Assert that a block of code is throwing

Assertions.java

public final class Assertions {

	/**
	 * This class is not intended to be instantiated.
	 */
	private Assertions() {
		throw new AssertionError(Assertions.class.getName() + " cannot be instantiated.");
	}

	public static RunnableAssert assertThat(Runnable test) {
		return new RunnableAssert(test);
	}

	public static class RunnableAssert {

		final Runnable test;

		RunnableAssert(Runnable test) {
			this.test = test;
		}

		public void isThrowing(Class<? extends Throwable> expectedException, String expectedMessage) {
			if (expectedException == null) {
				throw new IllegalArgumentException("expectedException is null");
			}
			try {
				test.run();
				throw new AssertionError(expectedException.getName() + " not thrown");
			} catch (AssertionError x) {
				throw x;
			} catch (Throwable x) {
				if (!expectedException.isAssignableFrom(x.getClass())) {
					throw new AssertionError("Expected exception assignable to type "
							+ expectedException.getName()
							+ " but was "
							+ x.getClass().getName()
							+ " with message "
							+ Strings.toString(x.getMessage())
							+ ".");
				}
				final String actualMessage = x.getMessage();
				final boolean isOk = (actualMessage == null) ? (expectedMessage == null)
						: actualMessage.equals(expectedMessage);
				if (!isOk) {
					throw new AssertionError("Expected exception message "
							+ wrapString(expectedMessage)
							+ " but was "
							+ wrapString(actualMessage)
							+ ".");
				}
			}
		}

		private String wrapString(String s) {
			return (s == null) ? "null" : '"' + s + '"';
		}
	}
}

AssertionsTest.java

import org.junit.Test;

public class AssertionsTest {

	@Test
	public void shouldAssertIsThrowing() {

		Assertions.assertThat(() -> {
			throw new RuntimeException("Error");
		}).isThrowing(RuntimeException.class, "Error");

	}

}