AssertJ is an alternative matching library to the widely used Hamcrest matchers. For my own projects I in fact have changed to solely use AssertJ — I just find the fluid interfaces and extensibility quite appealing.

You can write custom assertions as follows:

Imagine a coffee with a strength and a drink type, such as Espresso or Latte. A custom CoffeeAssert validates coffee instances based on their custom business logic — in this case their properties.

public class CoffeeAssert extends AbstractAssert<CoffeeAssert, Coffee> {

 public CoffeeAssert(Coffee actual) {
 super(actual, CoffeeAssert.class);
 }

 public static CoffeeAssert assertThat(Coffee actual) {
 return new CoffeeAssert(actual);
 }

 public CoffeeAssert hasType(Coffee.Type type) {
 isNotNull();

 if (actual.getType() != type) {
 failWithMessage("Expected the coffee type to be <%s> but was <%s>", type, actual.getType());
 }

 return this;
 }

 // hasStrength(Strength) omitted ...

 public CoffeeAssert isNotDecaf() {
 isNotNull();

 if (actual.getStrength() == Coffee.Strength.DECAF) {
 failWithMessage("Expected a coffee but got decaf!");
 }

 return this;
 }
}

Coffee instances can then simply be validated using the custom assertion. The static import of the assertThat has to refer to CoffeeAssert.

import static com.example.coffee.CoffeeAssert.assertThat;
...

Coffee coffee = new Coffee();
coffee.setStrength(Strength.STRONG);
coffee.setType(Type.ESPRESSO);

assertThat(coffee)
 .hasType(Type.ESPRESSO)
 .isNotDecaf();

The use of custom assertions can vastly improve the quality of your test code.

This post was reposted from my newsletter issue 012

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