src/main/java/org/fest/assertions/internal/Matchers.java
public <T> void assertIs(Description description, T actual, Matcher<? super T> matcher) {
    checkNotNull(matcher);
    if (!matcher.matches(actual)) {
      String format = "expecting:<%s> to be:<%s>";
      throw failures.failure(description, new BasicErrorMessageFactory(format, actual, matcher));
    }
  }
public <T> void assertIs(Description description, T actual, Matcher<? super T> matcher) {
    checkNotNull(matcher);
    if (!matcher.matches(actual)) {
      String format = "expecting:<%s> to be:<%s>";
      throw failures.failure(description, new BasicErrorMessageFactory(format, actual, matcher));
    }
  }
Asserts that the <em>actual</em> value satisfies the given {@link Matcher}.

@param <T> the type of the actual value and the type of values that given {@code Matcher} takes.
@param description the description of the <em>actual</em> object.
@param actual the <em>actual</em> value.
@param matcher the given {@code Matcher}.
@throws NullPointerException if the given {@code Matcher} is {@code null}.
@throws AssertionError if the <em>actual</em> value does not satisfy the given {@code Matcher}.