Creating specific assertions

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Creating its own assertions is simple : create a class inheriting from AbstractAssert and add your custom assertions method.
To be easy to use, you should also add a static method assertThat to provide an handy entry point to your new assertion class.

Let's see how to do that on an example !

The example is taken from fest-examples and more specifically TolkienCharacterAssert.java.

The class we want to make assertion on is TolkienCharacter :

// getter/setter not shown for brevity
public class TolkienCharacter {
  private String name;
  private Race race;
  private int age;
}

We define TolkienCharacterAssert by inheriting from AbstractAssert and specifying the generic parameters, first is the class itself (needed for assertion chaining) and second is the class we want to make assert on : TolkienCharacter.

// javadoc omitted for brevity
// 1 - inherits from AbstractAssert !
public class TolkienCharacterAssert extends AbstractAssert<TolkienCharacterAssert, TolkienCharacter> {

  // 2 - Write a constructor to build your assertion class from the object you want make assertions on.
  public TolkienCharacterAssert(TolkienCharacter actual) {
    super(actual, TolkienCharacterAssert.class);
  }

  // 3 - A fluent entry point to your specific assertion class, use it with static import.
  public static TolkienCharacterAssert assertThat(TolkienCharacter actual) {
    return new TolkienCharacterAssert(actual);
  }

  // 4 - a specific assertion !
  public TolkienCharacterAssert hasName(String name) {
    // check that actual TolkienCharacter we want to make assertions on is not null.
    isNotNull();

    // we overrides the default error message with a more explicit one
    String errorMessage = format("Expected character's name to be <%s> but was <%s>", name, actual.getName());

    // raw assertion : no WritableAssertionInfo usage - simpler but not friendly with custom comparison strategy (i.e. usingComparator method)
    if (!actual.getName().equals(name)) { throw new AssertionError(errorMessage); }

    return this; // return the current assertion for method chaining
  }

  // 4 - another specific assertion !
  public TolkienCharacterAssert hasAge(int age) {
    // check that actual TolkienCharacter we want to make assertions on is not null.
    isNotNull();

    // more complex than previous assertion but integrate better with custom comparison strategy (i.e. usingComparator method)
    WritableAssertionInfo info = new WritableAssertionInfo();
    info.overridingErrorMessage(format("Expected character's age to be <%s> but was <%s>", age, actual.getAge()));
    Objects.instance().assertEqual(info, actual.getAge(), age);
    
    return this; // return the current assertion for method chaining
  }
}