Java8-09-functional 函数式编程

淡淡的烟草味﹌ 2022-02-28 13:08 250阅读 0赞

### 文章目录 ###

*  Functional Programming in Java
 *  Usage of collection
 *  List translate
 *  Collection filter
 *  Optional
 *  MapReduce
 *  String Iterator
 *  Collect
 *  拓展阅读
 *  目录导航

# Functional Programming in Java #

Lambda expressions are lightweight, highly concise anonymous methods backed by functional interfaces in Java 8.  
You can use them to leap forward into a whole new world of programming in Java.

优点：

*  声明式
 *  提倡不可变性
 *  避免副作用
 *  优先使用表达式而不是语句
 *  使用高阶函数进行设计

> [Functional Programming in Java][]

> [blog zh\_CN][blog zh_CN]

> [deep mind][]

# Usage of collection #

对于列表

static final List<String> stringList = Arrays.asList("hello", "world", "hello", "lambda");

传统的打印方法如下：

@Test
    public void commonTest() { 
        for(String string : stringList) { 
            System.out.println(string);
        }
    }

引入新的方法如下：

@Test
    public void oneTest() { 
        stringList.forEach(new Consumer<String>() { 
            @Override
            public void accept(String s) { 
                System.out.println(s);
            }
        });
    }
    
    @Test
    public void twoTest() { 
        stringList.forEach((final String s) -> System.out.println(s));
    }
    
    @Test
    public void threeTest() { 
        stringList.forEach((s) -> System.out.println(s));
    }
    
    @Test
    public void fourTest() { 
        stringList.forEach(System.out::println);
    }

# List translate #

想将上述列表全部转化为大写。

*  common way

@Test
    public void commonTest() { 
        List<String> strings = new LinkedList<>();
        for (String string : stringList) { 
            strings.add(string.toUpperCase());
        }
        System.out.println(strings);
    }

*  new way

@Test
    public void innerIterTest() { 
        List<String> strings = new LinkedList<>();
        stringList.forEach(s -> strings.add(s.toUpperCase()));
        System.out.println(strings);
    }
    
    @Test
    public void streamTest() { 
        stringList
                .stream()
                .map(s -> s.toUpperCase())
                .forEach(s -> System.out.println(s));
    }
    
    @Test
    public void methodReferenceTest() { 
        stringList
                .stream()
                .map(String::toUpperCase)
                .forEach(System.out::println);
    }

# Collection filter #

@Test
    public void filterTest() { 
        stringList
                .stream()
                .filter(s->s.startsWith("g"))
                .forEach(System.out::println);
    }

一、复用 lambda

static final List<String> colorList = Arrays.asList("red", "green", "hack", "yellow");
    
    @Test
    public void reuseLambdaTest() { 
        final Predicate<String> startsWithG = name -> name.startsWith("g");
        stringList.stream().filter(startsWithG).forEach(System.out::println);
        colorList.stream().filter(startsWithG).forEach(System.out::println);
    }

这里复用的依然不够彻底。

二、场景2

查找以 g/w 开头的字符串

/** * 查找以 g/w 开头的字符串 */
    @Test
    public void filterTest() { 
        final Predicate<String> startsWithG = name -> name.startsWith("g");
        final Predicate<String> startsWithW = name -> name.startsWith("w");
    
        stringList
                .stream()
                .filter(startsWithG)
                .forEach(System.out::println);
        stringList
                .stream()
                .filter(startsWithW)
                .forEach(System.out::println);
    }

1、存在冗余。可以使用静态方法消除。

/** * 判断是否以某字符开始 * * @param letter * @return */
    private static Predicate<String> checkIfStartsWith(final String letter) { 
        return name -> name.startsWith(letter);
    }
    
    @Test
    public void staticMethodTest() { 
        stringList
                .stream()
                .filter(checkIfStartsWith("g"))
                .forEach(System.out::println);
        stringList
                .stream()
                .filter(checkIfStartsWith("w"))
                .forEach(System.out::println);
    }

2、使用函数，替换静态方法

@Test
    public void functionTest() { 
        final Function<String, Predicate<String>> startsWithLetter = (String letter) -> { 
            Predicate<String> checkStarts = (String name) -> name.startsWith(letter);
            return checkStarts;
        };
    
        stringList
                .stream()
                .filter(startsWithLetter.apply("g"))
                .forEach(System.out::println);
        stringList
                .stream()
                .filter(startsWithLetter.apply("w"))
                .forEach(System.out::println);
    }

上述方法可以使用 lambda 简化如下:

@Test
    public void lambdaFuncTest() { 
        final Function<String, Predicate<String>> startsWithLetter =
                (String letter) -> (String name) -> name.startsWith(letter);
    
        //...
    }

# Optional #

/** * 查询符合条件的字符串内容 * @param names * @param startingLetter */
    private static void pickName(
            final List<String> names, final String startingLetter) { 
        final Optional<String> foundName =
                names.stream()
                        .filter(name ->name.startsWith(startingLetter))
                        .findFirst();
        System.out.println(String.format("A name starting with %s: %s",
                startingLetter, foundName.orElse("No name found")));
    }

测试如下:

@Test
    public void optionalTest() { 
        pickName(stringList, "h");
        pickName(stringList, "Z");
    }

result:

A name starting with h: hello
    A name starting with Z: No name found

# MapReduce #

/** * 计算全部字符串的长度之和 */
    @Test
    public void sumTest() { 
        int total = stringList.stream().mapToInt(s->s.length()).sum();
        System.out.println(total);
    }
    
    /** * 找到最长的一个字符串 */
    @Test
    public void longestTest() { 
        final Optional<String> aLongName = stringList.stream()
                .reduce((name1, name2) ->
                        name1.length() >= name2.length() ? name1 : name2);
        aLongName.ifPresent(name ->
                System.out.println(String.format("A longest name: %s", name)));
    }
    
    /** * 列表内容进行拼接 */
    @Test
    public void joinTest() { 
        String string = stringList.stream()
                .map(String::toUpperCase)
                .collect(Collectors.joining(","));
        System.out.println(string);
    }

# String Iterator #

// 104
    // 101
    // 108
    // 108
    // 111
    @Test
    public void commonTest() { 
        final String string = "hello";
        string.chars().forEach(System.out::println);
    }
    
    // h
    // e
    // l
    // l
    // o
    @Test
    public void toCharTest() { 
        final String string = "hello";
        string.chars()
                .mapToObj(ch-> ((char) ch))
                .forEach(System.out::println);
    }

# Collect #

*  Person.java

public class Person { 
    
        private String name;
    
        private int age;
    
        public Person(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 "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

*  CollectTest.java

/** * 构建列表 * @return */
    private static List<Person> buildPersonList() { 
        List<Person> personList = new LinkedList<>();
        personList.add(new Person("Apple", 12));
        personList.add(new Person("Box", 12));
        personList.add(new Person("Cat", 20));
        personList.add(new Person("Dog", 26));
        personList.add(new Person("Eye", 62));
        return personList;
    }
    
    //[Person{name='D', age=26}, Person{name='E', age=62}]
    @Test
    public void older20Test() { 
        List<Person> older20List = new LinkedList<>();
        buildPersonList().stream().filter(person -> person.getAge() > 20)
                .forEach(person -> older20List.add(person));
        System.out.println(older20List);
    }
    
    //[Person{name='D', age=26}, Person{name='E', age=62}]
    @Test
    public void older20CollectTest() { 
        List<Person> older20List = buildPersonList().stream()
                .filter(person -> person.getAge() > 20)
                .collect(Collectors.toList());
        System.out.println(older20List);
    }
    
    // Grouped by age: {20=[Person{name='C', age=20}], 26=[Person{name='D', age=26}], 12=[Person{name='A', age=12}, Person{name='B', age=12}], 62=[Person{name='E', age=62}]}
    @Test
    public void groupByAgeTest() { 
        Map<Integer, List<Person>> peopleByAge =
                buildPersonList().stream()
                        .collect(Collectors.groupingBy(Person::getAge));
        System.out.println("Grouped by age: " + peopleByAge);
    }
    
    // People grouped by age: {20=[C], 26=[D], 12=[A, B], 62=[E]}
    @Test
    public void getNameGroupByAgeTest() { 
        Map<Integer, List<String>> nameOfPeopleByAge =
                buildPersonList().stream()
                        .collect(Collectors.groupingBy(Person::getAge, Collectors.mapping(Person::getName, Collectors.toList())));
        System.out.println("People grouped by age: " + nameOfPeopleByAge);
    }
    
    
    // Oldest person of each letter:
    // {A=Optional[Person{name='Apple', age=12}], B=Optional[Person{name='Box', age=12}], C=Optional[Person{name='Cat', age=20}], D=Optional[Person{name='Dog', age=26}], E=Optional[Person{name='Eye', age=62}]}
    @Test
    public void groupByFirstLetterTest() { 
        Comparator<Person> byAge = Comparator.comparing(Person::getAge);
        Map<Character, Optional<Person>> oldestPersonOfEachLetter =
                buildPersonList().stream()
                        .collect(Collectors.groupingBy(person -> person.getName().charAt(0),
                                Collectors.reducing(BinaryOperator.maxBy(byAge))));
        System.out.println("Oldest person of each letter:");
        System.out.println(oldestPersonOfEachLetter);
    }

# 拓展阅读 #

[java 函数式编程-系列教程][java _-]

# 目录导航 #

> [目录导航][Link 1]

[Functional Programming in Java]: https://pragprog.com/book/vsjava8/functional-programming-in-java
[blog zh_CN]: http://www.iteye.com/blogs/subjects/Java8-FP
[deep mind]: http://it.deepinmind.com/
[java _-]: https://www.iteye.com/blogs/subjects/Java8-FP
[Link 1]: http://blog.csdn.net/ryo1060732496/article/details/79424538