【持续记录】java8流常用代码

深藏阁楼爱情的钟 2022-11-18 02:17 189阅读 0赞

对于java8中的stream，由于工作中不常使用，所以很容易忘记。但一旦有相应场景，使用起来很方便，在这里记录一下我使用过的常用流。  
我对流操作的感受：流操作可以类比我们的sql语句，用流可以实现where、group by等。  
实体类：

class Person{ 
            private String name;
            private int age;
            private String province;
            private String city;
            
            public Person(String name, int age, String province, String city) { 
                this.name = name;
                this.age = age;
                this.province = province;
                this.city = city;
            }
    
            public Person(String name, int age) { 
                this.name = name;
                this.age = age;
            }
    
    		// 节约博客空间，省略getter、setter、toString
        }

### group by ###

private void groupBy(){ 
            List<Person> personList = new ArrayList<>();
            Person person1 = new Person("张三",13,"吉林","长春");
            Person person2 = new Person("陈二",12,"吉林","四平");
            Person person3 = new Person("李四",14,"辽宁","沈阳");
            personList.add(person1);
            personList.add(person2);
            personList.add(person3);
            Map collect = personList.stream().collect(Collectors.groupingBy(Person::getProvince, Collectors.groupingBy(Person::getCity)));
            System.out.println(collect);
        }

结果：

{吉林={四平=[Person{name='陈二', age=12, province='吉林', city='四平'}], 长春=[Person{name='张三', age=13, province='吉林', city='长春'}]}, 辽宁={沈阳=[Person{name='李四', age=14, province='辽宁', city='沈阳'}]}}

指定group by结果集，一个group by:

private void groupBy(){ 
            List<Person> personList = new ArrayList<>();
            Person person1 = new Person("张三",13,"吉林","长春");
            Person person2 = new Person("张五",13,"吉林","长春");
            Person person3 = new Person("陈二",12,"吉林","四平");
            Person person4 = new Person("李四",14,"辽宁","沈阳");
            personList.add(person1);
            personList.add(person2);
            personList.add(person3);
            personList.add(person4);
            Map<String,List<Person>> collect = personList.stream().collect(Collectors.groupingBy(Person::getProvince/*, Collectors.groupingBy(Person::getCity)*/));
            System.out.println(collect);
        }

指定group by结果集，两个group by：

private void groupBy(){ 
            List<Person> personList = new ArrayList<>();
            Person person1 = new Person("张三",13,"吉林","长春");
            Person person2 = new Person("张五",13,"吉林","长春");
            Person person3 = new Person("陈二",12,"吉林","四平");
            Person person4 = new Person("李四",14,"辽宁","沈阳");
            personList.add(person1);
            personList.add(person2);
            personList.add(person3);
            personList.add(person4);
            Map<String,Map<String,List<Person>>> collect = personList.stream().collect(Collectors.groupingBy(Person::getProvince, Collectors.groupingBy(Person::getCity)));
            System.out.println(collect);
        }

### filter ###

使用stream().filter()来过滤一个List对象，查找符合条件的对象集合。  
下面示例的等效SQL：age>13 AND province=‘吉林’

private void filter(){ 
            List<Person> personList = new ArrayList<>();
            Person person1 = new Person("张三",12,"吉林","长春");
            Person person2 = new Person("陈二",14,"吉林","四平");
            Person person3 = new Person("陈五",15,"吉林","松原");
            Person person4 = new Person("李四",16,"辽宁","沈阳");
            personList.add(person1);
            personList.add(person2);
            personList.add(person3);
            personList.add(person4);
            List<Person> collect = personList.stream().filter(person -> person.getAge() > 13 && person.getProvince().equals("吉林")).collect(Collectors.toList());
            System.out.println(collect);
        }

结果：

[Person{name='陈二', age=14, province='吉林', city='四平'}, Person{name='陈五', age=15, province='吉林', city='松原'}]

### sort ###

public void sort(){ 
            List<Person> personList = new ArrayList<>();
            Person person1 = new Person("张三",13);
            Person person2 = new Person("陈二",9);
            Person person3 = new Person("李四",14);
            personList.add(person1);
            personList.add(person2);
            personList.add(person3);
            // 按照年龄排序，默认升序
            List<Person> collect = personList.stream().sorted(Comparator.comparing(Person::getAge)).collect(Collectors.toList());
            System.out.println(collect);
        }