JDK8
Lambda
Lambda expression ennable u to treat functionality as method argument, or code as data.
将 function 作为方法参数，或将代码视为数据
参考文档：^1
package com.tyrantqiao.algorithms.lambda;
import java.util.List;
/**
* @author tyrantqiao
* @ClassName PersonBusiness
* @date 2022/2/13 15:41
* @version 1.0
* @Description 个人业务的报表查询
*/
public class PersonBusiness {
public static String filterPersonsWithinAgeRange(List<Person> personList, int low, int high){
StringBuilder stringBuilder=new StringBuilder();
for(Person person:personList){
if(person.getAge()>=low && person.getAge()<=high){
stringBuilder.append(person.getName()).append("|");
}
}
return stringBuilder.toString();
}
}
System.out.println(PersonBusiness.filterPersonsWithinAgeRange(personList, 0, 100));
像以前就是来一份业务写一份 static 以及涉及到的业务代码，方法等都要写死
但根据抽象时，我们可以发现这部分代码要实现的逻辑都是过滤
if(filterBoolean(personList)){
stringBuilder.append(xxx);
}
由于 Java 不支持将 function 作为参数传递，所以此处我们需要将 filterBoolean 转为 interface 接口
interface FilterPerson {
boolean filter(Person person);
}
public static String filterPersonsWithinAgeRange(List<Person> personList, FilterPerson filterPerson) {
StringBuilder stringBuilder = new StringBuilder();
for (Person person : personList) {
if (filterPerson.filter(person)) {
stringBuilder.append(person.getName()).append("|");
}
}
return stringBuilder.toString();
}
@Test
void testFilterPersonsWithinAgeRange() {
System.out.println(PersonBusiness.filterPersonsWithinAgeRange(personList, new PersonBusiness.FilterPerson() {
@Override
public boolean filter(Person person) {
if (person.getAge() >= 5 && person.getAge() <= 100) {
return true;
}
return false;
}
}));
}
可以将上面的匿名接口类抽出来实现类再操作，但这样子的代码就显得十分臃肿了
@Test
void testFilterPersonsWithinAgeRangeByLambda() {
System.out.println(
PersonBusiness.filterPersonsWithinAgeRange(personList,
person -> person.getAge() >= 5 && person.getAge() <= 100
)
);
}
JDK 针对以上的这些场景也设计了一些基本业务场景的@FunctionInterface
@FunctionalInterface
public interface Predicate<T> {
/**
* Evaluates this predicate on the given argument.
*
* @param t the input argument
* @return {@code true} if the input argument matches the predicate,
* otherwise {@code false}
*/
boolean test(T t);
/**
* Returns a composed predicate that represents a short-circuiting logical
* AND of this predicate and another. When evaluating the composed
* predicate, if this predicate is {@code false}, then the {@code other}
* predicate is not evaluated.
*
* <p>Any exceptions thrown during evaluation of either predicate are relayed
* to the caller; if evaluation of this predicate throws an exception, the
* {@code other} predicate will not be evaluated.
*
* @param other a predicate that will be logically-ANDed with this
* predicate
* @return a composed predicate that represents the short-circuiting logical
* AND of this predicate and the {@code other} predicate
* @throws NullPointerException if other is null
*/
default Predicate<T> and(Predicate<? super T> other) {
Objects.requireNonNull(other);
return (t) -> test(t) && other.test(t);
}
Predicate 起到断言，test 获得 true、false 的内容
在看刚刚的代码，我们也可以发现对于过滤后的对象操作，也属于一件过滤后加工的行为，也可以进行一个抽象，这里用到的是Consumer<T> block.accept(t)
public static void filterPersonsWithinAgeRangeByLambda(List<Person> personList, Predicate<Person> filterPerson,
Consumer<Person> consumer) {
for (Person person : personList) {
if (filterPerson.test(person)) {
consumer.accept(person);
}
}
}
@Test
void filterPersonsWithinAgeRangeByLambda() {
PersonBusiness.filterPersonsWithinAgeRangeByLambda(personList,
person -> person.getAge()>=5,
person-> System.out.println(person.getName()));
}
在处理对象时，我们可以看到是通过 person.getName()进行打印名字，假如说我们需要打印年龄，又会需要再写一个方法进行调用，像这种需要对对象进行操作，来获得一个值的操作可以使用Function<T,R> mapper.apply(t);
public static void filterPersonsWithinAgeRangeByFunction(List<Person> personList,
Predicate<Person> filterPerson,
Function<Person, String> functionPerson,
Consumer<String> consumer) {
for (Person person : personList) {
if (filterPerson.test(person)) {
consumer.accept(functionPerson.apply(person));
}
}
}
@Test
void filterPersonsWithinAgeRangeByFunction() {
PersonBusiness.filterPersonsWithinAgeRangeByFunction(personList,
person -> person.getAge() >= 5,
person -> String.valueOf(person.getAge()),
msg -> System.out.println(msg));
}
总结来说：这段代码实现的内容为
- 获取
Collection数据源，实现了 List 和 Iteration 的接口 - 通过 Predicate 进行过滤
- 通过 Maps 对过滤后对象进行映射为想要的值
- 对于映射后的值做相应 Consumer 的处理操作
personList.stream()
.filter(p->p.getAge()>=5)
.map(p->p.getName())
.forEach(name->System.out.print(name));
interface
default 默认方法
由于接口一定要实现方法，但是有时需要增加一些默认方法，但不需要所有实现接口的方法都实现一次时，可以通过默认方法进行补充实现
interface FilterPerson{
boolean filter(Person person);
default boolean filterPersonByAge(Person person){
return person.getAge()>=5;
}
}
@FunctionalInterface
public interface Consumer<T> {
/**
* Performs this operation on the given argument.
*
* @param t the input argument
*/
void accept(T t);
/**
* Returns a composed {@code Consumer} that performs, in sequence, this
* operation followed by the {@code after} operation. If performing either
* operation throws an exception, it is relayed to the caller of the
* composed operation. If performing this operation throws an exception,
* the {@code after} operation will not be performed.
*
* @param after the operation to perform after this operation
* @return a composed {@code Consumer} that performs in sequence this
* operation followed by the {@code after} operation
* @throws NullPointerException if {@code after} is null
*/
default Consumer<T> andThen(Consumer<? super T> after) {
Objects.requireNonNull(after);
return (T t) -> { accept(t); after.accept(t); };
}
}
在实现了两个接口，含有同名方法时，需要 override，不然无法执行对应的函数
package com.tyrantqiao.algorithms.interview;
import com.tyrantqiao.algorithms.lambda.Person;
/**
* @author tyrantqiao
* @ClassName JDK8InterfaceMethod
* @date 2022/2/13 17:15
* @Version 1.0
* @Description jdk8 interface 的默认方法和静态方法
*/
public class JDK8InterfaceMethod {
interface House {
void live(Person person);
default void log(Person person) {
System.out.println("live person:" + person.getName());
}
}
interface Car {
void drive(Person person);
default void log(Person person) {
System.out.println("drive car:" + person.getName());
}
}
class NewJob implements House, Car {
@Override
public void live(Person person) {
}
/**
* 因为实现的两个接口都有default的同名方法，所以需要override方法，实现自己独有的内容
*
* @param person person
*/
@Override
public void log(Person person) {
System.out.println("new job contains house and car," + person.getName());
}
@Override
public void drive(Person person) {
}
}
}
静态方法
静态方法其实和默认方法差不多，不同就是来自于 static 的差异了，可以通过类进行执行，实现方法不能进行 override，保证了方法可以作为应用的统一类型接口
static boolean isNull(Person person) {
return Objects.isNull(person);
}
比如像 Comparator 的比较方法，作为比较的基类方法进行提供，同时有抽象的能力，这样子实现的接口不用再实现自己的逻辑，避免这个方法受到污染。
public static <T, U> Comparator<T> comparing(
Function<? super T, ? extends U> keyExtractor,
Comparator<? super U> keyComparator)
{
Objects.requireNonNull(keyExtractor);
Objects.requireNonNull(keyComparator);
return (Comparator<T> & Serializable)
(c1, c2) -> keyComparator.compare(keyExtractor.apply(c1),
keyExtractor.apply(c2));
}
