Lambda表达式初学

Java  λ表达式

λ表达式本质上是一个匿名方法。

       记录常用方法（简单）

stream

@Test
        public void test2() {
            List<Integer> nums = Lists.newArrayList(1, 1, null, 2, 3, 4, null, 5, 6, 7, 8, 9, 10);

            System.out.println("sum is:" + nums.stream().

                    filter(num -> num != null).//过滤

                    distinct().//去重

                    mapToInt(num -> num * 2).//map处理

                    peek(System.out::println).//处理流，不改变原来流

                    skip(2).

                    limit(4).

                    sum()
            );

    }
           

运行结果

2
4
6
8
10
12
sum is:36
           

再来个例子 

Employee实体类

public class Employee {
   private String name;
    private int age;
    private String gender;
    private double salary;
//get and set
//toString
//contructor
   }
           

public class Notes {
    private List<Employee> employeeList=Lists.newArrayList();

    @Before
    public void setUp(){
        employeeList.add(new Employee("James",16,"M",10000));
        employeeList.add(new Employee("Kobe",17,"M",200000));
        employeeList.add(new Employee("AB",19,"F",2000));
        employeeList.add(new Employee("ABC",18,"F",20000));
        employeeList.add(new Employee("ABCD",20,"F",20000));
        employeeList.add(new Employee("ABCDE",21,"M",20000));
        employeeList.add(new Employee("ABCD",15,"M",20000));
        employeeList.add(new Employee("ABCD",15,"M",20000));
        employeeList.add(new Employee("ABCD",15,"M",20000));
    }

//排序
    @Test
    public void testSort(){
        employeeList.stream().sorted((a,b) ->(a.getName().length()-b.getName().length())).forEach(System.out::println);
    }
    @Test 
    public void testGroupingBy(){
      Map<String,List<Employee>> employeeMap=   employeeList.stream().distinct().
                collect(Collectors.groupingBy(p ->p.getGender()));
        System.out.println(employeeMap.get("M"));
        System.out.println(employeeMap.get("F"));
    }

    @Test/
    public void testPartitioningBy(){
        Map<Boolean,List<Employee>> employeeMap=   employeeList.stream().
                collect(Collectors.partitioningBy(p -> p.getAge()>=18));
        System.out.println(employeeMap.get(true));
        System.out.println(employeeMap.get(false));
    }

           

//list 转 Map 保证key不重复
  Map<String, String> areaMap = areaList.stream().collect(Collectors.toMap(Area::getAreaCode, Area::getAreaName));
           

//list去重      

@Test
public void test3() {
    List<TcPlace> tcPlaces = new ArrayList<>();
    tcPlaces.add(new TcPlace(13L, "大毛"));
    tcPlaces.add(new TcPlace(14L, "二毛"));
    tcPlaces.add(new TcPlace(15L, "三毛"));
    tcPlaces.add(new TcPlace(13L, "四毛"));
    List<TcPlace> tcPlaces22 = tcPlaces.stream().collect(
            Collectors.collectingAndThen(Collectors.toCollection(
                    () -> new TreeSet<>(Comparator.comparing(o -> o.getId()))), ArrayList::new)
    );
}