A simple example

Let’s start with a simple example showing a few important things.

import static org.assertj.core.api.Assertions.assertThat; (1)

import org.junit.jupiter.api.Test;

public class SimpleAssertionsExample {

  @Test
  void a_few_simple_assertions() {
    assertThat("The Lord of the Rings").isNotNull()  (2) (3)
                                       .startsWith("The") (4)
                                       .contains("Lord") (4)
                                       .endsWith("Rings"); (4)
  }

}

1	Statically import `org.assertj.core.api.Assertions.assertThat`
2	Pass the object under test as the sole `assertThat()` parameter
3	Use code completion to discover and call assertions
4	Chain as many assertions as you need

Except for isNotNull which is a base assertion, the other assertions are String specific as our object under test is a String.