lambda表达式，java双冒号（::）示例详解

Posted 2022-02-22 blouson

双冒号（::）主要使用形式包括：

类名::实例方法

对象::实例方法

下面通过代码示例，详细解说。

双冒号（::）和 箭头函数（->）一并展示如下：

如：HashMap::new  等同于  ( ) -> new HashMap()

 1 public class Test 
 2 
 3     // 实例对象引用实例方法
 4     Supplier<String> supplier1 = "lowerCase"::toUpperCase;
 5     Supplier<String> supplier1_1 = () -> "lowerCase".toUpperCase();
 6 
 7     // 类引用(无参)构造函数
 8     Supplier<String> supplier2 = String::new;
 9     Supplier<String> supplier2_1 = () -> new String();
10 
11     // 类引用(有参）构造函数
12     Function<String, String> function1 = String::new;
13     Function<String, String> function1_1 = (String str) -> new String(str);
14 
15     // 类引用实例方法，入参为传入实例对象，入参、出参同类型
16     Function<String, String> function2 = String::toUpperCase;
17     Function<String, String> function2_1 = (String str) -> str.toUpperCase();
18 
19     // Predicate<T>可理解为特殊的Function<T, Boolean>
20 
21     Person person = new Person();
22     // 须为无参静态方法
23     Supplier<Boolean> supplierBln = Person::isTest;
24     Supplier<Boolean> supplierBln_1 = () -> Person.isTest();
25 
26     // 实例对象调用实例方法
27     Supplier<String> supplierStr = person::getName;
28     Supplier<String> supplierStr_1 = () -> person.getName();
29 
30     // 无参构造函数
31     Supplier<Person> supplierPerson = Person::new;
32     Supplier<Person> supplierPerson_1 = () -> new Person();
33 
34     // 有参构造函数
35     BiFunction<String, String, Person> biFunction = Person::new;
36     BiFunction<String, String, Person> biFunction_1 = (name, gender) -> new Person(name, gender);
37 
38     // 类名调用实例方法，入参为传入实例对象
39     Function<Person, Person> functionP = Person::toOpposite;
40     Function<Person, Person> functionP_1 = person -> person.toOpposite();
41 
42     Consumer<String> consumer = System.out::println;
43     Consumer<String> consumer_1 = (String str) -> System.out.println(str);;
44 
45     public static void main(String[] args) 
46         List<String> list = Arrays.asList("1", "2", "3");
47         boolean bl = list.stream().anyMatch("1"::equals);
48         List<String> retval = list.stream().collect(Collectors.toCollection(LinkedList::new));
49 
50         List<Person> persons = Arrays.asList(new Person(10, "Jack", "M"));
51         Person person = new Person(20, "Lily", "F");
52         persons.stream().filter(Person::isMale).filter(person::isUnder).collect(Collectors.toCollection(ArrayList::new));
53     
54 

 

Person类代码如下：

 1 public class Person 
 2     int age;
 3     String name;
 4     String gender;
 5 
 6     public Person() 
 7     
 8 
 9     public Person(String name) 
10         this.name = name;
11     
12 
13     public Person(String name, String gender) 
14         this.name = name;
15         this.gender = gender;
16     
17 
18     public Person(int age, String name, String gender) 
19         this.age = age;
20         this.name = name;
21         this.gender = gender;
22     
23 
24     public String getName() 
25         return this.name;
26     
27 
28     public Person toOpposite() 
29         if (gender.charAt(0) == ‘M‘)
30             gender = "F";
31         else
32             gender = "M";
33         return this;
34     
35 
36     public static boolean isTest() 
37         return true;
38     
39 
40     public boolean isUnder(Person person) 
41         return person.age > this.age;
42     
43 
44     public boolean isMale() 
45         return gender.equals("M");
46     
47