尝试返回带有泛型的值时出错

Question

我得到这个编译错误"required T, found UseCaseTest"当我试图返回INSTANCE类UseCaseTest的方法keepStateBetweenTests。我知道我可以很容易地将它转换为T，但是

另外，我不知道为什么这行

if (INSTANCE == null) INSTANCE = getInstance(activityTestRule);

可以编译，因为前面的错误。

我再提供两个类来提供特定的上下文。

UseCaseTest

public abstract class UseCaseTest {
    private static UseCaseTest INSTANCE;
    public abstract static class Builder<T extends UseCaseTest> {
        private final ActivityTestRule activityTestRule;
        protected Builder(ActivityTestRule activityTestRule) {
            this.activityTestRule = activityTestRule;
        }
        public T keepStateBetweenTests() {
            if (INSTANCE == null) INSTANCE = getInstance(activityTestRule); 
            return INSTANCE; //compile error: required T, found UseCaseTest
        }
        public T releaseStateBetweenTests() {
            return getInstance(activityTestRule);
        }
        protected abstract T getInstance(ActivityTestRule activityTestRule);
    }
    private final ActivityTestRule activityTestRule;
    public UseCaseTest(ActivityTestRule activityTestRule) {
        this.activityTestRule = activityTestRule;
    }
}

SessionUseCaseTest

public final class SessionUseCaseTest extends UseCaseTest {
    public static Builder<SessionUseCaseTest> with(ActivityTestRule activityTestRule) {
        return new Builder<SessionUseCaseTest> (activityTestRule) {
            @Override protected SessionUseCaseTest getInstance(ActivityTestRule activityTestRule) {
                return new SessionUseCaseTest(activityTestRule);
            }
        };
    }
    private SessionUseCaseTest(ActivityTestRule activityTestRule) {
        super(activityTestRule);
    }

    public void signUp() {}
    public void logout() {}
}

SessionTest

@FixMethodOrder(MethodSorters.NAME_ASCENDING)
public class SessionTest extends BaseTest {
    @Test public void _1_SingUp() {
        SessionUseCaseTest.with(mActivityRule)
                .keepStateBetweenTests()
                .signUp();
    }
    @Test public void _2_Logout() {
        SessionUseCaseTest.with(mActivityRule)
                .keepStateBetweenTests()
                .logout();
    }
}

Answer 1

静态方法/字段与泛型结合的主题在这篇文章中有很好的描述

尽管如此，一个简单的类型转换似乎使编译器很高兴，同时允许保留某种级别的泛型:

public T keepStateBetweenTests() {
    if (INSTANCE == null)
        INSTANCE = getInstance(activityTestRule); 
    return (T)INSTANCE;
}

你的问题的一部分是为什么这可以编译，虽然有错误:

if (INSTANCE == null) INSTANCE = getInstance(activityTestRule);

根据我的理解，编译器知道INSTANCE本身实际上是UseCaseTest，它满足了Builder类上定义的限制:<T extends UseCaseTest>

Answer 2

你需要让你的类意识到类型T，即

public abstract class UseCaseTest <T> {

你可以这样做:-

    class UseCaseTest<T> {
        final Class<T> useCaseTestType;
        public UseCaseTest(Class<T> useCaseTestType) {
            this.useCaseTestType = useCaseTestType;
        }
//Then write your methods which are generic
}