Question

我正在使用Hamcrest 1.2库编写一些匹配器，但我在使用Java通配符时遇到了困难。当我尝试编译以下代码时

public class GenericsTest {

    public void doesNotCompile() {
        Container<String> container = new Container<String>();

        // this is the desired assertion syntax
        assertThat(container, hasSomethingWhich(is("foo")));
    }

    // these two are a custom made class and matcher; they can be changed

    public static class Container<T> {
        public boolean hasSomethingMatching(Matcher<T> matcher) {
            T something = null; // here is some application logic
            return matcher.matches(something);
        }
    }

    public static <T> Matcher<Container<T>> hasSomethingWhich(final Matcher<T> matcher) {
        return new TypeSafeMatcher<Container<T>>() {
            @Override
            protected boolean matchesSafely(Container<T> container) {
                return container.hasSomethingMatching(matcher);
            }
        };
    }

    // the following signatures are from the Hamcrest 1.2 library; they cannot be changed

    public static <T> void assertThat(T actual, Matcher<? super T> matcher) {
    }

    public static <T> Matcher<? super T> is(T value) {
        return null;
    }

    public interface Matcher<T> {
        boolean matches(Object item);
    }

    public static abstract class TypeSafeMatcher<T> implements Matcher<T> {
        @SuppressWarnings({"unchecked"})
        @Override
        public final boolean matches(Object item) {
            return matchesSafely((T) item);
        }

        protected abstract boolean matchesSafely(T item);
    }
}

它产生编译错误

$ javac GenericsTest.java
GenericsTest.java:7: <T>assertThat(T,GenericsTest.Matcher<? super T>) in GenericsTest cannot be applied to (GenericsTest
.Container<java.lang.String>,GenericsTest.Matcher<GenericsTest.Container<capture#928 of ? super java.lang.String>>)
        assertThat(container, hasSomethingWhich(is("foo")));
        ^
1 error

如何修改代码以便编译？我在Container类和hasSomethingWhich方法的签名中尝试了? super和? extends的不同组合，但是无法使其编译（不使用显式方法类型参数，但是产生丑陋的代码：GenericsTest.<String>hasSomethingWhich）。

欢迎使用创建简洁且可读的断言语法的替代方法。无论语法是什么，它都应该接受Container作为参数，以匹配Container内的元素。

Answer 1

is(T)匹配器返回一个签名为Matcher<? super T>的匹配器。

因此，如果你解构了assertThat(container, hasSomethingWhich(is("foo")))行，你真正拥有的是：

Matcher<? super String> matcher = is("foo");
assertThat(container, hasSomethingWhich(matcher));

第二行有编译错误，因为hasSomethingWhich方法的签名需要参数Matcher<T>。要匹配hamcrest的is(T)的返回类型，您的签名应该是：

public static <T> Matcher<Container<T>> hasSomethingWhich(final Matcher<? super T> matcher)

（区别在于将参数从Matcher<T>更改为Matcher<? super T>。

这会强制您将hasSomethingWhich()的签名更改为同时接受Matcher<? super T>：

public boolean hasSomethingMatching(Matcher<? super T> matcher)

Here是您发布的原始代码的完全修改版本，可以为我成功编译。

Answer 2

matt正好在<? super T>

中的hasSomethingMatching()/hasSomethingWhich()

使其有效：

    Matcher<Container<String>>  tmp = hasSomethingWhich(is("foo"));
    assertThat(container, tmp);

tmp变量是必要的，javac只会推断该赋值中的T == String，而不是任意表达式。（或者，您可以在调用方法时将T显式指定为String。）

如果eclipse放宽了推理规则，那就违反了语言规范。让我们看看这个例子为什么不合适：

public static <T> Matcher<Container<T>> 
hasSomethingWhich(final Matcher<? super T> matcher)

这种方法具有内在的危险性。给定Match<Object>，它可以返回Matcher<Container<Foo>>，其中Foo可以是任何内容。除非调用者明确提供T，否则编译器必须从上下文推断出T，否则无法知道he T是什么。

语言规范定义了上述赋值语句中的推理规则，因为开发人员的意图绝对清楚T应该是String

更多推理规则的倡导者必须提供他们想要的确切规则集，证明这些规则是安全可靠的，并且对于凡人来说是可理解的。

Answer 3

我能够创建一些解决方法来实现所需的语法。

选项1

一种解决方法是为assertThat方法创建替换，以便将Container<T>作为参数。当方法在不同的类中时，替换断言方法甚至应该能够具有相同的名称。

这需要在? super的返回类型中添加hasSomethingWhich的奇怪内容，并且必须放宽hasSomethingMatching的类型参数。所以代码变得难以理解。

public class GenericsTest {

    public void doesNotCompile() {
        Container<String> container = new Container<String>();

        // this is the desired assertion syntax
        assertThat2(container, hasSomethingWhich(is("foo")));
    }

    public static <T> void assertThat2(Container<T> events, Matcher<? super Container<T>> matcher) {
        assertThat(events, matcher);
    }

    // these two are a custom made class and matcher; they can be changed

    public static class Container<T> {
        public boolean hasSomethingMatching(Matcher<?> matcher) {
            T something = null; // here is some application logic
            return matcher.matches(something);
        }
    }

    public static <T> Matcher<Container<? super T>> hasSomethingWhich(final Matcher<? super T> matcher) {
        return new TypeSafeMatcher<Container<? super T>>() {
            @Override
            protected boolean matchesSafely(Container<? super T> container) {
                return container.hasSomethingMatching(matcher);
            }
        };
    }

    // the following signatures are from the Hamcrest 1.2 library; they cannot be changed

    public static <T> void assertThat(T actual, Matcher<? super T> matcher) {
    }

    public static <T> Matcher<? super T> is(T value) {
        return null;
    }

    public interface Matcher<T> {
        boolean matches(Object item);
    }

    public static abstract class TypeSafeMatcher<T> implements Matcher<T> {
        @SuppressWarnings({"unchecked"})
        @Override
        public final boolean matches(Object item) {
            return matchesSafely((T) item);
        }

        protected abstract boolean matchesSafely(T item);
    }
}

选项2

另一个更简单的解决方案是放弃类型参数，只使用<?>。如果存在类型不匹配，测试将在运行时发现，因此编译时类型安全性几乎没用。

public class GenericsTest {

    public void doesNotCompile() {
        Container<String> container = new Container<String>();

        // this is the desired assertion syntax
        assertThat(container, hasSomethingWhich(is("foo")));
    }

    // these two are a custom made class and matcher; they can be changed

    public static class Container<T> {
        public boolean hasSomethingMatching(Matcher<?> matcher) {
            T something = null; // here is some application logic
            return matcher.matches(something);
        }
    }

    public static Matcher<Container<?>> hasSomethingWhich(final Matcher<?> matcher) {
        return new TypeSafeMatcher<Container<?>>() {
            @Override
            protected boolean matchesSafely(Container<?> container) {
                return container.hasSomethingMatching(matcher);
            }
        };
    }

    // the following signatures are from the Hamcrest 1.2 library; they cannot be changed

    public static <T> void assertThat(T actual, Matcher<? super T> matcher) {
    }

    public static <T> Matcher<? super T> is(T value) {
        return null;
    }

    public interface Matcher<T> {
        boolean matches(Object item);
    }

    public static abstract class TypeSafeMatcher<T> implements Matcher<T> {
        @SuppressWarnings({"unchecked"})
        @Override
        public final boolean matches(Object item) {
            return matchesSafely((T) item);
        }

        protected abstract boolean matchesSafely(T item);
    }
}

Java泛型和通配符：如何编译此代码？

3 个答案: