AssertJ cheatsheet

AssertJ is a an assertions library for unit tests in Java that is well worth considering. Here are some examples.

Basic

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

assertThat(something).isNotNull();
assertThat(something).isNull();
assertThat(something).isEqualTo(expectation);
assertThat(something.getId()).isGreaterThan(0);

Exceptions

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

assertThatThrownBy(() -> obj.action())
    .isInstanceOf(IOException.class);

assertThatThrownBy(() -> obj.otherAction())
    .isInstanceOf(TimeoutException.class)
    .hasMessage("oops")
    .hasFieldOrPropertyWithValue("code", HttpStatus.GATEWAY_TIMEOUT);

Dates

import java.time.OffsetDateTime;
import java.time.temporal.ChronoUnit;
import static org.assertj.core.api.Assertions.within;

assertThat(obj.getCreatedAt())
    .isCloseTo(OffsetDateTime.now(), within(500, ChronoUnit.MILLIS));

Projections

assertThat(result.getParcels())
    .extracting(Parcel::getWeight)
    .isEqualTo(Arrays.asList(new BigDecimal(10), new BigDecimal(20)));