Assertj verify that a field of each items in a collection is always null

Question

I'm looking for a solution to check that each items in a collection have the field expectedNullField null.

The following doesn't work:

assertThat(aCollection).extracting("expectedNullField").isNull();

Note that the following works as expected: assertThat(aCollection).extracting("expectedNotNullField").isNotNull();

Anybody to help me ?

Thanks.

Answer 1

If you know the size (let's say it is 3) you can use

assertThat(aCollection).extracting("expectedNullField")
                       .containsOnly(null, null, null);

or if you are only interested in checking that there is a null value

assertThat(aCollection).extracting("expectedNullField")
                       .containsNull();

Note that you can't use:

    assertThat(aCollection).extracting("expectedNullField")
                           .containsOnly(null);

because it is ambiguous (containsOnly specifying a varargs params).

I might consider adding containsOnlyNullElements() in AssertJ to overcome the compiler error above.

Answer 2

You can use a Condition

Condition<YourClass> nullField = new Condition<>("expecting field to be null") {
  @Override
  public boolean matches(YourClass value) {
    return value.getField() == null;
  }
};

assertThat(aCollection).have(nullField);

which might be easier to read than the other solution

assertThat(aCollection).filteredOn("expectedNullField", not(null)).isEmpty();