一、函数式编程思想：主要关注对数据进行什么操作，易于并发编程
二、Lambda表达式：是函数式编程的体现。
例：替换匿名内部类
//匿名内部类
new Thread(new Runnable(){
@Override
public void run(){
System.out.println("test");
}
}).start();
//函数式编程 -> 只关注方法实现本省，去掉冗余代码
new Thread(()->{
System.out.println("test");
}).start();
省略规则：
参数可省略；方法体中只有一句代码时大括号return和唯一一句代码的分号可以省略；方法只有一个参数时小括号可以省略
三、Stream流：java8的Stream使用的是函数式编程模式，被用来对集合或数组进行链状流式操作。
创建流： public class Author {
private String id;
private String name;
private int age;
private String intro;
private List book;
public Author(List book) {this.book = book;}
public void setBook(List book) {this.book = book;}
public List getBook() {return book;}
public String getId() {return id;}
public void setId(String id) {this.id = id;}
public String getName() {return name;}
public void setName(String name) {this.name = name;}
public int getAge() {return age;}
public void setAge(int age) {this.age = age;}
public String getIntro() {return intro;}
public void setIntro(String intro) {this.intro = intro;}
}
public class Book {
private String id;
private String name;
private String category;
private String score;
private String intro;
public String getId() {return id;}
public void setId(String id) {this.id = id;}
public String getName() {return name;}
public void setName(String name) {this.name = name;}
public String getCategory() {return category;}
public void setCategory(String category) {this.category = category;}
public String getScore() {return score;}
public void setScore(String score) {this.score = score;}
public String getIntro() {return intro;}
public void setIntro(String intro) {this.intro = intro;}
}
//单列集合
ArrayList authors = new ArrayList<>();
Stream stream = authors.stream();
//数组
Integer[] arr = {1,2,3,4,5};
Stream stream1 = Arrays.stream(arr);
//双列集合
Map map = new HashMap<>();
map.put("1",1);
map.put("2",2);
map.put("3",3);
Stream> stream2 = map.entrySet().stream();
中间操作：
authors.stream()
.distinct()
.filter(bean->bean.getAge()<18)
.forEach(bean-> System.out.println(bean.getName()));
authors.stream()
.map(bean->bean.getAge()+10)
.forEach(x-> System.out.println(x));
authors.stream()
.distinct()
.forEach(bean-> System.out.println(bean.getName()));
authors.stream()
.sorted(Comparator.comparing(bean->bean.getAge()))
.forEach(bean-> System.out.println(bean.getAge()));
authors.stream()
.distinct()
.sorted(Comparator.comparing(bean->bean.getAge()))
.limit(2)
.forEach(bean->bean.getName());
authors.stream()
.distinct()
.sorted(Comparator.comparing(bean->bean.getAge()))
.skip(1)
.forEach(bean->bean.getName());
authors.stream()
.flatMap(bean->bean.getBook().stream())
.forEach(book->book.getName());
authors.stream()//打印数据所有分类，要求对分类进行去重，按照“，”分割开
.flatMap(bean->bean.getBook().stream())
.distinct()
.flatMap(book->Arrays.stream(book.getCategory().split(","))) //把数字转换成流
.distinct();
终结操作：
authors.stream()
.map(bean->bean.getName())
.distinct()
.forEach(name-> System.out.println(name));
authors.stream()
.flatMap(bean->bean.getBook().stream())
.distinct()
.count();
Optional max = authors.stream()
.flatMap(bean->bean.getBook().stream())
.map(x->x.getScore())
.max((score1,score2) -> score1 - score2);
authors.stream()
.map(bean->bean.getName())
.collect(Collectors.toList());
authors.stream()
.flatMap(bean->bean.getBook().stream())
.collect(Collectors.toSet());
authors.stream()
.collect(Collectors.toMap(new Function() {
@Override
public String apply(Author author) {
return author.getName();
}
}, new Function>() {
@Override
public List apply(Author author) {
return author.getBook();
}
}));
Map> collect = authors.stream()
.distinct()
.collect(Collectors.toMap(author -> author.getName(), author -> author.getBook()));
//判断是否有年龄在28以上的作家
authors.stream()
.anyMatch(bean->bean.getAge()>28);
//判断是否所有的作家都是成年
authors.stream()
.allMatch(bean->bean.getAge() >= 18);
//判断所有的作家都没有超过100
authors.stream()
.noneMatch(bean->bean.getAge()>100);
//获取任意一个大于18的作家，如果存在就输出
authors.stream()
.filter(bean->bean.getAge()>18)
.findAny().ifPresent(bean-> System.out.println(bean.getName())); //如果不为null，则输出名字，为null则不输出
//获取一个年龄最小的作家，并输出他的名字
authors.stream()
.sorted(Comparator.comparing(bean->bean.getAge()))
.findFirst().ifPresent(bean->bean.getName());
//使用reduce求所有作者年龄的和
authors.stream()
.map(bean->bean.getAge())
.reduce(0, new BinaryOperator() {
@Override
public Integer apply(Integer result, Integer element) {
return result+element;
}
});
authors.stream()
.map(bean->bean.getAge())
.reduce(0, (result,element) -> result + element);
//使用reduce求所有作者中年龄最大的
authors.stream()
.map(bean->bean.getAge())
.reduce(Integer.MIN_VALUE,(result,element) -> result< element?element:result);
//使用reduce求所有作者中年龄最小的
authors.stream()
.map(bean->bean.getAge())
.reduce(new BinaryOperator() {
@Override
public Integer apply(Integer result, Integer element) {
return result > element ? element : result;
}
}).ifPresent(age-> System.out.println(age));
authors.stream()
.map(Author::getAge)
.reduce((result, element) -> result > element ? element : result);
Optional空值处理：
函数式接口
方法引用
高级用法
并行流
