有时您可能希望限制允许传递给类型参数的类型.例如，对数字进行操作的方法可能只想接受Number或其子类的实例.这是有界类型参数的用途.
要声明有界类型参数，请列出类型参数的名称，然后是extends关键字，后跟其上限.
示例
以下示例说明了如何在一般意义上使用extends来表示"extends"(如在类中)或"implements"(如在interfaces中).这个例子是Generic方法，用于返回三个Comparable对象中最大的一个 :
public class MaximumTest { // determines the largest of three Comparable objects public static > T maximum(T x, T y, T z) { T max = x; // assume x is initially the largest if(y.compareTo(max) > 0) { max = y; // y is the largest so far } if(z.compareTo(max) > 0) { max = z; // z is the largest now } return max; // returns the largest object } public static void main(String args[]) { System.out.printf("Max of %d, %d and %d is %d\n\n", 3, 4, 5, maximum( 3, 4, 5 )); System.out.printf("Max of %.1f,%.1f and %.1f is %.1f\n\n", 6.6, 8.8, 7.7, maximum( 6.6, 8.8, 7.7 )); System.out.printf("Max of %s, %s and %s is %s\n","pear", "apple", "orange", maximum("pear", "apple", "orange")); }} 这将产生以下结果 :
输出
Max of 3, 4 and 5 is 5Max of 6.6,8.8 and 7.7 is 8.8Max of pear, apple and orange is pear