Can you explain what the AssertJ library is and what it is used for in testing?

AssertJ is a library for writing convenient and readable assertions in Java tests. It is used to verify expected results in automated tests, providing a rich and expressive API that allows writing checks in a fluent interface style. This makes tests more understandable and maintainable compared to using standard assert.

Example of using AssertJ:

import static org.assertj.core.api.Assertions.*;

@Test
public void testList() {
    List<String> list = Arrays.asList("apple", "banana", "cherry");
    assertThat(list)
        .isNotEmpty()
        .hasSize(3)
        .contains("banana")
        .doesNotContain("orange");
}

Thus, AssertJ helps to write more expressive and readable tests, improving automation quality.