count

返回流中数据的数量

List<String> list = new ArrayList<>();
Collections.addAll(list,"李雷", "韩梅梅", "lucy", "lisi","李小四");
long count = list.stream()
        .filter(item -> item.startsWith("李")) // 过滤姓李的数据
        .count();
System.out.println("总共元素："+count);

max

获取最大值

元素是数字类型

List<Integer> list = new ArrayList<>();
Collections.addAll(list,1,2,3,4);
Optional<Integer> res = list.stream()
        .min(Integer::compare); // 指定比较方式
System.out.println(res.get());

元素是对象类型

如果 List 中的元素是类对象，处理起来要稍微复杂些

Student 类：

package std;

public class Student{
    String name;
    int age;

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    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;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

核心代码：

List<Student> list = new ArrayList<>();
Collections.addAll(list,
        new Student("李雷",20),
        new Student("韩梅梅",19),
        new Student("lili",24),
        new Student("lucy",22)
);
Optional<Student> res = list.stream()
        .max(Comparator.comparingInt(Student::getAge)); // 指定比较 age

System.out.println(res.get());

提示：

Comparator.comparingInt 的底层代码如下：

可知底层调用的是：Integer.compare() 方法

min

获取最小值

List<Integer> list = new ArrayList<>();
Collections.addAll(list,1,2,3,4);
Optional<Integer> res = list.stream()
        .max(Integer::compare);
System.out.println(res.get());