What is the purpose of the AssertJ library when writing tests?

AssertJ is a library for writing readable and expressive assertions in Java tests. It provides a convenient and extensible API for checking values, collections, exceptions, and other objects, making tests more understandable and concise compared to traditional JUnit assertions.

Example of using AssertJ:

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

@Test
void testString() {
    String actual = "Hello, AssertJ!";
    assertThat(actual)
        .startsWith("Hello")
        .contains("AssertJ")
        .endsWith("!");
}

Thus, AssertJ helps write more expressive and readable tests, facilitating error diagnosis.