The Java™ Tutorials — Generics ：Generic Methods and Bounded Type Parameters 泛型方法和受限类型参数

原文地址：https://docs.oracle.com/javase/tutorial/java/generics/boundedTypeParams.html

关键点

• 泛型算法实现的关键：利用受限类型参数

全文翻译

Bounded type parameters are key to the implementation of generic algorithms. Consider the following method that counts the number of elements in an array T[] that are greater than a specified element elem.

受限泛型参数是实现泛型算法的关键。看看下面的方法，它会计算泛型数组中比指定元素elem更大的元素的个数。

public static <T> int countGreaterThan(T[] anArray, T elem) {
    int count = 0;
    for (T e : anArray)
        if (e > elem)  // compiler error
            ++count;
    return count;
}

The implementation of the method is straightforward, but it does not compile because the greater than operator (>) applies only to primitive types such as short, int, double, long, float, byte, and char. You cannot use the > operator to compare objects. To fix the problem, use a type parameter bounded by the Comparable<T> interface:

方法的实现非常直接，但编不过。因为大于操作符（>）仅对原始类型如short、int，double、long、float、byte和char有效。你不能用“>”去比较对象。为了搞定这个问题，你需要使用一个受Comparable<T>接口限定的类型参数。

public interface Comparable<T> {
    public int compareTo(T o);
}

The resulting code will be:

代码就改写为了这样：

public static <T extends Comparable<T>> int countGreaterThan(T[] anArray, T elem) {
    int count = 0;
    for (T e : anArray)
        if (e.compareTo(elem) > 0)
            ++count;
    return count;
}