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java8 stream的reduce()方法


reduce是用来执行聚合操作的。
参数BinaryOperator接口继承了BiFunction接口,简单的说就是
传入2个参数,返回一个参数。 2个入参,和一个出参类型都相同。
直接上代码:

@Data
public class User {

private String username;
private String password;
private Integer age;
public User(){

}

public User(String username, String password, Integer age) {
this.username = username;
this.password = password;
this.age = age;
}

public static void main(final String... args) {
ArrayList<User> list = new ArrayList<>();

list.add(new User("liubei","111",40));
list.add(new User("zhangfei","222",30));
list.add(new User("guanyu","333",35));

//reduce 需要传入一个BinaryOperator接口,继承了BiFunction接口
//简单说 就是需要传入2个同类型参数,返回同类型的参数
BinaryOperator<Integer> add = (n1, n2) -> n1+n2;
BinaryOperator<Integer> minus = (n1, n2) -> n1-n2;
list.stream().map(n->n.getAge()).reduce((a,b)->a*b).ifPresent(n-> System.out.println(n)); // 乘法
list.stream().map(n->n.getAge()).reduce(add).ifPresent(n-> System.out.println(n)); // 加法
list.stream().map(n->n.getAge()).reduce(minus).ifPresent(n-> System.out.println(n)); // 减法

}
}


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