* that are equal to each other but unequal to the objects in any other group. For example:
 *
 * <pre>
 * new EqualsTester()
 *     .addEqualityGroup(new User("page"), new User("page"))
 *     .addEqualityGroup(new User("sergey"))
 *     .testEquals();
 * </pre>
 *
 * <p>This tests:
 *
 * <ul>
 *   <li>comparing each object against itself returns true
 *   <li>comparing each object against null returns false

    new EqualsTester()
        .addEqualityGroup(FALSE, Predicates.alwaysFalse())
        .addEqualityGroup(TRUE)
        .addEqualityGroup(Predicates.not(FALSE))
        .testEquals();

    new EqualsTester()
        .addEqualityGroup(Predicates.isNull(), Predicates.isNull())
        .addEqualityGroup(Predicates.notNull())
        .addEqualityGroup(Predicates.not(Predicates.isNull()))
        .testEquals();

    new EqualsTester()
        .addEqualityGroup(f, Functions.constant("correct"))
        .addEqualityGroup(Functions.constant("incorrect"))
        .addEqualityGroup(Functions.toStringFunction())
        .addEqualityGroup(g)
        .testEquals();

    new EqualsTester()
        .addEqualityGroup(g, Functions.<@Nullable Object>constant(null))
        .addEqualityGroup(Functions.constant("incorrect"))

  public void testWrap() {
    new EqualsTester()
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("world"))
        .addEqualityGroup(LENGTH_EQUIVALENCE.wrap("hi"), LENGTH_EQUIVALENCE.wrap("yo"))
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.<@Nullable String>wrap(null),

  public void testWrap() {
    new EqualsTester()
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("hello"),
            LENGTH_EQUIVALENCE.wrap("world"))
        .addEqualityGroup(LENGTH_EQUIVALENCE.wrap("hi"), LENGTH_EQUIVALENCE.wrap("yo"))
        .addEqualityGroup(
            LENGTH_EQUIVALENCE.<@Nullable String>wrap(null),

    new EqualsTester()
        .addEqualityGroup(Hashing.concatenating(asList(Hashing.md5())))
        .addEqualityGroup(Hashing.concatenating(asList(Hashing.murmur3_32())))
        .addEqualityGroup(
            Hashing.concatenating(Hashing.md5(), Hashing.md5()),
            Hashing.concatenating(asList(Hashing.md5(), Hashing.md5())))
        .addEqualityGroup(

    new EqualsTester()
        .addEqualityGroup(Range.all())
        .addEqualityGroup(Range.atLeast(1), Range.downTo(1, CLOSED))
        .addEqualityGroup(Range.greaterThan(1), Range.downTo(1, OPEN))
        .addEqualityGroup(Range.atMost(7), Range.upTo(7, CLOSED))
        .addEqualityGroup(Range.lessThan(7), Range.upTo(7, OPEN))
        .addEqualityGroup(Range.open(1, 7), Range.range(1, OPEN, 7, OPEN))

    FreshValueGenerator generator = new FreshValueGenerator();
    T instance = generator.newFreshProxy(interfaceType);
    new EqualsTester()
        .addEqualityGroup(wrapperFunction.apply(instance), wrapperFunction.apply(instance))
        .addEqualityGroup(wrapperFunction.apply(generator.newFreshProxy(interfaceType)))
        // TODO: add an overload to EqualsTester to print custom error message?
        .testEquals();
  }

    new EqualsTester()
        .addEqualityGroup(f, Functions.constant("correct"))
        .addEqualityGroup(Functions.constant("incorrect"))
        .addEqualityGroup(Functions.toStringFunction())
        .addEqualityGroup(g)
        .testEquals();

    new EqualsTester()
        .addEqualityGroup(g, Functions.<@Nullable Object>constant(null))
        .addEqualityGroup(Functions.constant("incorrect"))

    new EqualsTester()
        .addEqualityGroup(Range.all())
        .addEqualityGroup(Range.atLeast(1), Range.downTo(1, CLOSED))
        .addEqualityGroup(Range.greaterThan(1), Range.downTo(1, OPEN))
        .addEqualityGroup(Range.atMost(7), Range.upTo(7, CLOSED))
        .addEqualityGroup(Range.lessThan(7), Range.upTo(7, OPEN))
        .addEqualityGroup(Range.open(1, 7), Range.range(1, OPEN, 7, OPEN))