The assertion error messages from AssertJ will use the toString() method of an object to give more insight about why the assertion could have failed. If an object doesn’t override the toString() method the default implementation is Object#toString(). The default implementation will print out the class name and an hexadecimal value of hashCode separated by a @. This doesn’t give much information about the actual object. For classes we have control over we can always implement a toString() method, but for third party classes we may not be able to do that. In order to customize how an object is represented in assertion error messages, AssertJ allows us to provide a custom representation class for an object. The custom representation class must implement the org.assertj.core.presentation.Representation interface. The interface has one method String toStringOf(Object) that should return a String representation of the object. If we want to keep the default behavior for other classes and only override it for our own class, we can extend the StandardRepresentation class and override the method String fallbackToStringOf(Object).

For a lot of types AssertJ has special assertion methods. Also for the type Optional. If we want to assert an Optional value we can use several methods that AssertJ provides. For example to check if an Optional is present we can use isPresent() or the alias isNotEmpty(). To check if the Optional is empty we can use isEmpty() or the alias isNotPresent(). Checking the value of an Optional (if it is indeed set) can be done with hasValue() or contains(). For more fine grained assertions on the value we can use hasValueSatisfying(Condition) or hasValueSatisfying(Consumer). With the map(Function) and flatMap(Function) methods we can map the Optional, if not empty, to another value and assert that value.

AssertJ has a lot of custom assertion methods for different types. For example to assert an URL object AssertJ gives us some specific methods. We can check for different components of the URL instance with different methods. For example we can check if the protocol is equal to the protocol we expect with hasProtocol(String). Similarly we can write assertions for the host, port, authority, path and anchor. To assert query parameters we can use hasQueryParameter(String) to check if query parameter is set and with hasQueryParameter(String, String) we can check if the query parameter has an expected value. To check the whole query string we can use hasQueryString(String).

Each of the assertion methods also has version to assert a component is not present. For example hasNoQuery() to assert a query is not defined for an URL.

AssertJ has some nice methods to verify string values. If we want to verify a string value is Base64 encoded we can use the isBase64String() method. We can leave out the padding of the value as it is optional. With the method asBase64Decoded() we can decode the value and write our assertions for the decoded value. The method asBase64Decoded() returns a byte[] object and we can use the asString() to convert it into a string value again.

To compare string values we can use the isEqualTo(String) method in AssertJ. But if we want to verify that a string contains a certain variable value we can use string templates. This makes the assertion more readable as we can see what value we expect in the string. To use string templates we must the method isEqualTo(String, Object…​). The first argument is the string template and the following arguments will be the actual values that should be used in the template. Actually the String.format(String, Object…​) method is used behind the scenes to format the string template, but we don’t have to clutter our assertions with that call.

With the returns method in AssertJ we can verify an object using a function. This allows us to verify an object in a very flexible way. We can chain multiple returns method calls to verify multiple aspects of our object. The first argument of the returns method is the expected value of the function call. And the second argument is a function that calls a method on the object we want to verify. A simple function call would be a method reference using the class of the object. But we can also write our own function, where the argument of the function is actual object we are writing the assertion for. To verify the function doesn’t return an expected value we can use the method doesNotReturn.
We can also pass the function to the from method, available in the Assertions class. It can make the assertion more readeable as we can now read the code as: we expect the following value from calling this function.

Writing assertions using the nice fluent API of AssertJ is a joy. Besides some of the basic assertions like isEqualTo AssertJ also has specific assertions for specific types. For example if we want write an assertion to check if a String value starts or ends with an expected value we can use the startsWith(String) or endsWith(String) methods. If we don’t care that a character is upper or lower case we can also use startsWithIgnoringCase(String) or endsWithIgnoringCase(String). Each of the methods also has a counterpart method to check the String value doesn’t start or end with an expected value. For example we can use doesNotStartWith(String) to assert a value does not start with the expected value.

AssertJ has many useful methods to write assertions using a fluid API. If we want to test the toString() method implementation of an object we can of course invoke the toString() method inside an assertThat expression and check with the assert method isEqualTo(String) the value. But AssertJ can make that easier so we don’t have to invoke toString() method ourselves. We can use the assert method hasToString(String) on the object we defined in the assertThat expression and specify our expected value as argument to the method. If we want to assert the toString() method doesn’t return an expected value we can use the assert method doesNotHaveToString(String).