How to use AssertJ for unit test assertions

AssertJ is a fluent assertion library for writing more readable and maintainable Java unit test assertions. It provides multiple assertion methods to verify the expected results of testing and can be used in conjunction with testing frameworks such as JUnit and TestNG. The main features of AssertJ include: 1. Use smooth API: AssertJ's assertion method adopts a chain call approach, making the assertion statements more readable and reducing the repetition of test code. 2. Provided rich assertion methods: AssertJ provides a large number of assertion methods for verifying the state, behavior, etc. of objects. For example, assertion methods can be used to verify the value of an object, the size of a collection, the content of a string, and so on. 3. Provided readable error information: When the assertion fails, AssertJ provides detailed error information, allowing developers to quickly locate the problem and fix it. Before using AssertJ for unit test assertions, it is necessary to first add the dependencies of AssertJ. In the Maven project, the following dependencies can be added to the pom.xml file: <dependency> <groupId>org.assertj</groupId> <artifactId>assertj-core</artifactId> <version>3.21.0</version> <scope>test</scope> </dependency> Below are several commonly used AssertJ assertion methods and their usage examples: 1. assertThat: Used to assert an object and determine whether a certain condition is met. import static org.assertj.core.api.Assertions.assertThat; public class ExampleTest { @Test public void testAssertThat() { String text = "Hello World"; assertThat(text).isEqualTo("Hello World"); assertThat(text).contains("Hello"); assertThat(text).startsWith("Hello"); assertThat(text).endsWith("World"); } } 2. assertAll: Used to combine multiple assertions and test whether multiple conditions are met. import static org.assertj.core.api.Assertions.*; public class ExampleTest { @Test public void testAssertAll() { Person person = new Person("John", 30); assertAll( () -> assertThat(person.getName()).isEqualTo("John"), () -> assertThat(person.getAge()).isGreaterThan(20), () -> assertThat(person.getAge()).isLessThan(40) ); } } 3. assertThrows: Used to test whether a code block will throw a specific exception. import static org.assertj.core.api.Assertions.*; public class ExampleTest { @Test public void testAssertThrows() { assertThatThrownBy(() -> { // some code that throws an exception throw new IllegalArgumentException(); }).isInstanceOf(IllegalArgumentException.class); } } The above are some commonly used methods of AssertJ, through which diverse assertions can be made to better write unit tests.