了解Java中的有限泛型.有什么意义呢?

问题描述

我试图理解有界类型，但并不太了解它们的要点.

I am trying to understand bounded types and not quite grasping the point of them.

有一个示例，其中提供此用例:

There is an example of bounded generics on which provides this use case:

public class NaturalNumber<T extends Integer> {

    private T n;

    public NaturalNumber(T n)  { this.n = n; }

    public boolean isEven() {
        return n.intValue() % 2 == 0;
    }

    // ...
}

如果要限制可以作为参数化类型的类，为什么不将所有参数化都忘了，并拥有:

If you are going to restrict the classes that can be the parameterized type, why not just forget the parameterization all together and have:

public class NaturalNumber {

    private Integer n;

    public NaturalNumber(Integer n)  { this.n = n; }

    public boolean isEven() {
        return n.intValue() % 2 == 0;
    }

    // ...
}

那么扩展/实现Integer的任何类都可以与此类一起使用.

Then any class that extends/implements Integer can be used with this class.

还有一个附带的问题:当Java Integer类为final时，在第一个示例中T如何扩展Integer?

Also, a side question: How is T extending Integer in the first example when the Java Integer class is final?

推荐答案

T只能是Integer，因此此处的扩展"纯粹是符号性的. (我从旁注开始，因为，实际上，这是一个泛型无用的示例.我真的不知道为什么本教程认为这是一个有用的演示.不是.)

T can only be Integer, so the "extends" here is purely symbolic. (I'm starting with the side-note because, indeed, it's an example where generics are useless. I truly have no idea why the tutorial thinks this is an informative demonstration. It's not.)

假设T extends Number:

class Example<T extends Number> {
    private T num;

    void setNum(T num) { this.num = num; }
    T    getNum()      { return num;     }
}

因此，泛型的要点通常是可以执行以下操作:

So the point of generics in general, is that you can do this:

Example<Integer> e = new Example<>();
e.setNum( Integer.valueOf(10) );
// returning num as Integer
Integer i = e.getNum();
// and this won't compile
e.setNum( Double.valueOf(10.0) );

泛型是参数多态性的一种形式，从本质上讲，它使我们可以重用具有通用性的代码涉及的类型.

Generics are a form of parametric polymorphism, essentially it lets us reuse code with a generality regarding the types involved.

那么界限的意义是什么?

So what's the point of a bound?

这里的绑定意味着T必须是Number或Number的子类，因此我们可以在T的实例上调用Number的方法.不幸的是，Number本身是一个通常没用的基类(由于精度方面的考虑)，但是它可能会让我们做一些有趣的事情，例如:

A bound here means that T must be Number or a subclass of Number, so we can call the methods of Number on an instance of T. Number is unfortunately a generally useless base class on its own (because of precision concerns), but it might let us do something interesting like:

class Example<T extends Number> extends Number {
//                              ^^^^^^^^^^^^^^
    ...
    @Override
    public int intValue() {
        return num.intValue();
    }
    // and so on
}

例如，更常见的是找到T extends Comparable<T>，这使我们可以对T做一些更有意义的事情.我们可能会有类似的内容:

It's more common, for example, to find T extends Comparable<T> which lets us do something more meaningful with T. We might have something like:

// T must be a subclass of Number
// AND implement Comparable
Example<T extends Number & Comparable<T>>
        implements Comparable<Example<T>> {
    ...
    @Override
    public int compareTo(Example<T> that) {
        return this.num.compareTo(that.num);
    }
}

现在我们的Example类具有自然的顺序.即使我们不知道实际上在类体内是什么，我们也可以对其进行排序.

And now our Example class has a natural ordering. We can sort it, even though we have no idea what T actually is inside the class body.

如果我们结合这些概念，那就:

If we combine these concepts, that:

• 泛型允许外部世界"指定实际类型，并且
• 界限允许内部世界"使用共同点，

我们可以构建以下结构:

we could build constructs such as:

static <T extends Comparable<T>> T min(T a, T b) {
    return (a.compareTo(b) < 0) ? a : b;
}

{
    // returns "x"
    String s = min("x", "z");
    // returns -1
    Integer i = min(1, -1);
}

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