Java泛型和通配符：如何使这段代码编译？

Question

我正在使用 Hamcrest 1.2 库编写一些匹配器，但我遇到了困难与 Java 通配符。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);
    }
}

它会产生编译错误

$ 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

如何修改代码以使其能够编译？我尝试过 的不同组合？超级和<代码>？ extends 在 Container 类和 hasSomethingWhich 方法的签名中，但无法使其编译（不使用显式方法类型参数，但这会产生丑陋的代码：GenericsTest. ;hasSomethingWhich）。

还欢迎用于创建简洁且可读的断言语法的替代方法。无论语法如何，它都应该接受 Container 和 Matcher 作为参数，用于匹配 Container 内的元素。

Answer 1

is(T) 匹配器返回一个签名为 Matcher。

因此，如果您解构行 assertThat(container, hasSomethingWhich(is("foo"))) 您真正拥有的是：

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

第二行有一个编译错误，因为您的 hasSomethingWhich< 的签名/code> 方法需要 Matcher 参数。为了匹配 hamcrest 的 is(T) 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 更改为 Matcher< 所示

然后，这将迫使您更改 hasSomethingWhich() 的签名以接受 Matcher，如下

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

： a href="http://gist.github.com/604664" rel="nofollow">Here 是您发布的原始代码的完全修改版本，它对我来说可以成功编译。

Answer 2

马特关于 hasSomethingMatching()/hasSomethingWhich() 中的 super T>

使其工作：

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

tmp 变量是必需的，javac 只会推断 T==String赋值，而不是任意表达式。 （或者您可以在调用该方法时将 T 显式指定为 String）。

如果 eclipse 放松了推理规则，那就违反了语言规范。让我们在这个例子中看看为什么它不合适：

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

这种方法本质上是危险的。给定 Match

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明确，T 应该恰好是 String

更多推理规则的倡导者必须提供他们想要的精确规则集，证明这些规则安全、稳健，并且对于凡人来说都是可以理解的。

Answer 3

我能够创建一些解决方法来实现所需的语法。

选项 1

一种解决方法是创建 assertThat 方法的替代方法，以便它采用 Container 作为参数。当方法位于不同的类中时，替换断言方法甚至应该能够具有相同的名称。

这需要奇怪的添加？例如，hasSomethingWhich 的返回类型和 hasSomethingMatching 的类型参数必须放宽。所以代码变得很难理解。

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

另一种解决方案要简单得多，是放弃类型参数，只使用 。无论如何，测试都会在运行时发现是否存在类型不匹配，因此编译时类型安全几乎没有什么用处。

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);
    }
}