public void testStart_whileRunning() {
    stopwatch.start();
    assertThrows(IllegalStateException.class, stopwatch::start);
    assertTrue(stopwatch.isRunning());
  }

  public void testStop() {
    stopwatch.start();
    assertSame(stopwatch, stopwatch.stop());
    assertFalse(stopwatch.isRunning());
  }

  public void testStop_new() {
    assertThrows(IllegalStateException.class, stopwatch::stop);

  }

  public void testUnregister_notRegistered() {
    assertThrows(IllegalArgumentException.class, () -> registry.unregister(new StringSubscriber()));

    StringSubscriber s1 = new StringSubscriber();
    registry.register(s1);
    assertThrows(IllegalArgumentException.class, () -> registry.unregister(new StringSubscriber()));

    registry.unregister(s1);

    assertThrows(IllegalArgumentException.class, () -> registry.unregister(s1));
  }

    /* UnsupportedOperationException on indirect modifications. */
    NavigableSet<Integer> reverse = unmod.descendingSet();
    assertThrows(UnsupportedOperationException.class, () -> reverse.add(4));
    assertThrows(UnsupportedOperationException.class, () -> reverse.addAll(singleton(4)));
    assertThrows(UnsupportedOperationException.class, () -> reverse.remove(4));
  }

  void ensureNotDirectlyModifiable(SortedSet<Integer> unmod) {

import static com.google.common.collect.testing.features.CollectionSize.SEVERAL;
import static com.google.common.collect.testing.features.CollectionSize.ZERO;
import static com.google.common.collect.testing.testers.ReflectionFreeAssertThrows.assertThrows;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractCollectionTester;
import com.google.common.collect.testing.features.CollectionFeature;

import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly;
import static org.junit.Assert.assertThrows;

import com.google.common.util.concurrent.ClosingFuture.ClosingCallable;
import com.google.common.util.concurrent.ClosingFuture.DeferredCloser;
import java.io.Closeable;
import java.util.concurrent.ExecutionException;

import static java.util.concurrent.Executors.newSingleThreadExecutor;
import static java.util.concurrent.TimeUnit.SECONDS;
import static org.junit.Assert.assertThrows;

import com.google.common.util.concurrent.ClosingFuture.ClosingCallable;
import com.google.common.util.concurrent.ClosingFuture.DeferredCloser;
import com.google.common.util.concurrent.ClosingFuture.ValueAndCloser;

    assertThrows(IllegalStateException.class, () -> sha1.putInt(42));
  }

  public void testHashTwice() {
    Hasher sha1 = Hashing.sha1().newHasher();

    assertEquals(
        "2fd4e1c67a2d28fced849ee1bb76e7391b93eb12",
        sha1.putString("The quick brown fox jumps over the lazy dog", UTF_8).hash().toString());
    assertThrows(IllegalStateException.class, () -> sha1.hash());
  }

import static com.google.common.collect.testing.features.MapFeature.SUPPORTS_PUT;
import static com.google.common.collect.testing.testers.ReflectionFreeAssertThrows.assertThrows;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractMapTester;
import com.google.common.collect.testing.Helpers;
import com.google.common.collect.testing.SampleElements;

import static com.google.common.collect.testing.features.CollectionSize.SEVERAL;
import static com.google.common.collect.testing.features.CollectionSize.ZERO;
import static com.google.common.collect.testing.testers.ReflectionFreeAssertThrows.assertThrows;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractCollectionTester;
import com.google.common.collect.testing.features.CollectionFeature;

import static com.google.common.collect.testing.features.MapFeature.SUPPORTS_PUT;
import static com.google.common.collect.testing.testers.ReflectionFreeAssertThrows.assertThrows;

import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.testing.AbstractMapTester;
import com.google.common.collect.testing.features.CollectionSize;