简单的泛型循环形参
/**
* 返回Collections中的最大值（取决于compare方法的实现）
* @author kkyeer
* @date 2018/10/18 21:57
* @param collection 要比较的入参
* @param <E> recursive parameter type
* @return E Collections中的最大值
*/
public static <E extends Comparable<E>> E max(Collection<E> collection) {
if (collection.isEmpty()) {
throw new IllegalArgumentException();
}
Iterator<E> iterator = collection.iterator();
E current = iterator.next();
while (iterator.hasNext()) {
E next = iterator.next();
if (next.compareTo(current)>0) {
current = next;
}
}
return current;
}
方法定义中用到了泛型中循环形参表示法<E extends Comparable<E>>，按Effective Java的说法，此表示法的意义是，E为任何可与自身比较的类
The type bound <E extends Comparable<E>> may be read as “any type E that can
be compared to itself,” which corresponds more or less precisely to the notion of
mutual comparability
个人认为，同样可以理解为，E表示任何实现了Comparable接口的类
此处包含以下几重含义
- 使用时传入的实际类，应实现Comparable接口
- 使用时传入的类，在实现Comparable接口时使用的为泛型定义的方式，非RawType
以上两点可以保证在while循环中，使用next.compareTo(current)方法时是类型安全的
Builder模式中的循环形参
Effective Java 中有介绍一种Builder写法，具体如下
- 父类
/**
* @author kkyeer@gmail.com
* @date 2018/10/1 17:49
*/
public abstract class AbsPerson {
int age;
String name;
String id;
AbsPerson(AbsBuilder absBuilder) {
this.age = absBuilder.age;
this.name = absBuilder.name;
this.id = absBuilder.id;
}
protected abstract static class AbsBuilder<T,B extends AbsBuilder<T,B>> {
int age;
String name;
String id;
public AbsBuilder(int age, String name) {
this.age = age;
this.name = name;
}
@SuppressWarnings("unchecked")
public B id(String id) {
this.id = id;
return self();
}
protected abstract B self();
abstract T build();
}
}
- 子类
/**
* @author kkyeer@gmail.com
* @date 2018/10/2 21:29
*/
public class Student extends AbsPerson {
private int classNo;
private Student(StuBuilder builder) {
super(builder);
this.classNo = builder.classNo;
}
public static class StuBuilder extends AbsPerson.AbsBuilder<AbsPerson,StuBuilder> {
int classNo;
public StuBuilder(int age, String name, int classNo) {
super(age, name);
this.classNo = classNo;
}
public StuBuilder classNo(int classNo) {
this.classNo = classNo;
return this;
}
@Override
protected StuBuilder self() {
return this;
}
@Override
Student build() {
return new Student(this);
}
}
@Override
public String toString() {
return "Student{" +
"classNo=" + classNo +
", age=" + age +
", name='" + name + '\'' +
", id='" + id + '\'' +
'}';
}
}
注1：此处泛型保证AbsBuilder的使用泛型的子类，必须在泛型处声明此builder对应build出的类型T，第二个形参为子类本身，保证id方法返回值为子类类型
如以下声明是可以编译通过的
public static class StuBuilder extends AbsPerson.AbsBuilder<AbsPerson,StuBuilder>{...}
下面的是有编译报错的
public static class WrongExampleBuilder extends AbsPerson.AbsBuilder<WrongExample, AbsBuilder>
