Java泛型和通配符：如何使此代码编译？

2019-10-23 21:12:36 阅读：243 来源： 互联网

标签：compiler-errors generics hamcrest java wildcard

我正在使用Hamcrest 1.2库编写一些匹配器,但Java通配符却很难.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

如何修改代码使其可以编译？我尝试过不同的组合吗？超级和？扩展了Container类和hasSomethingWhich方法的签名,但无法使其进行编译(不使用显式方法类型参数,但是会产生难看的代码：GenericsTest. 最佳答案

is(T)匹配器返回签名为Matcher&lt ;?的Matcher.超级T.

因此,如果您对行assertThat(container,hasSomethingWhich(is(“ foo”)))进行解构,您真正拥有的是：

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

第二行出现编译错误,因为hasSomethingWhich方法的签名需要Matcher< T>参数.为了匹配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>.

然后,这将迫使您将hasSomethingWhich()的签名更改为也接受Matcher<？.超级T像这样：

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.

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