Foo.

EqualsVerifier will recursively try to instantiate objects of every class it encounters, but cannot do so if there is a loop in the object graph. In this case it’s necessary to add predefined instances (“prefab values”) of one of the classes involved in the loop. If EqualsVerifier mentions more than one class, it doesn’t matter which one you choose, although if one of these classes is the class that you are testing, it is probably better to choose another.

Example:

EqualsVerifier.forClass(Foo.class)
    .withPrefabValues(Bar.class, new Bar(1), new Bar(2))
    .verify();

If Bar is an interface or an abstract class, it’s fine to use instances from any implementation of Foo.

If you are using withIgnoredFields, you still have to add prefab values. EqualsVerifier will instantiate all fields, even if they are ignored, because it can’t know if these fields aren’t called in equals or hashCode anyway. For example, a class could contain a cache that is consulted in the equals method, even if it’s not part of the equals contract.

Note that static fields are ignored only if they are also final.

Generics

In rare cases involving generics, withPrefabValues might not work.

For instance, Android has a SparseArray type that has an equals method that iterates over its elements and assigns each one to a variable of its generic type. If you have a class with two SparseArrays with different generic parameters that are both checked by equals, EqualsVerifier will fail. For example:

class SparseArrayContainer {
    private final SparseArray<String> strings;
    private final SparseArray<Integer> ints;

    // leaving out everything else for brevity
}

// ...

EqualsVerifier.forClass(SparseArrayContainer.class)
    .withPrefabValues(SparseArray.class, new SparseArray(1), new SparseArray(2, 3))
    .verify();

This will fail because there’s no prefab values for the String variant of SparseArray.

In these cases, you can use withGenericPrefabValues:

EqualsVerifier.forClass(SparseArrayContainer.class)
    .withGenericPrefabValues(SparseArray.class, SparseArray::new)
    .verify();

In this case, EqualsVerifier will generate values of type Integer and String, and provide them to the lambda which can use that value to construct a SparseArray of the correct generic type.