Java：Lambda表达式、Stream

原创

彭世瑜 2022-02-28 17:00:21 ©著作权

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函数式接口 @FunctionalInterface

package com.demo;

import java.util.Arrays;
import java.util.Comparator;

public class Demo {
    public static void main(String[] args) {
        String[] array = new String[]{"pig", "cat", "dog"};

        Arrays.sort(array, new Comparator<String>(){
            @Override
            public int compare(String s1, String  s2) {
                return s1.compareTo(s2);
            }
        });

        System.out.println(Arrays.toString(array));
        // [cat, dog, pig]
    }
}

Lambda表达式

package com.demo;

import java.util.Arrays;

public class Demo {
    public static void main(String[] args) {
        String[] array = new String[]{"pig", "cat", "dog"};

        Arrays.sort(array, (s1, s2) -> {
            return s1.compareTo(s2);
        });

        System.out.println(Arrays.toString(array));
        // [cat, dog, pig]
    }
}

引用静态方法

package com.demo;

import java.util.Arrays;

public class Demo {
    public static void main(String[] args) {
        String[] array = new String[]{"pig", "cat", "dog"};

        Arrays.sort(array, Demo::compareTo);

        System.out.println(Arrays.toString(array));
        // [cat, dog, pig]
    }

    public static int compareTo(String s1, String s2){
        return s1.compareTo(s2);
    }
}

引用实例方法

// public int compareTo(String anotherString)
// 相当于：
// public static int compareTo(String this, String anotherString)
Arrays.sort(array, String::compareTo);

引用构造方法

class Person {
    private String name;

    public Person(String name) {
        this.name = name;
    }
}

package com.demo;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;


public class Demo {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Tom", "Jack", "Steve");
        List<Person> persons = new ArrayList<>();

        for (String name : names) {
            persons.add(new Person(name));
        }
    }
}

简写

package com.demo;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;


public class Demo {
    public static void main(String[] args) {
        List<String> names = Arrays.asList("Tom", "Jack", "Steve");

        List<Person> persons = names.stream()
                .map(Person::new)
                .collect(Collectors.toList());
    }
}

创建Stream


String[] pets = new String[]{"pig", "dog", "cat"};
Stream<String> stream = Arrays.stream(pets);


Stream<String> stream = Stream.of("pig", "dog", "cat");


Stream<String> stream = Arrays.asList("pig", "dog", "cat").stream();

// 映射
Stream<Integer> stream = Stream.of(1, 2, 3);
stream.map(x -> x * x).forEach(System.out::println);
// 1 4 9


// 过滤，留下偶数
Stream<Integer> stream = Stream.of(1, 2, 3);
stream.filter(x -> x % 2 == 0).forEach(System.out::println);
// 2


// 聚合
Stream<Integer> stream = Stream.of(1, 2, 3);
int ret  = stream.reduce(0, (sum, x) -> sum + x);
System.out.println(ret);
// 2