如果 Package Map并且只允许放置值为 SummaryGenerator<T> 匹配 Class<T> . 得到 SummaryGenerator 您需要强制转换，但由于您确保只添加了实际上可以强制转换到的条目 SummaryGenerator<T> ，这样做是安全的。如果您试图添加不兼容的 SummaryGenerator 对于给定的实现 Car ，这将导致编译错误。

public class SummaryGeneratorStorage {

    private static final Map<Class<? extends Car>, CarSummarizer.SummaryGenerator<? extends Car>> map = new HashMap<>();

    // provides type safety to only add a SummaryGenerator<T> for a key of Class<T>
    public static <T extends Car> void add(Class<T> clazz, CarSummarizer.SummaryGenerator<T> sg) {
        map.put(clazz, sg);
    }

    @SuppressWarnings("unchecked")
    public static <T extends Car> CarSummarizer.SummaryGenerator<T> get(T car) {
        // this cast is safe since the add-method only
        // allows a SummaryGenerator<T> to be added for a key of Class<T>
        return (CarSummarizer.SummaryGenerator<T>) map.get(car.getClass());
    }

    private SummaryGeneratorStorage() { }
}

为了得到一个 Summary ，则检索已注册的 SummaryGenerator 对于 Car 并致电 generateSummary 方法。如果没有 SummaryGenerator 为已通过的实现找到 Car ，只需抛出一个异常或以不同的方式处理它。请注意 add 需要 Class 而不是 Car 对象，因为此时不需要示例来引用的实现 Car .

public class CarSummarizer {

    public interface SummaryGenerator<T extends Car> {
        Summary generateSummary(T car);
    }

    static {
        // here you cannot pass incompatible implementations of Car (compile-time safety)
        SummaryGenerator<CarA> aGen = c -> new Summary(c.getASpecifics());
        SummaryGeneratorStorage.add(CarA.class, aGen); // with variable
        SummaryGeneratorStorage.add(CarB.class, c -> new Summary(c.getBSpecifics())); // direct
    }

    public static <T extends Car> Summary getSummary(T car) {
        SummaryGenerator<T> generator = SummaryGeneratorStorage.get(car);
        if (generator != null) {
            return generator.generateSummary(car);
        } else {
            throw new IllegalArgumentException("no summary generator found");
        }
    }
}

下面是上述代码的测试类。简单的 Summary 使用了包含名称字段的 Car 在其字段中返回a、b或c getXSpecifics() 方法：

public class CarSummarizerTest {

    @Test
    public void testCarA() {
        CarA car = new CarA();
        Summary summary = CarSummarizer.getSummary(car);
        Assert.assertEquals(car.getASpecifics(), summary.getName());
    }

    @Test
    public void testCarB() {
        CarB car = new CarB();
        Summary summary = CarSummarizer.getSummary(car);
        Assert.assertEquals(car.getBSpecifics(), summary.getName());
    }

    @Test
    public void testCarC() {
        CarC car = new CarC();
        Assert.assertThrows(IllegalArgumentException.class, () -> {
            CarSummarizer.getSummary(car);
        });
    }
}

对于其他类别的完整性：

public class CarA implements Car {
    public String getASpecifics() { return "A"; }
}

public class CarB implements Car {
    public String getBSpecifics() { return "B"; }
}

public class CarC implements Car {
    public String getCSpecifics() { return "C"; }
}

public class Summary {
    private final String name;
    public Summary(String name) { this.name = name; }
    public String getName() { return name; }
}