Java集合、泛型和抽象类:具体类信息丢失

Bar (可能还有许多其他类)扩展抽象类 AbstractFoo . 转换的实例时 酒吧 到 FooDTO ,检测到具体类。

但是,在转换 实例到 ,具体类信息将丢失,并根据 AbstractFoo公司

这里怎么了?

public class CollectionGenericsNGTest {


    public static abstract class AbstractFoo { }

    public static class Bar extends AbstractFoo { }

    public static class FooDTO {
        final boolean isBar;

        public FooDTO(AbstractFoo f) {
            this.isBar = false;
        }

        public FooDTO(Bar b) {
            this.isBar = true;
        }
    }

    public static class FooDTOList {
        List<FooDTO> list;

        public FooDTOList(Collection<? extends AbstractFoo> source) {
            list = source.stream()
                    .map(entry -> new FooDTO(entry))
                    .collect(Collectors.toList());
        }

        public List<FooDTO> getList() {
            return list;
        }
    }

    @Test
    public void testDTO() {
        Bar b = new Bar();
        FooDTO f = new FooDTO(b);

        assertTrue(f.isBar);
    }

    @Test
    public void testDTO_abstract() {
        AbstractFoo b = new Bar();
        FooDTO f = new FooDTO(b);

        assertTrue(f.isBar); // <-- fails, too
    }

    @Test
    public void testDTOList() {
        Bar b = new Bar();
        List<Bar> collection = Arrays.asList(b);
        FooDTOList list = new FooDTOList(collection);

        FooDTO f = list.getList().get(0);
        assertTrue(f.isBar); // <--- this fails!
    }

}