如何检查通用类型是否在java中实现特定类型的通用接口？ [英] How to check if a generic type implements a specific type of generic interface in java?

问题描述

可能重复：

运行时的常规类型的局部变量

我刚刚接触Java泛型，来自.NET世界，我习惯于能够编写一个这样的方法：

I'm new to Java generics, and coming from a .NET world, I'm used to being able to write a method like this:

public void genericMethod<T>(T genericObject)
{
    if (genericObject is IList<String>)
    {
        //Do something...
    }            
}

此方法接受通用类型的对象，该对象是否实现通用接口 IList<> 的特定版本，在这种情况下为 IList< String>

The method accepts an object of a generic type, and checks whether that object implements a specific version of the generic interface IList<>, in this case, IList<String>.

现在，在Java中，我可以这样做：

Now, in Java, I'm able to do this:

public <T> void genericMethod(T genericObject)
{
    if (genericObject instanceof Set<?>)
    {
       //Do something...
    }
}

但

Java不会 让我做 if（genericObject instanceof Set< String>）

Java does not let me do if (genericObject instanceof Set<String>)

从我知道的，因为类型擦除，通常在Java中，这将由一个类对象，我们将做类似下面的事情：

From what I know, because of type erasure, normally in Java this would be taken care of by a class object, and we would do something like the following:

public <T> void genericMethod(T genericObject)
{
    Class<OurTestingType> testClass = OurTestingType.class;
    if (genericObject.getClass() == testClass)
    {
       //Do something...
    }
}

但是因为我检查的类型是一个通用接口，你不能这样做：

but since the type I'm checking for is a generic interface, you can't do this:

Class< Set< String>> testClass = Set< String> .class

那么，在Java中如何检查一个通用对象是否实现了设置< String> ？

So, how, in Java, do I check if a generic object implements the specific type of Set<String>?

推荐答案

Java实现擦除，在运行时告诉如果 genericObject 是 Set< String> 的实例。保证这一点的唯一方法是使用你的泛型的边界，或者检查集合中的所有元素。

Java implements erasure, so there's no way to tell on runtime if genericObject is an instance of Set<String> or not. The only way to guarantee this is to use bounds on your generics, or check all elements in the set.

使用边界检查：

public <T extends SomeInterface> void genericMethod(Set<? extends T> tSet) {
    // Do something with tSet here
}

使用迭代和类型检查：

public <T> void genericMethod(T t) {
    Set<String> strs = new HashSet<String>();
    Set<?> tAsSet;
    if (t instanceof Set<?>) {
        tAsSet = (Set<?>) t;
        for (Object obj : tAsSet) {
            if (obj instanceof String) {
                strs.add((String) obj);
            }
        }
        // Do something with strs here
    } else {
        // Throw an exception or log a warning or something.
    }
}

编辑： Mark Peters的注释如下：如果你可以将它添加到你的项目，Guava也有你的方法：

As per Mark Peters' comment below, Guava also has methods that do this for you if you can add it to your project:

public <T> void genericMethod(T t) {
    if (t instanceof Set<?>) {
        Set<?> set = (Set<?>) t;
        if (Iterables.all(set, Predicates.instanceOf(String.class))) {
            Set<String> strs = (Set<String>) set;
            // Do something with strs here
        }
    }
}

语句 Iterables.all（set，Predicates.instanceOf（String.class））本质上与 set instanceof设置< String> 。

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