java stream-终结操作 count、max、min-马育民老师

# 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：

[![](/upload/0/0/1IX5KsJxdc74.png)](/upload/0/0/1IX5KsJxdc74.png)

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());
```

原文出处：http://malaoshi.top/show_1IX5HCciJkT3.html