<T extends Comparable<T>>表明T实现了Comaprable<T>接口，此条件强制约束，泛型对象必须直接实现Comparable<T>（所谓直接就是指不能通过继承或其他方式）
<T extends Comparable<? super T>> 表明T的任意一个父类实现了Comparable<? super T>接口,其中? super T表示 ？泛型类型是T的父类（当然包含T）,因此包含上面的限制条件，且此集合包含的范围更广
案例如下
// 有限制的泛型 只允许实现Comparable接口的参数类型
// 从集合中找出最小值
public static <T extends Comparable<T>> T min(List<T> list) {
Iterator<T> iterator = list.iterator();
T result = iterator.next();
while (iterator.hasNext()) {
T next = iterator.next();
if (next.compareTo(result) < 0) {
result = next;
}
}
return result;
}
// <T extends Comparable<? super T>> 限制
// 此方法 多一个 ？ super T 约束 表明可以是T或者T的某个父类实现Comparable接口
public static <T extends Comparable<? super T>> T min2(List<T> list) {
Iterator<T> iterator = list.iterator();
T result = iterator.next();
while (iterator.hasNext()) {
T next = iterator.next();
if (next.compareTo(result) < 0) {
result = next;
}
}
return result;
}
Dog.java
package com.effectJava.Chapter2;
public class Dog extends Animal {
private int age;
private String name;
private String hobby;
public Dog(int id,int age, String name, String hobby) {
super(id);
this.age = age;
this.name = name;
this.hobby = hobby;
}
public void setName(String name) {
this.name = name;
}
public void setHobby(String hobby) {
this.hobby = hobby;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public String getHobby() {
return hobby;
}
public int getAge() {
return age;
}
}
Animal.java
package com.effectJava.Chapter2;
public class Animal implements Comparable<Animal> {
// 唯一标识动物的id
protected int id;
public Animal(int id) {
this.id = id;
}
public Animal() {
}
public void setId(int id) {
this.id = id;
}
public int getId() {
return id;
}
@Override
public int compareTo(Animal o) {
return this.getId() - o.getId();
}
}
运行main函数
List<Dog> dogs = new ArrayList<>();
dogs.add(new Dog(1, 1, "1", "1"));
dogs.add(new Dog(2, 2, "2", "2"));
dogs.add(new Dog(3, 3, "3", "3"));
// 编译错误 由于Dog没有直接实现Comparable<Dog>接口
min(dogs);
// 由于Dog父类Animal实现了Comparable<Dog>接口
min2(dogs);
