具有通用返回类型的Java方法 [英] Java method with generic return Type

问题描述

Java中是否有一种方法可以通过一个方法的声明返回不同的类型？

Is there a way in Java to return different types with one declaration of a method?

public Object loadSerialized(String path) {
    Object tmpObject;

    try {
        FileInputStream fis = new FileInputStream(path);
        ObjectInputStream ois = new ObjectInputStream(fis);
        tmpObject = (Object) ois.readObject();

        ois.close();
        fis.close();

        return tmpObject;
    } catch (FileNotFoundException e) {
        return null;
    } catch (Exception e) {
    }
}

我希望此方法返回一个Object，然后在函数调用中将其云转换为正确的类型。那是我的想法，但这种方式无法正常工作。我需要某种通用的返回类型来做到这一点吗？
解决此问题的最佳方法是什么？

I want this method to return an Object and I cloud cast it to the right type at the function call. That was what i thought but it doesn't work like this. Do I need some kind of generic return Type to do this? What would be the best way to solve this problem?

推荐答案

要安全地执行此操作，您需要传递所需的类型作为Class对象：

To do this safely, you need to pass in the desired type as a Class object:

public <T> T loadSerialized(String path, Class<T> targetType) {
    try (ObjectInputStream ois = new ObjectInputStream(
        new BufferedInputStream(
            new FileInputStream(path)))) {

        Object tmpObject = (Object) ois.readObject();
        return targetType.cast(tmpObject);
    } catch (FileNotFoundException e) {
        return null;
    } catch (IOException | ClassNotFoundException e) {
        throw new RuntimeException(e);
    }
}

可以写 return（T）tmpObject; ，将生成编译器警告，因为它不安全：因为编译器只知道T可能是Object（或Object本身）的后代），编译器会生成（Object），这与不执行任何操作相同。该代码盲目地假定返回的对象是T类型，但是如果不是，则当程序尝试调用T中定义的方法时，您将得到一个意外的异常。最好在对对象进行反序列化后立即知道它是否为预期的类型。

While you could write return (T) tmpObject;, that will generate a compiler warning, because it is not safe: since the compiler only knows that T might be some descendant of Object (or Object itself), the compiler generates (Object), which is the same as doing nothing at all. The code blindly assumes the returned object is of type T, but if it isn’t, when the program tries to call a method defined in T, you’ll get a surprise exception. It’s better to know as soon as you have deserialized the object whether it was the type you expected.

如果对a执行不安全的转换，也会发生类似的情况列表：

A similar thing happens if you do an unsafe cast on, say, a List:

List<Integer> numbers = Arrays.asList(1, 2, 3);

List<?> list = numbers;
List<String> names = (List<String>) list;  // Unsafe!

String name = names.get(0);    // ClassCastException - not really a String!

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